AOD Conversion

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.


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.


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.)


59 comments to AOD Conversion

You must be logged in to post a comment.