Information about ECM \ PCM of Metro's ?

[DOCTORBILL]

I wanted to start a general discussion thread about ECM's.....

I never have seen much printed about automobile ECM's
(same as PCM's ?) other than the letters when one is referred to.

Let's deal with the Geo Metro's "ECM."

Compared to a modern PC or a modern hand held $8 "Scientific
Calculator", what are (for older cars) they capable of?

I read that the Voyager probe's (went to the edge of the Solar System)
computer was about as capable as a student's $10 scientific hand held
calculator of today.

Is an early 90's Geo Metro's ECM 'very much' of a computer?
How tough are they - handle heat & cold well?
How many kilobytes of memory? What kind of memory?

How does one know if one's ECM is worth a damn in an older car?
i.e that it is working?

Are they expensive to replace? Where can I get one?
How much $$ ?

DoctorBill

[hot_sd]

I have never opened up an ECM/PCM of a car but I suspect that if you do you will find that it is based around a standard microcontroller. A microcontroller is a much simplified version of a microprocessor and usually contains all the needed RAM/Flash and peripherals like UARTs on-chip. These days there may be specific microcontrollers being made for the automotive market with things like CAN bus interfaces on-chip.

In terms of processing power - the functions needed to control something like an engine or the powertrain inside a car is relatively light computing work as opposed to say streaming live digital/MPEG video which requires processing power several orders of magnitude higher. You PC will have far higher compuiting power than the ECM which in the world of processing is not that intensive. Also some of the signals generated by the PCM like PWM and ODB2 are still a relatively low data rate compared to other apps.

If an ECM goes faulty and you can open it up you can probably repair it for a fraction of what a new module would cost, unless the micro itself needed replacing in which case you would need a new one complete with the code which the manufacturer is unlikely to supply. The new micros of course are not programmed.

Testing an ECM is an interesting question. I guess that at the factory there will probbaly be a set of tests that are run before each ECM is shipped out. Typically these shoud include stimulus to simulate the various sensors and the controls signals would be read and logged in some way (scope or some ATE). However, I doubt that these will be available to the general public. I guess the only way would be to try it out.

[bhrollin]

I have known several people who replaced pcm's. However, I have never had or known anyone who actually had a bad pcm. They are extremely long lived devices. Problems are almost always mechanical in nature. Even when sensors or solenoids go bad, in my experience it is ussually something mechanical in them that fails. The only exception seems to be oxy sensors.