1991 Pontiac Grand Prix SE 3.1 LH0 ecm issue

[mdpierce8]

Hello all. I’m having an issue with my GP where it seems to keep resetting the ecm on its own without removing the battery or fuse. I’ll go through what I think is the idle relearn procedure where it starts and runs at like 2k rpms for around 10 mins with the CEL on, then it’ll just die all on its own, then once restarted it runs perfect, idles perfect, CEL is off. But if I shut it off for an extended period of time and restart it it wants to go through the whole idle relearn again. Like the ecm has lost power or ground somehow? I’ve checked around for loose wires and grounds but to no avail. Any insight at all on this would be greatly appreciated! Thank you!

[Schurkey]

Obtain the service manual set for your vehicle.  Look in the wiring-diagram section, find out how the ECM is powered.

Verify that wiring for loose connections, weak fuse-blade terminals, broken/intermittent wires, etc.

 

I would want to connect a scan tool, and see what other info I could get that might be useful.  Verify EVERY sensor and computer output.

Wouldn't hurt to check the battery connections, too.  Assure that the battery isn't losing voltage while parked.

[mdpierce8]

10 hours ago, Schurkey said:

Obtain the service manual set for your vehicle.  Look in the wiring-diagram section, find out how the ECM is powered.

Verify that wiring for loose connections, weak fuse-blade terminals, broken/intermittent wires, etc.

 

I would want to connect a scan tool, and see what other info I could get that might be useful.  Verify EVERY sensor and computer output.

Wouldn't hurt to check the battery connections, too.  Assure that the battery isn't losing voltage while parked.

I do have a scan tool which is just an aldl usb cable and tunerpro and I can only get it to connect after it finishes the idle relearn. During the idle relearn it won’t connect at all.

[mdpierce8]

Everything is checking out so far. Wiring is testing good, good hot-at-all-times voltage to ecm, good grounds; But it did seem like the pins in the connectors to the ECM were bulged out, I'm starting to think the pins in the connectors are the issue. I'm gonna see about depinning the connectors and see if I can reshape them to make good contact on the ECM pins.

[mdpierce8]

Well still continuing this project. Starting to wonder if the ecm is the cause now. I did notice that it misfires sometimes only when relearning the idle. I’ve also discovered that a lot of the sensors on this car are faulty. I’ve replaced the TPS (Bosch), IAC (Bosch), MAP (AcDelco), Crankshaft Position Sensor (CarQuest), Oil Pressure Sensor (Delphi), and Coolant Temperature Sensor (carquest). The ECM was also replaced with a 88999195 AcDelco ecm. The owner before me replaced the ignition coils and module and he thought it needed a crankshaft position sensor which the car came with the new carquest one when I bought it. Still getting the same issues though. There’s a small wiring harness that runs from the crank sensor to the ignition block and it is very oily. Wondering what shape those wires are in. It’s the only thing that makes sense to me right now.

[mdpierce8]

nope. That harness checked out. I’m at my wits end at this point. Without a back probe kit, I’m not sure there’s much else I can do. I’m suspicious of a sensor being shorted and putting the ecm into limp mode. SES light is on and there’s no communication with the ecm when in this mode with the aldl cable. It misfires only at a specific rpm range between 700-900 rpm. I’ll have to take some time off and think about this for a while because I’m not sure what to do.

[mdpierce8]

Been a while since I updated here. Still in the same boat. I’m starting to wonder if capacitors inside the ECM can be replaced. The ecm I have installed will work perfectly for about 2 mins if left unplugged for a couple days then it turns on the SES light, the speedometer and odometer quit working, it quits managing the idle speed, and quits communication through the aldl connector all at the same time. I have another ecm open just to look around and see what I can see. I don’t see anything out of the ordinary. I can see 4 capacitors. I’m wondering if anyone here knows anything about these 1227727 ECMs and can help. Message me. Thanks!

Edited October 26 by mdpierce8

[Schurkey]

I am not an electronics expert, and I've had zero training.  That said, I've replaced a bunch of capacitors in loudspeaker crossovers, a Bose 901 equalizer, a CD player, a couple of dbx 118s, and so forth.  I have a "project" where I'll be replacing electrolytic caps in four different GMT400 ('88--'98 GM pickups) HVAC dashboard controllers.

MOSTLY the problem capacitors are the electrolytics.  They have a known service life of 20--30 years; (some sources say as little as 6 years!) and multiple ways they can fail.  They can leak their electrolyte, which then corrodes the circuit board.  They can drift out-of-spec, either the capacitance value, or the internal resistance, or both.  And they can sometimes actually EXPLODE.  Some years ago, the Chinese did some Industrial Espionage, tried to steal the electrolyte formulation from another capacitor company.  They got it wrong, and caused widespread capacitor failure--the infamous Capacitor Plague.

https://en.wikipedia.org/wiki/Capacitor_plague

What you have look like Tantalum or dipped Ceramic caps, not electrolytic.  I'm not saying they "can't" be defective, but the odds are against it.

 

 

Beyond replacing capacitors, I've fixed multiple electronic devices--remote controls, windshield wiper control boards, and lots of other stuff--by looking at the circuit board with a magnifying glass, and visually verifying the solder joints.  Doing nothing more than re-flowing solder on various circuit boards has brought them "back to life".  Sometimes I find a printed-circuit copper trace that's melted or cracked, so I solder-in a jumper wire across the break.

But doing that on a computer circuit-board seems unreasonably difficult.  You'd spend entire days doing nothing but following circuit traces.

Edited October 27 by Schurkey

[mdpierce8]

Thanks for the reply. I am still trying to decide how deep I want to get into this instead of just buying a new one. Mine has the AUSJ code but I can't seem to find another with the same code. Is that code that important?

[Vegeta]

Sticker code on the ECM doesn't matter, just use your current memcal in the new computer.

[mdpierce8]

So last week I bit the bullet and bought a reman ECM from AutoZone, (They’re partnered with a company called Blue Streak Electronics who remans a lot of OBD1 stuff), and now my Grand Prix runs perfectly now finally. I should have bought it a long time ago but I was afraid that wouldn’t be the problem. Happy to have this car on the road now! Runs and drives almost perfect. Only issue I ran into now is the cruise control cuts out sometimes. One step at a time though. Thank you everyone for the help!