Some background: My beloved 1990 Cherokee Laredo was totaled by a drunk 7/1/13, I kept her for parts and went in search of another Cherokee. I have a complete, running, not drivable 4.0L Cherokee with a bent frame and axle.

I found a 1991 Cherokee cheap and bought it, its ugly but it ran great when I got it, the only problems I noticed were a missing headliner and loose steering column. It sat for about 10 days while I was out of town, when I got back it was giving me a hard time starting.

Once it starts it runs great, and will start up again as long as it's still warm. Usually it takes a few tries to get it going once it cools off. This morning was a bit colder than usual and it wouldn't start at all.

I messed around with it previously and couldn't get it fixed. I took it to my mechanic and he tried a few things before determining the ECM was bad. I bought a used one but it didn't work. Before I buy a reman I am wondering if there is a wiring harness mod to put the 1990s ECM in the 1991.

I plan to see if I can swap the wiring harness from the firewall to the ECM this weekend but I don't know if the connectors are the same. A reman ECM is $300 (including core charge) and I would rather not potentially waste the money on a non-refundable part if I can use the ECM from the 1990.

They only used this ECM in the 1991 and 1992. Part number 56027528

 

The ECU/PCM on these things almost never die. Whoever told you it was dead likely doesn't know what he's doing, is lazy, or both. The computers between 90 (Renix) and 91 (Chrysler) are totally incompatible. You can transfer ALL of your wiring and sensors too if you like...

So what's going on with the Jeep? Why was the PCM blamed? Your brief description could be a vacuum leak, bad battery, or failing check valve in the fuel pump.

 

I suspect its a fuel issue, I spoke with the prev owner today and he neglected to mention that he thought it needed a fuel pump. I am going to replace the fuel filter this weekend (if i can get to it in my driveway) and replace the plug/wires/cap/rotor.

The crank sensor was swapped from my other jeep already and that didn't help, I know he tried a few other things. The battery is fine and no evidence of vaccum leaks, runs great once it gets going.

My mechanic found a bunch of literature on the exact problem i have. one suggested putting the ECM in the freezer and seeing if that helped, since the jeep isnt starting COLD he heated the ECM up and it worked perfectly. He has spent a considerable amount of time and effort trying to solve this for me and hasn't charged me a dime since it's not fixed yet. Lazy def isn't the problem.