EFI: The ECM and MEMCAL

THE ECM AND MEMCAL

The Beretta uses the type GMP4 ECM. It connects to your scan tool via the ALDL connector. The ALDL uses a serial protocol that runs at 8192 baud. It uses 10 bit words with one start, one stop and eight bits of data. A packet consists of a device byte, a length byte, N bytes of data and a checksum byte. Since the protocol isn't documented rolling your own scan tool isn't practical (unless you have a few man years to throw at it!)

[photo] The memory and calibration unit is a removable part of the ECM. MEMCAL is GM-talk for PROM and it contains a 27C256 ROM, PROM, or EPROM. It also houses back-up fuel control system which consists of an IC and some analog circuitry.

I've heard that a number of these ECMs were defective. To test for this, tap lightly on the aluminum case with the handle of a screwdriver while the engine is running. If this induces changes in RPM or other symptoms, then you've probably got a bad one. This is one of the rare cases where replacing the ECM isn't a waste of money.

I've also heard stories about cars that seem to eat ECMs. This can happen when there are problems with the sensed or controlled circuits connected to the ECM. This might be something like a sensor shorted to ground or 12 volts. Some people recommend testing all the ECM circuits prior to installing a new ECM to avoid blowing it up. This may sound tedious but trashing two or three ECMs can get pretty expensive. The service manual explains how to test for bad sensors and solenoids with a DVM.

The PROM contains only data. This consists mostly of the "calibrations" needed for the specific vehicle but other values such as the rev and speed limiters are also stored here. For any specific make, model and year, there might be dozens of different PROMs available. The particular PROM depends on engine, transmission, sales location (California or federal), etc. It's very important to use the correct one for your car.

As a side note, the speed limiter is set by the auto maker to avoid litigation by idiots who drive faster than their tires can handle. In the case of my 1989 Beretta GT, it's about 114 MPH (I would imagine). This is probably based on a T rated tire (118 MPH) which the non-GT came with; it seems likely Chevrolet didn't bother making a special "GT" prom just to address the higher speed rating of the H rated tire (130 MPH) that were stock on the GT. (Also, it's not clear if the car with a stock engine and aerodynamics could actually achieve 130 MPH). Finally, most aftermarket performance chips remove the speed limit (I would imagine). I believe they also remove the rev limiter since frequently a performance chip installation is accompanied by engine enhancements that allow higher than stock red-line.

One thing you can do is check to make sure you have the most recent MEMCAL for your car. There are almost always updates to the original version and the newer ones usually are issued in response to specific problems.

If you go to the dealer to buy a new MEMCAL, you should bring the part numbers from the old one. This is because even though you ought to be able to look up the part number just by knowing the make, model, year, engine, transmission, etc., the best way to find the updated part number is to reference the old part number.

The ECM is located behind the passenger's side kick panel. To gain access to it, I usually remove the glove box (4 screws) and remove the three screws that hold the under dash panel to the dash board. (There are also nuts that hold this panel to the firewall but you don't need to remove them unless you're going to actually remove the ECM.) There are two sheet metal screws that hold the aluminum plate that covers the MEMCAL. It's not usually necessary to remove the MEMCAL to read the numbers off of it. If you do remove the MEMCAL, be sure to disconnect the battery first and perform the idle learn procedure after replacing it.

One thing I found out about my car, which has the five speed manual transmission, is that its MEMCAL is for a car with an automatic transmission. I believe this was a hack that GM, Chevrolet or my dealer used to solve some of the idle problems I was having. This bit of information became important when I installed a Hypertech chip in my car. I was having minor idle problems that improved when I switched to the automatic transmission powerchip.

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