Coding the ECM (Engine Control Module)
What does the ECM do?
Getting a code from your ECM can be very helpful when trying to figure out what's going on with many different problems with the engine. What happens is sensors in the engine send messages to the ECM. The ECM then takes that information and makes appropriate ajustment to components in the engine which it controls. For example if the air fuel mix is lean the ECM will tell the injector to stay open longer and so on. These sensors in the engine will also let the ECM know when things aren't running properly and something has malfunctioned , when that happens the ECM will store a code and sometimes turn on the Check engine light to let you know a malfunction has been detected.
How to check your ECM for Codes
1989-1993In the engine compartment in front of the battery you will see a thick wiring harness. In that harness you near the main fuse block you will find what looks like a plug with a protective cap on it. This is your Monitor Coupler. Pull off the protective cap and you should see four terminals , the terminals should be marked A,B,C and D. Terminal B is the diagnostic terminal , terminal C is the ground terminal. What you need to do now is make sure the key is in the off position , then take a small piece of wire and jump terminal B to terminal C. Now with the terminals jumped turn the key to the on position and watch the check engine light. The light will flash the code for example it will go flash , flash flash. Indicating a code 12 (everything normal). The code flash will cycle and repeat so don't panic if you miss it also you can wait and double check the you counted correctly. Once you have the code write it down so you don't forget and check the code meaning. When your done you can reset the ECM and clear the codes by disconnecting the battery for around 20 seconds.
1994-1995In the engine compartment in front of the battery you will see a thick wiring harness. In that harness you near the main fuse block you will find what looks like a plug with a protective cap on it. This is your Monitor Coupler. Pull off the protective cap and you should see 6 opening 5 of which have terminal connectors in them. The top row of the plug has the empty space and 2 terminals , the bottom row has 3 terminals. The first terminal on the top row (beside the empty shot) is terminal B. The first terminal in the bottom row (directly under the empty spot) is terminal D Terminal B is the diagnostic terminal , terminal D is the ground terminal. What you need to do now is make sure the key is in the off position , then take a small piece of wire and jump terminal B to terminal D. Now with the terminals jumped turn the key to the on position and watch the check engine light. The light will flash the code for example it will go flash , flash flash. Indicating a code 12 (everything normal). The code flash will cycle and repeat so don't panic if you miss it also you can wait and double check that you counted correctly. Once you have the code write it down so you don't forget and check the code meaning. When your done you can reset the ECM and clear the codes by disconnecting the battery for around 20 seconds.
1996 and upThese model year require a Scan Tool that plugs into the ECM to read the codes. You can usual get a scan tool from your local auto parts store.
