Help with Bizzare PCM problem on 90 Bonneville - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 05-23-2006, 07:17 PM   #1
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Default Help with Bizzare PCM problem on 90 Bonneville

Ok, Thought this problem was licked, but it keeps comming back. Here'* the skinny:

Driving down the road, with AC on, car is surging a bit (feels like the tourqe converter locking and unlocking, even with cruise on a flat road) Car stutters, Speedometer goes to zero, SES light comes on, Coolant light comes on, coolant temp gauge goes to zero. Split second later, everything returns to normal. Happened about 5 times today. Has happened before with the AC off, but much more frequent with AC on. Goes away for a few days, then comes right back.

This makes no sense according to the wiring diagrams, there'* nothing common to all these circuits. For example, the VSS sender doesn't use an engine ground, both leads of the transaxle pulse generator lead to the PCM and Speedo output is from the PCM. Coolant temp idiot light is from the block to the gauge cluster and doesn't touch the PCM. And on and on... there'* nothing I can see in common as to why they would all fail at once then start working again all by itself. (Even when the car stalls out, it starts right back up again) No blown fuses.

Problem began when the alternator went up. Looser mechanic in St Augustine changed it for me, but in the process threaded the serpentine belt wrong, leaving it hanging off the inside of the crank and AC compressor pulleys. Noticed it a day later when it started sawing into the radiator hose, corrected it, but a day later the compressor clutch sheared off (Me thinks that the side load from having the belt running half off the pulley did it in.) Ok, replaced the AC compressor, new belt, new clutch connector on the harness to replace the one melted by the belt.

So, thinking I may have an engine ground problem, I clean all the ground wires and check continuity between the bat ground, frame, and engine block. Looks good, just a few ohms of resistance. Also cleaned off the paint where the grounds attach to the fenders on driver and passenger sides. I don't see any damage to the harness around the belts...they look pretty well protected in the plastic jacket and steel cover plate.

Did I miss any grounds? Is there another braided one between the block and the chassis? Where does the grounds from the PCM connect?

I'm just completely baffled by this. (*looks longengly at '87 Camaro with ultra-simple wiring*)
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Old 05-23-2006, 07:37 PM   #2
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Did this all start after you installed the Type 2 ignition module? Or was it a problem before?

Let me also add if the RPM'* drop below 400, the PCM will not be in control... The car starts in Module mode and after 400 Rpms the ECM takes over...

But the strange part is that even if the engine has stalled out, you should still have a signal to the speedo letting you know you are still moving... Via the ECM

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Old 05-23-2006, 08:00 PM   #3
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Quote:
Originally Posted by jr's3800
Did this all start after you installed the Type 2 ignition module? Or was it a problem before?

Let me also add if the RPM'* drop below 400, the PCM will not be in control... The car starts in Module mode and after 400 Rpms the ECM takes over...

But the strange part is that even if the engine has stalled out, you should still have a signal to the speedo letting you know you are still moving... Via the ECM
Negative on ignition module, problem was there with the old ones. This has been going on off and on now for a month. Same goes for the plugs, plug wires, IAT motor..these were all items replaced during the course of chasing this down (figured it couldn't hurt to have new ones). At first I though it was just getting loaded down by the AC compressor and stalling, then I started noticing the stuff that was lighting up when it stalled or just hiccupped (It didn't stall all day, but tried about five times. Maybe the module mode is catching it fast enough before it falls on it'* face, but even so, the things going on at the same time are downright weird.

Also, when Nimnicht Pontiac tried to sort this out a few weeks ago, they found no stored codes and though the battery ground was just loose.

Yeah this is baffling, unless the PCM itself is the problem, but unless I'm reading this wiring diagram wrong, the coolant temp on the dash is not involved in the PCM circuits. (It is a Hayes manual, not the GM real deal service manual)
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Old 05-23-2006, 08:14 PM   #4
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Just a wild stab in the dark, but have you checked the positive cable connection to the battery. Is it loose in any way or is the positve post on the battery chewed up?

I only mention it, because when I first got my car, the positive post on the side mount battery was reamed out and occasionally, the car would up and die, dash lights would flicker and gauges drop, and then seconds later, be just fine. This may have absolutlely nothing to do with your problem, but a tight connection on the postive is a must and a new battery fixed this for me.
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Old 05-23-2006, 08:34 PM   #5
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Quote:
Originally Posted by Echo SSEI
Just a wild stab in the dark, but have you checked the positive cable connection to the battery. Is it loose in any way or is the positve post on the battery chewed up?
Yep, I know that thought. Positive cables look good, connections tight, and the Battery is new (*.O.P for me, when a GM craps an alternator, replace the battery too..just in case...every GM I've owned has coughed up the factory alternator before the 50k mile mark, wasn't expecting this one to be the exception)

I don't think I'm losing the positive side, or the negative to the chassis, cause all the rest of the circuits in the car are working. One time it hiccupped while making a left turn and the turn signal kept right on blinking. I've also not noticed the AC blower or radio having any problems when the motor hiccups or stalls, they keep right on going like nothing is wrong.

In a way, I wish it would just fail completely. I may have to call AAA for the flatbed ride home, but at least I'd have a hard fault to track down and solve for good instead of one that comes and goes. Car'* starting to get a bad reputation already.
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Old 05-24-2006, 01:05 PM   #6
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Default surge

My guess is a TPS voltage problem or vac leak although it could be many things.
Surging is also related to dirty throttle body, IAC, MAF, EGR, water in fual etc.
I would start by verifying TPS voltage and check for smooth operation throughout its range, then remove and clean all the sensor electrical connectors, including the ICM.
Has the PCV been changed and a standard tune up done?
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Old 05-24-2006, 07:34 PM   #7
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Default Re: surge

Quote:
Originally Posted by SSEBONNE4EVA
My guess is a TPS voltage problem or vac leak although it could be many things.
Surging is also related to dirty throttle body, IAC, MAF, EGR, water in fual etc.
I would start by verifying TPS voltage and check for smooth operation throughout its range, then remove and clean all the sensor electrical connectors, including the ICM.
Has the PCV been changed and a standard tune up done?
Still focusing on wiring/ecm related problem. The surging isn't like the engine hunting an rpm..it'* more like the transmission constantly changing gears, like when the car upshifts theres a brief pause while the clutches lock/unlock. It just does it a lot more than I think is should be doing. Transmission fluid looks good, nice red color, and the level is ok too.

IAC is new, MAF cleaned out two weeks ago along with the TB, reseated all the electrical connectors on the top end of the motor. Air filter and PCV valve changed with the plugs and plug wires. Water in the gas is a stretch, I burn about 7 gallons of fuel per day in the car.

Today, on my way to Bunnell, I picked up another ECM from a junk yard. Connected it up and drove the rest of the day on it. The car did not stall or try to stall today with the AC running full blast, though it still feels like the TCC is constantly locking and unlocking for no reason. Will take a few days to see if the rest of the bizzare stuff goes away on the new ECM. Also need to try resetting the TV cable in the morning.

BTW, how do you like the transmission recal kit you list in your mods? Done it to a bunch of 4L60/THM700R4'* but haven't done one on a transaxle yet. (Like the very positive upshifts I've been getting with the kits in F-Body trans, you getting the same thing in the Bonneville'* transaxle?)
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Old 06-03-2006, 10:58 AM   #8
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Default Re: surge

Follow up:

A week or so ago I changed the ECM from the original, to a junkyard one. I've been running the car since then and the bizzare problems with the car went away. While it still feels like the TCC is locking and unlocking a lot more than it should, the intermittant stalling accompanied by wild guage readings and SES lights have not happened.

I opened up the engine wiring harness in a many places as I could, didn't find any chafes or broken wires. So I guess I'll write this off to a ECM gone bad and start troubleshooting the TCC all by itself.
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Old 06-03-2006, 12:24 PM   #9
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I had a similar problem happen to my white 90 SSE just on Friday. Was driving along ( it had sat for a week before I drove it) and I noticed a slight miss, then I get to a set of lights and it stalled. tried to start it back up and nothing. Tried again, it started. I was only going a bit further up the road, so I made my turn and went WOT to see if maybe it was bad gas or something, come to make my corner and it was wanting to stall. I noticed that the rpm went to zero also, the dash warning lights came on also, then it got rpms back again.
Just up the road from where I was stopping, it was still loosing rpms and coming back again then it stalled again and I coasted into the driveway. I tried to start it back up and stalled right away. I let it sit for a couple hours and drove it home and it was fine.

I was thinking i twas a clogged fuel filter since I was loosing complete rpms but it would come back ( I had clogged fuel filter syndrome before on another car, lol)

so now the car sits and I don't wanna drive it anywhere. BUT we will see next week since I may be stealing the AC components from it to fix our Grey SSE.
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