Inside a MEMCAL

The MEMCAL is used inside the ECM on W-body cars from 1988 thru either 94 or 95. MEMCAL stands for MEMory CALibration module. It is a module that plugs into the ECM, and contains a ROM chip that has the software and parameters that control the ECM.

Click on image for a BIG PICTURE

The big chip on the left is a regular 27C256 28-pin EPROM. (UV erasable kind). On newer MEMCALs, the EPROM isn't marked with a standard number, they masked it with proprietary GM numbering, but they are the same. Yes, you could burn your own performance chips if you knew what parameters to change!

On the right is a GM proprietary circuit board. It is called an ESC filter. GM calls it an SNES, or Signal Noise Enhancement filter. It tries to filter out other noise, and let the knock frequency through. Then some delay circuit makes sure it is there for some minimum amount of time before it triggers the ECM to do anything. It consists of laser-trimmed resistors and tiny surface mount components. There also appears to be a bare silicon chip mounted onto it. The components on this board are protected by a rubbery silicone-like coating. Underneath it are two DIP components with proprietary GM numbering. One has 14-pins, the other has 16-pins. I believe they are resistor networks that provide the ECM with backup fuel information.

Thanks to Scot Sealander for info about the ESC filter.