172Heavy

I need some help; I have a 1996 GC 4.0. 180K mi. The problems started on a road trip pulling a trailer on a mild grade, suddenly and momentarily the engine quit and the transmission dropped out of OD and lock up and continued to do so intermittently for the rest of the trip, ( I held it in third for the rest of the grade and it ran fine) it may also go for several hundred miles without doing it. And then it just starts to miss and jump in and out of gear, it never completely stops running and it always starts, since then I have been replacing parts, as a mechanic with over twenty years of pulling on wrenches it’s something that I despise.
Here is a list of parts that I have replaced, header, it was cracked, fuel pump, sock and filter, wires ,plugs, cap and rotor, ignition coil Crank PS, rebuilt distributer with cam PS, and finally a rebuilt CPU. Most of these parts were due with 180K on the vehicle. The CPU was expensive and unwarranted.
I have also swapped around the relay for the ASD. One thing that’s driving me nuts is how intermittent this problem is. If it just died I could diagnose it. The vehicle is a dry country vehicle no corrosion.
Here are some things I have noticed; the tach stays stable during an episode, it does not do it when cold, only after about 15 to 20 mi of driving. It always starts.
Trouble codes are for multiple misfires, all cylinders, I have also had codes for Cat efficiency and most recent, leak detection pump switch or mechanical failure.
I am under the understanding that OD and lock up are controlled by the ECU that senses engine speed via the Crank Ps True or false?
Thanks for your time and looking forward to your reply. 172Heavy

dave1123

A transmission overheat will cause it the drop out of OD. It could be you cooked the spark plugs also. It's suggested you manually shift out of OD when starting up a grade loaded. I've also used the switch for downgrade engine braking.

The PCM (CPU) monitors throttle position, vehicle speed, and engine speed to control both OD and lock-up. It also monitors temps in the engine and trans. In severe cases, it will shut down the engine.

You may have a plugged convertor as well.

BTW, is this a 4.0?

GitRDunn

I have never had my engine shut down, 96 GC 4.0, NP242, but at one time I had an issue with OD not staying on. I would start the Jeep and the OD was off. I would turn OD back on. when I turned the Jeep back off and then start it again OD would be off. And of all things the battery was dead (like 9 years old) and the alternator was not charging enough when tested. Installed new battery and alternator and never had an issue with OD again. I think I also did the screw thing with the PCM also. This was at about 225,000.

Also, I didn't see it in the post but how is the oil pressure?

172Heavy

Oil psi is normal, and the trans was not overheated,( did a service on it immediately after I got home, the fluid was red and no debris in the pan) it jumps out of od and lock up at the moment the engine starts to miss, buck. The engine is a 4.0 inline six and runs very healthy. The missing is like the ignition is cycling on and off, it may do it for several miles and then back to normal, I can then stand on it and twist it tight through the gears, runs normal. I have seen plugged cats and baffles in mufflers that plug outlets restricting flow; they do not act like this.
Thanks for the input. This icon says it all.

dave1123

Hmmm?

172Heavy

The 96 ZJ is finally fixed, here are some things that I have learned along the way, and some suggestions along the way that echo in the back of my mind.
1. The symptoms may not be an indicator of the real problem, missing and jumping out of gear intermittently. Problem: bad cat and downstream O2 sensor, a bad cat will never give these symptoms, it will never be intermittent, it will either plug or burn out; that was what happened in my case, it burnt out, the vehicle should have continued to run normally, it should have given a constant, repeated Cat efficiency code.
2. I have discovered and I’m certain that others know this, but it’s new to me. Clearing a code only clears the code, it does not (reset) the CPU, this means that even though the problem has been corrected, the vehicle may still miss, jump out of gear and throw erroneous codes till the CPU has been reset by draining the electrical system .
3. Suggestions that echo in my mind. “Believe the codes they are real”, “you may need a Cat”, “it’s very important to short the battery cables together when replacing parts”,” the Cat code is delayed to ensure that the code is real”, Hmmm,
4. So, I’m only guessing here, the missing and jumping out of gear is a program built into the CPU that indicates a malfunctioning Cat, forcing the driver to repair the problem due to drivability issues or destroy the transmission, the fact that it only occasionally threw a Cat efficiency code but would frequently jump out of gear, miss and throw multi cylinder code, almost as if the CPU was programed to be intentionally misleading. So what’s up with that? But once again I’m only guessing.