In this article I’ll cover the general things that apply to all Fiero versions, then later I’ll break out the version specific and detail items.
Contrary to popular belief, vacuum plumbing problems are a major issue. Even small vacuum leaks can cause major headaches.

A major problem? Why?

The ECM will try to compensate for leaks up to a certain size. The leak will often cause performance problems without setting DTCs in the ECM. Depending on exactly where the leak is, it can unbalance the entire engine.
Let’s say you’ve got an Iron Duke and the EGR is leaking slightly around the base. This allows air to enter the intake right near the middle cylinders, leaning them out just a little. The ECM tries to make up for this by increasing the fuel feed. The end result is that the end cylinders are probably running rich. The car may seem to run pretty normally but you’ll take a hit in fuel economy and most likely you’ll have strange drivability problems that are often hard to duplicate.
Any vacuum leak is letting air in after the IAC and Throttle Plate. The ECM tries to make up for this by closing the IAC and likely increasing the fuel flow. This can cause a number of odd things to happen, like goofy Idle and hesitation. Ultimately, there is no such thing as an acceptable vacuum leak. All leaks must be found and fixed.

Tools:

Spray can of “throttle body” and Fuel Injection cleaner. (This is safe for all EFI. Carburetor cleaners can sometimes do damage to EFI parts. Don’t use it.) ((I do NOT recommend most people use the propane search method. It is far to easy to start a fire with this method, especially if done with improvised equipment.))
Assorted line plugs and port caps.
Hand operated vacuum pump such as “Mighty Vac” or equivalent. If you are buying one of these, just get the whole vacuum brake bleeding kit. Then you’ll get more use from it and bleed the brakes or clutch allot faster.

General:

Eliminate the nonessential items first. Start the car after each of the following and see if it has solved your problem.
Disconnect the 2 lines to the Fuel Vapor Recovery canister. Plug the ports on the throttle body and/or manifold.
Disconnect the Cruise Control line and plug its manifold port.
Disconnect the EGR line from the throttle body and cap its port. (I’ll get more into the EGR later)
In the front compartment disconnect and plug the line on the brake booster. DO NOT attempt to drive the car with the booster disabled! The brake booster can leak internally. Carefully inspect the booster vacuum line and all it’s clamps. All joints in this line should be clamped and the rubber hose sections free of cracks or other damage. Make sure there are no rusted sections of steel line. (The section inside the cabin is likely OK.) If any section of the line looks questionable spray the suspect area with EFI cleaner. Check the line to the MAP sensor carefully. Hard line doesn’t like being bent, especially when cold. (DO NOT spray the MAP sensor with solvent when searching for a vacuum leak!)
If the above have not solved the problem then carefully begin searching with the spray cleaner. Start the car and spray the throttle body mountings and work your way around whatever isn’t disconnected. DO NOT spray the distributor, coil(s), sensors or IAC with cleaners or solvents! You will likely damage them or start a fire. Use as little spray as possible to find the leak.
Don’t soak things. Test an area, let it dry then test another. The RPMs should change one way or the other when you spray the leak.

Fuel Vapor Recovery Canister:

This device catches vapor from the fuel tank. It uses 2 vacuum lines. The large (purge) line is connected to manifold vacuum. This line is used to ventilate the canister. The smaller (control) line is connected to the main bore of the throttle body and only opens the purge valve beyond a certain amount of throttle.
Problems with the canister can include failure of the purge valve to close and cracks in the purge valve body or canister itself.
There is also a fiber filter in the bottom of the canister that should be replaced periodically.
Another problem that this device can develop is flooding. Flooding is caused when liquid fuel is forced up the vent line into the canister. A flooded canister can cause rich mix problems with the engine. A canister that has been flooded should be replaced. To prevent flooding avoid “topping off” the fuel tank when you fill up. If the nozzle of a retail fuel pump is defective, and they often are, then you can pressurize the fuel tank and force fuel up the vent line into the vapor canister. It doesn’t take allot of liquid fuel to screw up the canister. Even if the pump doesn’t force fuel into the canister, fuel expansion in the tank can and often will. Either way, topping off the tank is a bad idea.

Quick test….

Disconnect both lines and connect a hand vacuum pump to the large line. Give the pump a squeeze and make sure the line holds vacuum. 10 inches is plenty of vacuum for this test.
Now use the pump on the small line. Draw only 3-4 inches maximum on this line! More than a few inches vacuum can damage it. While you have vacuum on the control line, blow into the purge line. Air should move freely. While blowing into the purge line release the vacuum on the control line. The valve should close and block all airflow in the purge line.

Cruise Control:

Leaks in the cruise control servo diaphragm cannot be fixed. Some people have had luck cleaning the valves on the servo and/or the dump valve. (The dump valve is a backup valve that kills the cruise when the brake is pressed and is mounted separately from cruise servo.)
There is also a vacuum reserve canister in the cruise system. Older style canisters are often metal and are prone to rusting out. Replace these with new style plastic canisters.
There are 2 styles of canister. The ones with 2 fittings usually have integral check valves. The ones with single ports are used with a check valve in the manifold line.
You can replace the can and inline check valve with a 2 port vacuum ball, but you must also insure that the plumbing is redone to work correctly. I think the larger port on these is usually the manifold side but test it to make sure. The exact size of the canister is not very critical. Any canister of approximately the same volume or greater will work. You can get good used plastic canisters for about nothing at any salvage yard. The plastic ones will last nearly forever. Just make sure the check valve is working before you leave the yard. (Blowing into one port is very easy. The other is not going to let you if the valve works.)
BTW: You can get these from many American made cars. It doesn’t have to be a GM product.

As you can see, changing to the vacuum ball also simplifies the vacuum lines quite a bit. If you make this change, and cruise stops working, then you may have put the lines on the Vac Ball wrong.

Brake booster:

The brake booster unit should not be disassembled. If you believe it leaks then it will need to be replaced. You generally can’t get parts for the booster and there is a very heavy spring inside it that likes to send thing flying.
The check valve that the vacuum line connects to can go bad. This usually manifests as brakes that work right only at idle. The check valve shell can crack and cause a vacuum leak. You can replace the valve by itself. Try MotorMite’s HELP! or VacuTite lines.
Don’t forget to check the brake filter on cars that have them. The plastic shell of them is pretty tough but it can crack or melt. The brake filter does need to be replaced on occasion. If it plugs up you will have brake problems.

Hard lines:

Hard vacuum line can be a pain to service. You can get the line but finding the correct fittings can be difficult at best. (The rubber fittings are reusable if they are in good condition and sealing tightly to both the line and the port.) If you replace hard line with soft line make sure the new lines are not pinched or collapsed from bending too tightly. The soft line should be sized to properly slide onto the fittings and stay tight. Don’t stretch undersized line to fit. It will likely split in short order.
New Hard Line can be shaped or straightened by careful use of heat. (Try dipping the line in very hot water.) Be careful not to pinch or stretch the softened line. Don’t try to form old lines. They usually break even when warmed.
Note: If you replace the MAP sensor line with soft line then keep the line as short as possible. The soft line will have a larger internal volume per inch of line than the hard line. This increased volume can affect the reaction time of the MAP sensor. A slow MAP sensor will likely hurt performance. Even when using hard line it is best to keep the MAP feed short as possible.
Make sure the steel PCV valve line is not rusted through anyplace and that both elbows are sealed. GM recommends clamps on the elbows. Be careful with the clamps that you don’t cut into the soft elbows.

4 Cylinder specific:

The 4 cylinder EGR is bolted directly to the intake manifold. Make sure it’s tight. The bolts have a nasty habit of coming loose. Replace the gasket if the bolts have come loose at all. (The FelPro EGR gasket costs about $1.00 at most parts stores.)
The EGR valve is hard to test for leakage. Make sure the valve isn’t binding.
One way to test EGR problems is to make a block off plate and seal the manifold openings. Then run without it for a little bit. Note: Permanently running the car without the EGR is a crime in the United States. Running the car without the EGR can cause detonation, which can damage the engine. This is a testing procedure only.

87-88 TBI:

Be careful not to spray the MAT sensor’s plastic parts with solvent. Depending on the solvent used, you can damage the sensor. The MAT sensor is located to the right of the TBI unit and is screwed into the intake manifold. Avoid intake backfires in these engines. If the car backfires into the intake, the MAT sensor should be inspected for damage. If the MAT sensor has been cooked it should be replaced. (The MAT sensor is used in calculating fuel/air ratio.) Don’t over tighten the MAT sensor or you may split the intake manifold.
Check the base of the TBI for leaking, especially if there is any evidence the air cleaner has hit the trunk wall. (Read the torque strut article.) There is a specific sequence to tighten the 700 TBI to the manifold. Read the TBI 700 article. The emissions vacuum line map for these years is in the TBI 700 article. That map doesn’t include cruise.

V6 specific:

You’ll need to check the EGR plumbing very carefully. Use extreme caution if you decide to use spray or gas searching on this. The V6 EGR plumbing gets hot enough to light off many solvents on contact.
The V6 EGR control valve must also work properly. If it’s acting up, try electronics parts cleaner. Don’t use the cleaners with lubricant in them. The lubricants will hold dirt and quickly clog the valve up again. (Most “Tuner wash” and contact cleaners have lubricants in them.)
To test this valve, with the engine off, blow into the line connected to the EGR valve. The air should exit the vent hole. Cap the vent hole and blow again into the EGR port. No air should come out the manifold port.
To test during run, connect a vacuum gauge to a T in the EGR valve line. You should be able to see the vacuum rise and fall when the ECM has activated the EGR valve. Using an ECM scanner with the vacuum gauge may be helpful. I think the ECM will tell you when it’s trying to open/shut the EGR. You should see a corresponding change on the gauge. (I think the ECM may also report the EGR vacuum sensor status. I can never remember just what ECM reports what.) You must also carefully inspect the entire plenum. If bolts have come loose you may be able to simply tighten them but likely you will need to replace the gaskets.
Rodney Dickman now offers formed metal vacuum lines for the V6.

Who’s There?

Ping.
Ping Who?
Ping I just knocked a hole in your piston….

Engine Knock, Fuel Octane and other myths

Thing One: Never use higher Octane fuel than the engine needs to prevent knock.

I’m going to try to greatly simplify a very complex topic here. They write encyclopedias about this stuff. I’m going to try to keep it to a few pages.

What is Knock/Ping?

When the engine is running normally, the spark plug fires and the flame spreads evenly thru the cylinder. The mix burns evenly until consumed. When the engine Knocks or Pings it means the fuel was set off at the wrong time and/or the fuel was set off from multiple sources. Both conditions are bad for the motor and rob you of performance and fuel economy.

What causes it?

There are many things that can cause it. Most of them you can and should fix.
Here’s a list of common items. (In no particular order.)

• Over advanced ignition timing. (Very easy to do with mechanical distributors but still possible with electronic ignition.)
• Vacuum leaks or EGR problems. (These are way more common than people think.)
• Exhaust leaks. (Even small upstream leaks can screw up O2 sensor accuracy.)
• Wrong spark plugs or other bad ignition parts.
• Carbon buildup in cylinders. This alters engine compression and can create hot spots.
• Engine sensors dirty/bad or damaged sensor wiring. If a sensor is lying to the ECM the car will never run properly.
• Compression increased as a result of engine or head rebuild. (This can be intentional or a side effect.)

What is Fuel Octane Rating?

That number you see on the pump comes from a bunch of tests that determine how much the fuel helps resist engine knock. The higher the number the more it resists knock and it usually means the fuel is also less volatile. This last bit is the big rub… Less volatile fuel will often perform worse, not better as many people think. How much you may notice this depends on the specific engine in question. In any case, using more Octane than needed will not increase engine performance. Higher-octane fuel sells for more than regular fuel. Compared to most 87 Octane fuels, you pay 10-20 cents more per gallon for a product that costs them almost nothing more to make. That extra dollar or two per tank can add up really fast. Why spend it if you don’t have to… The carmakers all list the minimum Octane rating for fuels in the owner’s manuals. For most Fiero I’m pretty sure it’s 87 Octane. (I know the 87 owners manual lists 87 for both the 4 and 6 cylinder.) Oh, and the factory 2.8 has lower compression than the 4 cyl, so don’t go thinking bigger engine means more Octane. (2.8 is 8. something:1 while L4 is 9:1) Even motors with 100,000 miles or more can run the specified Octane if they are in good shape. Mine has over 150,000 miles and is running wonderfully on 87 Octane fuel. (Even WaWa fuel that is usually one of the lowest priced fuels around the region.) Note: Someone in the forums pointed out the European Octane ratings are higher than the U.S. numbers. Keep in mind other regions of the world use different methods and rules for grading fuel. While the European Octane number might be higher, that doesn’t mean they are automatically better than U.S. fuel at preventing knock.

Bending the rule, additives in Pump Fuel

All U.S. fuel is required by the EPA to have cleaning additives in it. Fuel meeting the minimum requirements is perfectly fine for most cars. Some fuel vendors promote their fuel based on the additive package. These additives won’t add power so much as restore or maintain it by removing carbon and dirt. These additives have contributed to confusion over octane ratings. These cleaning additives, which are unrelated to Octane rating, may help performance in some vehicles. This can be a function of brand, grade, or both. Some brands use the same additive package in all grades of fuel, while others put the “good stuff” only in higher grades. As an example, Texaco puts “Cleansystem3®” in all their fuel. Other brands may put their best additive package only in premium grades. What results you’ll get from any additive packages will depend greatly on the condition of your engine. None of them are likely to clean up a really dirty motor but they may help an engine expel minor carbon buildup and other dirt. Some no doubt do this better than others. Other than cost, it won’t hurt anything to shop fuels based on their additive package. If one product works better for you than something else that’s great. Otherwise don’t buy more than you need. To use Texaco again… Since they put Cleansystem3® in all grades, you’d still run the lowest Octane fuel the engine will take. If Texaco 87 Octane fuel works, their 89 and higher isn’t going to do anything but cost more. (Texaco is just a handy national brand for an example… I use a regional brand most of the time.)

The car knocks/pings…

We’re talking generally stock engines here. Even a regular rebuild should run on the OE octane or maybe one step higher. Obviously if you’ve done a big build up or super/turbo setup, you could need more octane. If a car that was built to run 87 Octane only runs on 90+, the engine may just be dirty or you could have mechanical problems. If the car knocks even on the highest Octane pump fuel you almost certainly have engine problems that need repair. These are some things I look for when an engine knocks. Triple check for vacuum leaks. Even a tiny leak can lean out a cylinder enough to cause knocking without tripping ECM errors. (You’d be surprised how easy it is to have a vacuum leak that only messes with one cylinder.) Check the cooling system very carefully. Knock/Ping can also be an early sign of overheating. Don’t assume the temperature gauge is correct. (Even if it is correct, don’t assume there are no cooling problems. A common problem here is a head gasket installed wrong. This can make some cylinders hot even if coolant temp reads ok on the gauge.) Check timing on distributor motors very carefully. The difference between knocking and not can be just a couple degrees of advance. (Defective or improperly configured Mechanical Distributors are great for this.) Use OE EGR valves (GM, AC Delco, Motorcraft, etc.) whenever possible. If you must use a “universal” style valve, make sure it’s a good brand. A crappy EGR valve can really screw up an engine. (I have seen aftermarket valves die in as little as a year.) Is the engine suffering carbon buildup? You might want to try one of the carbon removal processes. Carbon buildup can be the result of past fuel system problems and other things. It usually won’t go away without help. Is the engine sucking oil? This can both alter the octane of the mix inside the engine and cause carbon buildup. It’s probably time to rebuild some or all this engine or find another. Higher Octane fuel or carbon removal is at best a temporary fix. 87-88 Fiero L4 motors with DIS may need a different PROM in the ECM. GM had an update to tweak ignition timing a bit. (The new PROM numbers are in DIS notes. This kind of stuff is common with ECM/PCM controlled motors.) This should be considered ONLY AFTER all other mechanical items are checked out.

An example

Awhile back, my car started knocking under certain load conditions. (87SC L4 w/ DIS) I’d also lost some fuel economy. This went on for weeks and since the problem wasn’t consistent, it was really a pain to find. It did it even on 90+ Octane fuel. (Since it hadn’t done this previously, I didn’t think the PROM update would help.) In searching for a vacuum leak, I took off the EGR to see if maybe the middle of the gasket had partly blown out. (A common problem with this type of EGR setup.) The EGR wasn’t that old, and like many aftermarket units not easy to test. Since it wasn’t very old I didn’t really expect it to be bad but just out of habit I checked to see if the pintle was moving ok while it was off. It wasn’t moving at all… It turned out that the 2-year-old aftermarket EGR valve had failed totally. It had jammed in such a way as to not open and still leak slightly. (It never set any kind of code in the ECM during all this.) The spring in the top had actually broken and you could hear it rattling around when shaken. Replacing the EGR with a good used GM valve solved both the knock problem and cranked fuel economy back up to 26 mpg even in really crappy traffic. (The EGR gasket cost about a dollar.) The lesson, yet again, is never to assume recently installed parts are good.

Fuel additives

Octane Boosters and Cleaners

Octane boosting additives don’t help much except maybe for when you have a modified/race engine and can’t find suitable fuel at the pump. A stock vehicle that won’t run on pump gas has something wrong. Octane booster products, like higher Octane fuels, usually will not increase performance. How much, if anything, a cleaning product does will depend greatly on how messed up the engine is. Don’t expect any miracles from most of them. Even professional cleaning products may not remove heavy buildup. Some add in Octane boosters and cleaning products may actually damage the older styles of port fuel injectors used by GM and some others. In the older injectors, used thru the late 90’s, the solenoid coil is exposed to the fuel. (This helps cool the coil.) Most products are probably ok but make sure they are used as directed on the container. Mix them stronger than directed and they might eat your injectors. (The newer GM injectors are redesigned and do not expose the coil to the fuel just because of this problem.)

I’ve heard fuel changes by season….

Yes fuel does change by season and it also changes by city and region. This double whammy can make buying fuel quite confusing. If you suddenly drop or gain 2-4 MPG, you might be a victim of Reformulated or Oxygenated fuel. In some vehicles these fuels may also induce knock/ping that forces you to a higher, more expensive, grade of fuel. What many people call Winter Gas is defined as Oxygenated or Reformulated Gas (RFG) depending on where you live. They are supposed to reduce air pollution. These fuels contain MTBE and other chemicals to increase the oxygen content of the fuel. They cost more, often cause odd problems, bite performance, and you get less MPG. There is little you can do about the problems caused by RFG and Oxygenated fuels. On HEI engines, try retarding the timing very slightly if you have ping/knock.

Acceptable Alcohol Fuel Content

Methyl Tertiary-Butyl Ether
Fuel containing Methyl Tertiary-Butyl Ether (MTBE) may be used, providing there is no more than 15% alcohol by volume. MTBE is under investigation in many places. Some states are likely to ban it in future. MTBE is intended to reduce air pollution but has been shown to be a serious problem when it gets in the water. Small amounts of MTBE can pollute large amounts of water and the stuff is hard to filter from drinking water.

Ethanol
Fuel containing ethanol (ethyl) or grain alcohol may be used, providing there is no more than 10% ethanol alcohol by volume. Ethanol plays a roll in Octane rating and pollution control. Its presence in motor fuels is likely to increase greatly as MTBE (Methyl Tertiary Butyl Ether) is phased out in many regions. Higher concentrations of Ethanol will alter the fuel beyond what the ECM/PCM can deal with, and may be too strong for some fuel system parts to handle. Pre-computer engines may need considerable carburetor and distributor adjustments to run right.

Methanol
GM recommends against any methanol use. It can damage the fuel system parts. (Nearly all carmakers say stay away from methanol even now.) “Dry” gas typically contains Methanol but not enough to hurt anything. Fuel containing methanol (methyl) or wood alcohol may be used, providing there is no more than 5% methanol by volume. Use of fuel (gasohol) that contains more than 5% methanol can corrode metal fuel system components and damage plastic and rubber parts.

The TH440-T4 (4T60, RPO code ME9) is the only GM Automatic Over Drive transmission that will easily drop into otherwise stock Fiero. I’ve been interested in this conversion myself for a long time. I’ve pulled together some of the more pertinent data into this article for those considering the swap. Maybe I’ll get around to trying it someday.

Why Change The Tranny?

There are 2 ways to look at this, fuel economy and performance. The TH440-T4 can gain some of each. As you’ll see when you read the Space Coast article, (linked further down) the AOD 1st is slightly lower than the TH125 and the 4th is over driven. If you keep the same final gearing as you have, this gives a little better launch and better fuel economy. If during the swap you also go to a lower final drive ratio than your TH125 has, you should get measurably better launches and still turn slower in high gear than if you had changed only the TH125’s final gears. The exact launch vs. fuel economy result will vary depending on the finals chosen, but you’ll still get better than the TH125c with the same final gears. As with any transmission, lowering the finals will boost performance while lowering fuel economy but the AOD unit won’t beat you in the head like the TH125 will on a long trip.

It is worth the effort & cost?

Keep in mind that the gain in fuel economy etc is only going to happen if you aren’t constantly flogging the hell out of the car. Highway mileage is going to be affected much more than city. Your range on a long trip can go up quite a bit since the transmission will stay over driven most of the time.

City MPG is dependant almost entirely on the low gears and finals. If most of your driving is city miles, an AOD transmission will be of little or no benefit most of the time. The transmission just won’t spend enough time over driven to matter. It’s fairly common to have to manually shift an AOD to third in city traffic, which effectively leaves you with a lower geared TH125. Even if you keep the same finals, any improvement in fuel economy may be enough to make the swap worth doing. A big factor is if you can get the parts cheap and install it yourself. If you have to pay for it to be done then it may not make sense in terms of install cost vs. saved fuel cost. (Although that may change if fuel prices keep going up.) If you’ve upgraded V6 or V8 then the extra trip range alone may be enough to make it worth the effort but it will depend allot on what final gears are involved just how much improvement you get. In this case, even if you don’t save any money, you’ll gain the convenience of fewer fill ups. What finals are available?

See the Ratio article in the Axle & Transmission section.

The Math

Here’s a comparison of engine RPM to reach 60 and 70 mph with various finals thru OD and straight high gears. I used my stock 195/75R14 tire size to calculate this. Third gear in both TH125 and TH440 is 1:1. Fourth gear in the TH440 is .705 over driven. (This table doesn’t cover all possible combinations.)

As you can see, even with 3.33:1 finals you should gain some fuel economy compared to the TH125 with the 2.84:1 final.

Are there any hidden problems?

There is a catch some people may have to watch out for… How loaded is the electrical system? If you are frequently pushing the alternator to full load, have under driven the system, or worse both, you could run into problems covered in “Watt Story.” The AOD’s lower cruising RPM may not be fast enough for a heavily loaded electrical system to keep up with demand, which could kill the battery and strand you. (With an under drive setup, the water pump may also have trouble keeping up enough coolant flow. That part is a tougher call.) You’ve got to watch out for the Torque Converter if you change the tranny from one engine to another. In most cases the different engines use different TC’s. There are a couple variable besides the “stall” speed. If the TC still has the factory ID label, you can decode the label in the Torque Converter article. Installing a TC with the wrong stall speed or clutch can cause major headaches. The Space Coast article mentions that the AOD unit has more loss thru the tranny and it may not be a good choice on a 4 cylinder. That one is sort of a toss up. Most of the TH125’s for L4 have 2.84:1 final gears. If you went to a 3.06:1 final in the TH440, I would think it would more than make up any loss and still boost fuel economy. I don’t have anyway to really say for sure. It may also be less of an issue on latter L4’s, which are a little stronger to start with. It’s something to consider at any rate. Even if you install the transmission yourself, you may want to have a qualified transmission shop adjust the modulator and TV cable. The two controls work very closely together and getting them adjusted correctly is critical to proper transmission operation.

The Tranny (And fitting it in the car) When you collect the transmission, don’t forget to snatch the electrical connector for the TC lockup. There are apparently 3-4 versions of that connector.

There are a bunch of Fiero Forum threads on this item. (When searching forum threads, don’t forget to check in the TD&Q Archives!) Try Search words 440-T4 or 4T60.

The Space Coast Club has a good write up on the transmission itself. I’m not going to rehash all that. It covers some important things to watch for when choosing a donor, like the speedometer sender and the Neutral Start switch.

Big bunch of pictures.
This another good write up at Fiero Addiction. This one has more pictures on how to install the thing. Also covers specific issues of fitting one into an 88 Fiero.

More Axle & Shifter thoughts

This is pretty much covered in the sites linked just above…. Those sound like drop in solutions for the 4T60 in an otherwise stock Fiero. As yet I’ve not personally verified axles. Here are some things I ran across when digging around for this article. This data might be handy for people with engine conversions that need alternate/custom axles.

Axles

A combination listed in the Fiero Online Service Guide says use Pontiac Trans Port (Olds Silhouette) mini van inner CV and axle with Fiero Manual transmission outer CVs. I’m told CV Unlimited can make any axle you need. There are notes about what to order from them someplace in the forums but I can’t find it right now.
(I think the thread said the same as FOSG.)

One source sited in several threads is Pontiac 6000 with AOD and Light Duty Brakes. These are supposed to drop right in. There are several combinations of axle used in 6000 so it’s easy to get the wrong ones. By comparing notes from a number of articles/threads on axles and measuring the Fiero axle threads, I found the following in the CV Unlimited catalog. (You can download an Excel version of it from their site.)

Notice that both the 6000, and the Ciera in the Space Coast article are listed. This should help you isolate the correct used parts as well as new ones if needed.

The most obvious determining factor between the various AOD axles for the cars above seems to be the diameter of the threaded portion of the outer CV. A spare Fiero axle nut is a good tool for checking that. It was the nut size specification that I used to figure out the data above… 0.785 inch is roughly 20mm. The “Wrong” ones have are larger (23-24mm) and won’t fit the Fiero nut or hub opening.

Shifter

If the shifter won’t reach low gear, you don’t need to cut the extra detent unless you want to. Most people never use low gear on the three speed so I can’t imagine not reaching it on the 4 speed is that big a deal. (If you live in the land of snow and slop you may occasionally need it to get unstuck.) Many of the AODs I’ve run into in various vehicles don’t let you get to 1st gear. On the other hand, a new indicator window or a good way to mark it would be nice. I haven’t messed with that one at all.

TCC High Gear Interlock

I didn’t particularly like the idea of not having high gear switch(s) for the TC lockup circuit. The guy that did the Space Coast article said he doesn’t seem to have problems, but I wanted a way to emulate that function just in case so I came up with this…

The above circuit change can be done completely in the engine bay, which simplifies the wiring and allows easy service if the relay goes bad. The new relay needs to be a diode suppressed one. A Fuel Pump or AC clutch Relay will work. For a more finished look, it can be mounted next to the existing FP & AC relays and the wiring tucked into the flex conduit. The diagram shows using a Fiero Fuel Pump/AC relay. Nearly any relay with the spike suppression diode would work. Relay lead C goes to B, C, or both on the transmission depending on how you need lockup to work. (I don’t know off hand which gear switch is 3rd and 4th.)

Torque Converter Lockup.

Why is TC locking important?

The biggest reasons are fuel economy and heat control. The TC is the car’s single biggest heat generating device besides the engine. The transmission will run significantly hotter if the TC doesn’t lock. Before listening to people that tell you it doesn’t matter read this article from Borg Warner. Borg Warner Tech Tips, Transmission Fluid. It will also increase heat load on the radiator since it also cools the transmission. That could lead to easier overheating if the cooling system isn’t 100%. With the TC locked there is almost no heat from the transmission.

You’ll also waste gas. Likely allot. One of the advantages of having a locking torque converter is that it allows the TC to be built with more “slip” which increases the torque multiplier effect. This increased multiplication translates into quicker launches but horrible fuel consumption. Once the transmission reaches top gear, the TC is locked eliminating the slip that is now just wasting fuel and making heat. This is why today’s automatic transmissions both perform better and get nearly the same fuel economy as a manual transmission would. To learn more about the locking TC, go hear–> Motor Magazine Back Issues, and select the May 1999 issue then go to the bottom of the next page that opens. If you have acrobat loaded just get this–> Motor Magazine Torque Converter Article PDF.

Testing

TH125c has a locking torque converter system controlled by 4 electrical items. They are listed in the order of their location in the circuit. (Starting at the fuse end)

1. The Brake Pedal Switch. This switch opens to unlock the TC when you press the brake. It is on the brake pedal hardware.
2. The Third Gear Switch. This is inside the transmission. It closes only when the transmission is in third gear.
3. The Converter Lockup Solenoid. The device operates a valve that pressurizes a clutch and locks the converter.
4. The ECM. The ECM switch closes a connection to ground only when the car is doing a fairly constant speed above 35mph.

There are 2 tests for problems with the Torque Converter lockup system. These tests should BOTH be done before any shift problems are looked at. The first, and easy, test for TC lock is to press the brake while doing a constant speed over 35mph. (The test WILL NOT work if you are accelerating or slowing.) You only need press the pedal enough to activate a switch. (About 1/2″ ­ 3/4″) If the TC is working right you should see the tachometer jump up about 200RPM then drop again when you let off the brake.

The second TC test is to pull the TC lock’s electrical connector. It’s on the front of the transmission. Unplug it and see if you still have problems. The second test is very important if you are stalling when stopping the car or if the car bucks badly when slowing. There are known issues with the Torque Converter Lockup Control Solenoid. This device can hang the Torque Converter Clutch in the locked mode, usually when the transmission is hot.

If the second test stops the problems then the TC control solenoid likely needs to be replaced. This repair can be done with basic tools. It’s not hard just very time consuming. The Solenoid is under the side cover of the transmission. While replacing the solenoid it’s a good idea to replace the Third Gear Switch, which is next to it. Then you’ll have all new electrical parts in there. If you don’t see the RPM jump while doing the first test you may have a TCC apply problem. This could be electrical, TCC solenoid, TC clutch or hydraulic problems.

The test for the brake switch is in the Manual lockup section at the end of this page. The test for the Third gear switch is a bit different. If you watch ALDL terminal F with a voltmeter you’ll see it go to 12V when the TG switch closes but then you’ll see it drop again when the switch in the ECM closes. At this point you’ll be able to watch the ECM lock/unlock the TC depending on driving conditions. This only tells you that the electrical system is OK. You could still have a hung TC solenoid or other problems in the transmission.

Once the electrical problems are ruled out then it’s either solenoid hydraulic or the torque converter itself. Hydraulic or TC problems are beyond most people but if you do work on this yourself the best source of information on this is a GM document that covers the whole TC lockup system step by step. A very good general guide on this was provided to trainees at a GM seminar and is not part of the shop book. It is however on ALLdata. You may be able to get this from a friendly shop. I think ALLdataDIY.com may also have it.

Reading the TC ID Label There are different Torque Converters used with the various engine and transmission combinations. The V6 TC may not perform properly on an L4 and vise versa. I don’t think the 3T40 torque converters will work in the 4T60. (I don’t have any way to check.)

The ID label may still be on many converters, here’s how to read it. (The following TSB has been reduced to cover only 245mm units. The larger 298 mm units are used with inline transmissions.)

Torque Converter Identification
TRANSMISSION: GM All
BULLETIN: # 010 REVISED
SUBJECT: Torque Converters
DATE: September 1990
GM Torque Converter I.D.

The 245 mm dia torque converter I.D. code works the same way as the 298 mm converters, but with different meanings.

The first digit of a 245 mm converter refers to the application.

F = Front wheel drive
H = Rear wheel drive (except THM 180’s)
S = THM 180

The second digit is the “K” factor. The chart shows the “K” factor for each code along with the stall speed. Note: Multiply the “K” factor by 11.6438 to get the stall speed of a 245 mm torque converter.

CODE   K FACTOR       STALL SPEED
A      240            2795
B      220            2560
C      205            2385
D      180            2095
E      160            1860
F      148            1720
G      140            1630
H      130            1514
J      177            2061
K      237            2760
L      163            1897
M      131            1525
N      218            2538
Y      122            1420
Z      203            2363

The third digit refers to the clutch type.

CODE DAMPER TYPE
0    No converter clutch
1    12O ft/lb
2    104 ft/lb
3    170 ft/lb
4    215 ft/lb Heavy Duty
5    215 ft/lb *RTC
6    170 ft/lb Heavy Duty
7    215 ft/lb *RTC Heavy Duty
8    Viscous Clutch
9    240 ft/lb
A    215 ft/lb
B    215 ft/lb *RTC

* RTC = Reverse Torque Control (poppet valve)

The fourth digit (optional) refers to the type of cover used.

CODE COVER TYPE (APPLICATION)
A    FWD 208.2 mm Bolt Circle
B    FWD 237.0 mm Bolt Circle
C    FWD 237.0 mm Bolt Circle (viscous Clutch)
D    RWD 247.65 mm Bolt Circle
E    RWD 237.0 mm Bolt Circle, No Converter Clutch *
F    RWD 237.0 mm Bolt Circle *
G    RWD 247.65 mm Bolt Circle *
H    RWD 247.65 mm Bolt Circle, No Converter Clutch *
K    RWD 237.0 mm Bolt Circle, No Converter Clutch *
P    THM 180 ISUZU
R    THM 180 Opel
S    THM 180 BMW

* 180 degree weld on each lug

Look at the code on the torque converter in figure 6. You can easily identify it as a front wheel drive model. (The first code is an “F”). The stall speed is 2760 (the second code is a “K”), and it has a damper assembly rated at 120 ft/lb’s (the third code is a “1″.) Notice that the fourth code has been omitted.

Manual TC lockup.

Warning! Manual TC locking presents a number of problems and can damage the transmission. It is potentially dangerous to drive with a manual lock active. Manual locking should ONLY be considered when the ECM has been removed such as when a carbureted engine has been installed.

Notice! This is an EXPERT level article and it assumes the ECM was unplugged and that the wiring is intact. If the harness has been cut it is up to you to return it to operating condition. This topic is only covered as an aid to installation of non-stock engines.

Manual locking of the TC WILL NOT help performance. Manual locking is used only help gas mileage on cars without an ECM. Even then it is preferred to find an automatic solution to the problem. On cars with an operational ECM, delaying the locking of the TC can help performance. This company can tell you more about that-> Transmission Exchange Co.

Before you can install a manual lock. You need to test the brake and third gear switches. BOTH switches Must work. Without both switches you cannot wire the manual switch without damaging the transmisson.

There are 2 wires on the TC lockup connector. The first wire, according to GM schematics, comes from a switch on the brake pedal. The other wire coming from the transmission connector is ground. The ECM used to ground that wire to activate the TCC. Your new switch will replace the ECM.

The brake switch. Disconnect the transmission plug and hook a test light to the wire that is always hot with the key on and hit the brake pedal. The line should go dead. If it doesn’t you can’t wire the switch manually. The Brake Switch MUST work or you’ll have nothing but problems. Dangerous problems.

The third gear switch. This switch must also work or the TCC may try to activate as soon as you take your foot off the brake when stopped. This is not a good thing. (Same effect as dumping a clutch every time you take your foot off the brake. You WILL brake the tranny like that.)

To test the third gear switch connect a voltmeter between the TC ground wire and body ground. TCC ground is Terminal F on the ALDL. Put the meter where it can be seen while driving and have a passenger watch it. When the car goes into third the meter should swing up.

Warning! You MUST use a voltmeter for this test! Test lights may pass enough current to allow TC locking. This is not a good thing just now. This wire should only be hot when the transmission is in third gear. If you see a voltage on the line when stopped then the third gear switch is bad.

The manual lock switch is connected between the transmission ground wire and the vehicle ground. When the switch is closed the Torque Converter will lock anytime the transmission is in third gear.

Manual locking presents several problems!

You want to avoid using the manual lock except for highway driving. You don’t want the tranny locking as soon as it hits third gear, which is exactly what it will do with the switch on. If the TC locks at the same time you hit third you’ll likely tear up the tranny. (You also lose some performance that way.)

If you get into heavy traffic you’ll want to leave it unlocked, even if the tranny stays in third.

The most obvious method to do this is a simple toggle switch. Problem is a toggle switch doesn’t allow for the driver forgetting it’s on. If the toggle is on then the transmission will lock as soon as it hits third gear, which isn’t what we want for either reliability or performance.

The preferred way to restore TC locking is to make a speed sensor for the circuit. That way the TC will never lock below a specific speed. The ECM cutoff was no lock below 35. In most cases setting the cutoff for no lock below 35-45, and add Transmission Exchange Co.’s TCC delay unit, would return auto locking in the safest way possible. Ideally the speed sensor should only lock at relatively constant speed. It should unlock when speed changes rapidly.

A second option is to make a control with a push button. When the button is pressed the TC locks but when the brake is pressed or the transmission goes out of third gear the circuit resets and leaves the TC unlocked. This gives the same result as a simple toggle switch on the highway without some of the safety and drivability problems. (This circuit is Very easy to do and really cheap.)

Either control circuit would be fairly easy to do. There’s a 2000ppm feed back to the ECM connector from the dash. There’s any number of ways to make that feed close a switch at a particular speed or above.