Adding a heat sink to a GM ECU

The early GM ECU (Electronic Control Unit) is a pretty low-tech device. It doesn't generate a ton of heat, but since it is typically mounted below center consoles (on top of a hot floorboard) or under the dash, it can benefit from some extra heat dissipation. It's easy to do and good insurance.

Out-of-the box, the ECU has two internal heat sinks that touch the inside top of the metal case, which serves to further dissipate the heat generated by power transistors, voltage regulators, etc. Actually, I think some of them are injector drivers. But anyway, they get pretty hot.

Here's an interior view of a typical unit. This happens to be a 1228838 from an Isuzu Rodeo. This style is very, very common. Just about every GM car or truck from the 80s will have something similar. This one doesn't have socket for the CALPAK; others I've seen did. If you don't know, the PROM contains the program code to run the fuel injection and ignition for the vehicle. They are customized for different applications based on weight, gearing, etc. The CALPAK contains a resistor bank that contols some rudimentary default settings. This comes into play if there is a failure in the PROM, and provides a "limp home" mode.

















In this photo, you can see the two holes in the cover that line up with the threaded holes in the larger of the sinks. This ensures good contact and heat transfer. The smaller heat sink has no screws and simply touches the top cover. Note that on this particular unit, there wasn't any thermal grease on either heat sink from the factory.
















In the next shot, I'm showing an aluminum heat sink I had lying around. I believe it came off a Pentium II. These should be easy to find. I drilled two holes to line up with those in the cover, and found two longer machine screws with washers. Be careful that the screws you choose aren't so long as to hit the circuit board below and crack it or short something out. I was fortunate that the gaps between the fins happened to line up with the hole locations. Improvise as needed.
















Here I am coating the tops of the sinks with PC processor thermal grease. Note that this stuff is also good for the Ignition Control Modules in GM HEI (High Energy Ignition) distributors. The factory grease on those tends to dry up, especially on older vehicles.
















And the bottom of the new external heat sink:
















The finished product. The access panel screws have been replaced with PC case brass thumbscrews. This is good for a 10% horsepower increase :) Don't crank down on the screws, just snug them or you could break something!















That's it! Happy motoring.