Many of you may be aware of the stalling problem that various Jeep models and years have. My neighbor is experiencing this with his 1997 Grand Cherokee. In my effort to help him I learned of what some call “the voodoo fix,” replacing two 1/2" screws in the cover of the ECM with 1/4" screws, or using enough washers to accomplish the same thing. He, like many other Jeep owner’s replaced every possible part that might contribute to the stalling, which occurs when driving highway, city or stopped at a light. Engine usually starts right back up. After installing three washers on each screw to effectively shorten them, the stalling problem vanished! Until it came back three months later. Since the problem went away I figured we were on the right track and had localized the problem to the ECM and plugs. Next we used some CRC Electronic Cleaner on the plugs and pins at the ECM and zip-tied the plugs to the ECM. Problem vanished again, but only for three for or four weeks this time. Usually it would stall just about every time he drove it. Today we unplugged the connectors again and I attempted to deform the tiny round receptacles in the plugs to get a tighter fit to the pins in the ECM, used the CRC again and reassembled. Feedback will trickle in as he drives.
I was trying to puzzle out this behavior. We had obvious good results the first and second time as mentioned above. What was common about both attempts was that the the three plugs going to the ECM were unplugged, the first time to access the screws for the voodoo fix. It occurred to me when it started stalling again after three months that perhaps it was not adding three washers to each of the aforementioned screws that solved the stalling problem for three months, but the unplugging and reinserting the three connector plugs to the ECM, which did two things, made the computer reboot, and burnished the pins and receptacles in the ECM and plugs.
I’m just throwing this out to the Car Talk community to see if any of you have anything to throw at this. What I keep going back to is the very obvious fact that the first two attempts detailed above brought positive albeit temporary alleviation of the problem, so I cannot avoid coming to the conclusion that the problem has something to do with the connection of the plugs to the ECM.
what you’re describing has been called “transitional resistance” by some . . .
And we’re assuming that unplugging the ecm actually “fixed” the problem, and wasn’t a total coincidence
Intermittant electrical problems can be very difficult to replicate after the wiring harness has been disturbed by a non repair event like replacing the PCM screws.
With the engine at operating temperature place your hand on the engine wiring harness about 6" from the firewall and gently move the wiring up-down-left-right about 1/2 " to see if the engine will stall. I have found that those two engine harness to body harness connectors on the ZJ Grand Cherokee to have loose pins/connectors and cause the engine to die.
This particular PCM is the fifth that was installed by a Jeep dealership in an effort to solve the stalling problem. Three were installed one after the other before the problem went away for four years. This current one is the second to be installed by the dealership after the problem came back about six months ago. All were “reconditioned”, which to me means stick one in and see if it works, and if it don’t, stick in another one. I may be be mistaken on this point, by it’s probably not too far off the mark. I don’t think there is much one can do to these units to “recondition” them.
I tried getting the engine to stall the last time I worked on it by disturbing the wiring harnesses and plugs at the PCM like you described, but got nary a hiccup from the engine. Yet both times I pulled the plugs and reinstalled them, the Jeep ran without stalling for months the first time and weeks the second time where it was stalling daily before that, often 2 or 3 times in a day. Are you suggesting some other location to disturb the wiring harness other than right at the PCM? You referred to “about 6” from the firewall". I’ll have to get under the hood and see what location you might be referring to and give that section of wiring harness a jiggle.
This last time I worked on it, in an attempt to lower this "transitional resistance” at the plugs to the PCM, I jambed the tapered blade of a small pair of precision scissors into each metal receptacle in the plugs, after determining that they were extruded and not rolled, in an attempt to deform them from circles to ovals in order to get a tighter grip on the metals pins of the PCM that the plugs accept. It was the only thing I could do without major surgery to get each wire out of the plug to check on the crimping of the wire to what I am referring to as receptacles. It is possible to take the plugs apart but they are like Chinese puzzles. You have to know how they are put together. It may come to that if it continues to stall. I know from experience that both the crimped connection between the wire and receptacle and the contact connection between the receptacle and the pin can be a source of resistance that is too great for the system to handle. In one thread dealing with this very problem one guy said he went so far as to solder each individual wire to each individual pin in the PCM, which had to be devilishly difficult. I am detail oriented, but not to that degree, at least not any more Sadly, it did not fix the problem. This is a well documented problem with this model and vintage of Jeep. I came to discover that many are having the same stalling problem with much later models of Jeep, but that is theirs to enjoy, not mine
An update for future generations of 1997 Jeep Grand Cherokee owners. I advised my neighbor to lean on the warranty from Napa and take the PCM/ECM/Engine Computer, whatever you want to call it, back to Napa and get another one. He took my advice and I pulled the old one and installed the one they gave him and it’s been two months and no problems.