Warning! Don’t force anything into the ALDL connector! If you damage the connector then you can have problems when using ECM scanners and other tools that have the proper size terminals.
Note! It is possible that the ALDL may be installed with the Key up or down.
The above picture is Key Down. Use the key sticking off the ALDL by terminals C & D to determine which way the ALDL is positioned.
With the key down, when you are sitting in the drivers seat, Terminal A is on the bottom row on the passenger’s side. With the key up A is the top terminal closest you.
To read ECM codes short terminals A & B together and turn the key to run, codes will blink thru the Service Engine Soon light. (Don’t start the car!)
Terminal F is used for some tests of the Torque Converter Lockup System. See the TH125 article for more information.
Terminal G is the fuel pump test point. It will have 12V on it whenever the pump is on. It can also be used to feed 12V to the pump to determine if the pump or the controls are a problem.
None of the other terminals are used during normal service routines so leave them alone. If you short them out you could do major damage to the ECM. (Only a few terminals in the ALDL are even wired.)
You can find the complete pin out of this connector and others here –> Fiero Tech Zone He also has a brief ECM code list.
There is a cheap tool to read codes in the tool section of most parts stores. It is a simple stamped metal item and cost about $2. You can also use any wire that will make snug contact.
If you have an Autozone store near you ask the counter man for the book and key to read the codes. The book is titled “What Does It Mean When Your `Check Engine’ Light Comes On?” This small book lists common codes for GM Ford and Chrysler and how to test some of the ECM sensors. The book is free.
The best tool for dealing with the ECM is a scanner. These tools read the full data stream from the ECM and will report data from sensors and how the ECM is reacting to it. The AutoXray GM OBD-1 scanner is one of many tools available and cost about $150-200. Other scanners are available but most cost more.