ECM dementia

Perhaps a closer look at the wiring harness and devices is in order before wasting money on an ECM.

From memory (so double check it!) code 42 is an EST error (maybe bypass wire?). Look for a corroded or broken connection, grounded wire, etc.

It *could* turn out to be a faulty ECM, but not as likely.

Initially it indicated a single fault. In that case, I could see a harness problem. Later, it's tossing out errors at random from the trans to the speedometer. How could a harness problem cause all that?

I'm going to try the actual connectors at the ECM, but I think that even that is wishful thinking; I'll wind up needing a new unit, more than likely.

I have to back up the responder here. I mean there is the possibility of the ECM being at fault. But the codes don't seem so random. The codes it is setting is all the things that are messing up. A harness of wires, which could be loose, corroded, insulation of the wires worn with cracks in them, allowing voltage to escape. Any of this would cause voltage spikes and drops recognized by the ECM that then sets the codes you see, and furthermore is unsure of the exact condition the car needs to be running at. The car is about 14 years old. I don't think we're trying to disagree with you, but save you some money if we can.

Electrical gremlins are probably the hardest to track down. For example, I am currently having a problem with my 01 Montana. With the exterior lights off, the turn signals work normally. With the lights on, when I signal a right turn, the lisght flashes rapidly, causes the rest of the lights to dim, turns on the parking aid (really a joy at a light with someone close up behind me...) and freaks out the traction control -- setting the SES light, and knocking the traction control off-line. Haven;t really had time to trouble shoot it. I guess a bad ground, bad wire or bad circuit board in the tail light is causing power to back-feed down all those circuits from the one of the tail lamps.

If you have electrical schematics, check and see if everything that is throwing codes is in a common harness or is getting poer from the same place.

HTH,

---Matt chicago, IL

The connectors and especially the ground connection tend to loose contact. Make sure the ground at the ecu is good.

No, I appreciate the suggestions -- hope I didn't come off otherwise.

I can make a further inspection of the harness, especially in the area where it enters the dash (firewall), but everything I've seen looks fine. Not even a hint of any wear or damage to the insulation or the protective wrap.

I'm curious why you say that the codes don't seem so random, though. Some are from the transmission, and the 42 is the electronic spark advance (coil pack area); the problems with the speedometer don't involve any harness that travels under the hood. To examine each length of harness for these faults, I'd be going in entirely different directions... seems random to me.

Just as a followup, this gave me the kick in the a$$ to go out and take a look. Popped both tail lamp assemblies off and pulled and reseated all the bulbs. The problem went away! Reinstall both TLA and it was back! Removed right only and it went away again. Reinstalled and all is well. Didn't have time because it was starting to rain, but I suspect one of the wires feeding the tail light is chaffed and grounding out when installed, or the circuit board in the taillight is cracked causing intermittent problems. Further updates as events warrent.

After inspecting the harness from stem to stern, I bought and installed a new ECM. Problems ceased immediately.

Since it was an intermittent problem to begin with, I'm not 100% sure the problem is solved, but this morning (before the brain transplant) the car was almost undriveable. Now it's its old self again.

I'm just hoping it stays that way...

Marky wrote:

Where did you get the ECM & what did it cost? Mine is the original, and the car still runs like a dream. Just returned from Florida to Chicago (

2454 miles ) with no problems.

========= Harryface =========

1991 Pontiac Bonneville LE ~_~_~273,304 miles_~_~_

AutoZone, and $85. $95 for the core.

For the longest time, I heard a rattle up in that area. I suspect it was the computer itself, since it was so poorly mounted -- and since the rattle has suddenly disappeared. I couldn't believe it would come that way from the factory.

Perhaps the car was worked on sloppily. No idea why it failed so early (33,000 miles) but I think the vibrations killed it. I'm going to wrap it in foam rubber before I put it back up behind the glove box.

Ah! That parcel of information would have been valuable: someone would have recommended the old GM "rap test." GM had a problem once wherein they changed the solder used in the manufacture of ECMs and the resulting connections were brittle and often cracked, resulting in intermittent, mind-boggling, pull-your-hair-out episodes. The test was to drive the car while it was behaving, gently rap on the ECM, and see if things went nuts. I'm pretty sure it was rectified by '90 (ask Ian -> shiden_kai in A.A.GM), but if the sucker was banging around in there, I can see that happening to any ECM.

N> Harry Face wrote:

RB,

If the ECM is something that heats up I wouldn't wrap foam around it. I seem to remember the ECM in my Z - 24 getting very hot, I may be wrong though. Make sure you have all the mounting brackets and screws in place and it should be fine.

One other thought that might of occurred to your car is if any aftermarket alarm / radio installers got their hands on the car when it was new, they might have removed some bracket or screws securing the ecm and didn't replace them.

Happened to my car, damn alarm installers removed the steel plate under the steering column where the plug for the code scanner is mounted. On a trip to the junkyard this past summer I discovered 3 cars like mine as well as a few Le Sabre's that had this plate under there and I wondered why my car didn't have it.

I thought I'd buy the plate so I could at least anchor the code scanner connector to keep if from falling out from behind the black plastic dash trim. Once I go the car home I pulled off the lower dash trim, saw one of the alarm componenets wrapped around a wire loom and put two and two together. Uh huh - the alarm installer removed mine and didn't replace it.

I also discovered that with the steel plate under there, there are now 3 more attachment points to screw the black plastic trim cover up. Before there was only 2 screws on the outer edge holding it up and it bowed down in the center and would hit the tips of your shoes.

I'd check for a missing ecm braket or some other kind of anchor.

Good Luck !

========= Harryface =========

1991 Pontiac Bonneville LE ~_~_~273,304 miles_~_~_