Inside an 88-93 ECM/PCM

Updated March 12, 2000

The ECM is the computer that controls the engine. It stands for Engine Control Module. The PCM is an engine and transmission controller. It stands for Powertrain Control Module. On an 88-93 W-car, the only difference between an ECM and PCM is the fact that a PCM has firmware in it that allows it to control an electronic transaxle, while the ECM does not. Physically, the two computers are virtually identical in hardware. All 88-93 cars use a P4 ECM/PCM with the exception of some late-'93 cars with the 3100 Gen III V6. Those cars got a P6 PCM. The most common P4 ECM used is the GM #1227727 found in cars with the 2.8L and 3.1L engines. Cars with the 91-93 3.4L DOHC V6 use a 16149396 PCM, which is identical to the 1227727 ECM, but with additional SRAM installed into a space which is left empty on the 1227727. Cars with the Quad4 engine use a 1228708 ECM, while cars with the 3800 Series I engine use a 16132792 ECM. I don't know how they differ from the 1227727, but they are based on the same P4 ECM.

Here are a few pictures of the inside of the 1227727 ECM.

This is a picture of the ECM as removed from the car. Note the heavy duty, water-resistant aluminum housing. The ECM/PCM is installed underhood, in front of the RH strut tower, so it has to be tough to withstand the elements. Note the MEMCAL access cover on the top rear of the picture.

This is a picture of the "top" of the ECM with the MEMCAL access cover removed. Notice that the MEMCAL has been removed from its socket. In the car, the ECM is actually installed with the MEMCAL access cover on the bottom.

Here the ECM has been removed from its weatherproof aluminum housing. This picture shows the MEMCAL on the circuit board so you can see where it's supposed to go.

In this picture the MEMCAL has been removed, revealing the 66-pin connector. This image is really big so you can see the detail of the circuit board.

This is the bottom of the circuit board. Not really anything to see here except a few surface-mount components. This is a really big picture too.

Want to know what chip does what in the ECM? This picture shows a map of the ECM and what each chip does. Note the empty SRAM space that differentiates a 1227727 from the 16149396 PCM used in the 3.4L DOHC V6.

Interesting stuff, eh? I'll bet that just makes you want to learn how to be an ECM hacker!

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