1996 Jeep ZJ Dying (Usually) at or Near Idle
My 1996 Jeep Grand Cherokee (ZJ) 4.0L AT Laredo was repeatedly dying or coming close to dying at or near idle. It took me months and throwing countless parts at it before finally isolating the problem to the Powertrain Control Module (PCM) (brain).
I believe the flow solder on the board is faulty, and at around 165,000 miles it develops microscopic hairline fractures that only seem to lose contact at low frequencies (near idle). Thus, you can sometimes keep it from dying by either shifting to neutral as you approach a stop (even lower frequency than idle), or keeping your foot on the gas when you are stopped (higher frequency than idle, but very stressful and not particularly safe). Obviously this is not a good long term solution...
This is a write up to help others identify whether their ZJ has the same problem mine did, how to isolate it to the PCM, and how I ended up resolving it. I hope it helps someone avoid the incredible frustration (and expense) I experienced...
I believe the flow solder on the board is faulty, and at around 165,000 miles it develops microscopic hairline fractures that only seem to lose contact at low frequencies (near idle). Thus, you can sometimes keep it from dying by either shifting to neutral as you approach a stop (even lower frequency than idle), or keeping your foot on the gas when you are stopped (higher frequency than idle, but very stressful and not particularly safe). Obviously this is not a good long term solution...
This is a write up to help others identify whether their ZJ has the same problem mine did, how to isolate it to the PCM, and how I ended up resolving it. I hope it helps someone avoid the incredible frustration (and expense) I experienced...
Symptoms
It first happened when I left my car idling and the car suddenly died. Then it started happening at stop lights, stop-and-go traffic, coming to a stop in a parking lot, etc. Almost always, the car would start right back up. Over the next few months, the OBD2 gave random codes, and I replaced various sensors, distributor, etc., etc., but within 20-100 miles the problem would always resurface.
My Approach
Eventually, I decided I needed to stop throwing parts at it and take a more methodical approach. I found a few forums where other people were complaining about the same problem. Like me, most had tried multiple solutions but only a tiny few had been successful. Sadly, there was not one standout solution, and (since this is the internet) you never knew if someone was celebrating prematurely either.
So I made a list of everything people had tried, and kept track of what they said worked. Then I looked at what was most likely to work, and started with the cheapest/easiest repair. My list looked like this:
So I made a list of everything people had tried, and kept track of what they said worked. Then I looked at what was most likely to work, and started with the cheapest/easiest repair. My list looked like this:
I recorded # of attempts using hash marks on the right, and # of successes using Xs on the left. As you can see, people have a lot of ideas, but most of them did not actually work. There is a tendency to try what other people mention, but what you really want is what has the highest probability of success.
I had already replaced spark plug/wires, distributor, coil, and crank position sensor. Next on the list was fuel filter (cheap) but that did not work. So that left the PCM (computer/brain) and wire harness, which made sense since I could affect idle by wiggling the connector. I had seen some other attempts at dealing with this, like supporting the connector, but that did not work. I had also seen a video posted by a EE who had soldered the connector, so I decided to try that next because replacing the wire harness seemed even more painful.
I had already replaced spark plug/wires, distributor, coil, and crank position sensor. Next on the list was fuel filter (cheap) but that did not work. So that left the PCM (computer/brain) and wire harness, which made sense since I could affect idle by wiggling the connector. I had seen some other attempts at dealing with this, like supporting the connector, but that did not work. I had also seen a video posted by a EE who had soldered the connector, so I decided to try that next because replacing the wire harness seemed even more painful.
What to Test
While the car is running, grab the 3 connectors to the PCM, one at a time, and wiggle them up and down. Try it slowly and you can wiggle pretty hard. If your car's idle changes and nearly dies, chances are you have the same problem I did.
How I Confirmed
I had already replaced so many parts that I was reluctant to spend another $300 on a refurbished PCM. The PCM also has to be flashed to your VIN and mileage, so you can't just swap it with your buddy's PCM and go for a drive.
Before shelling out $300, I decided to try to confirm my PCM was bad. I took it apart, everything looked fine. No bulging capacitors, no evidence of arcing. So I re-soldered the connector, and put it back together. It ran for over 1000 miles before it died again, and since previously it would die within 20-100 miles, I knew I had isolated the problem.
Before shelling out $300, I decided to try to confirm my PCM was bad. I took it apart, everything looked fine. No bulging capacitors, no evidence of arcing. So I re-soldered the connector, and put it back together. It ran for over 1000 miles before it died again, and since previously it would die within 20-100 miles, I knew I had isolated the problem.
What I Soldered
This is the PCM after the coolant reservoir has been removed:
With car powered off (I suppose you should disconnect the negative terminal though I did not), unplug the three connectors going into the PCM. There are two clips per connector, be careful that you do not break the clips.
There are two long bolts holding the PCM against the firewall that you will need to remove. You can see one of them on the right side of the photo.
Once the PCM is off, you will need to pry open the 10 metal clips along the perimeter:
There are two long bolts holding the PCM against the firewall that you will need to remove. You can see one of them on the right side of the photo.
Once the PCM is off, you will need to pry open the 10 metal clips along the perimeter:
Inside will look like this:
(Do not remove any of the conformal coating.)
Undo the 2 screws on either side of the long connector in the center of the photo above (screws have been removed in the photo). You then split the bottom half apart from the rest of the PCM to expose the backside of the board. It will look like this:
Undo the 2 screws on either side of the long connector in the center of the photo above (screws have been removed in the photo). You then split the bottom half apart from the rest of the PCM to expose the backside of the board. It will look like this:
(Do not remove any of the conformal coating.)
There are about 25 solder joints under the long connector. Since the flow solder has microscopic hairline fractures that only seem to lose contact at low frequencies (near idle), I do not think it is possible to figure out which ones are faulty, so I soldered all of them. When done, reassemble the PCM and reinstall back into your vehicle. Make sure all 6 clips of the wire harness are seated (they should click into place).
I've done this on two cars so far. Mine lasted 1000 miles after soldering and then began to die at idle, so I replaced the PCM (ordered from CarComputerExchange.com) and have not had a problem in the ~25,000 miles I have added since. The other car (similar mileage) ran fine for a year, and then the owner sold the car. It was still running great when sold.
There are about 25 solder joints under the long connector. Since the flow solder has microscopic hairline fractures that only seem to lose contact at low frequencies (near idle), I do not think it is possible to figure out which ones are faulty, so I soldered all of them. When done, reassemble the PCM and reinstall back into your vehicle. Make sure all 6 clips of the wire harness are seated (they should click into place).
I've done this on two cars so far. Mine lasted 1000 miles after soldering and then began to die at idle, so I replaced the PCM (ordered from CarComputerExchange.com) and have not had a problem in the ~25,000 miles I have added since. The other car (similar mileage) ran fine for a year, and then the owner sold the car. It was still running great when sold.
Important Notes for Consideration
The unfortunate part of all this is that these PCMs are no longer being manufactured, so you can only buy refurbished ones. If you end up replacing yours there will be a core charge, and you may not get it back if you send the PCM back soldered like this. It is also unfortunate because those of us who love our ZJs know how important it is to keep these PCMs in circulation. My suggestion is, if you can wiggle the connectors and cause the car to die at idle, I'd say it is worth replacing the PCM even without "testing" by soldering the connector.
Good luck!
Good luck!
