I have been chasing a no start/no spark problem for a while now. Replaced ignition module, pickup, and coil. Cap and wires are new as well as plugs.

The only thing I have not changed that appears on the ignition wiring diagram is the MPCU (Computer).

I have searched and searched and can find no definite answer to where it is on an 85 XJ. Best suggestion is under dash near center above the transmission tunnel. Anyone know if that is correct.

I did have a melted spot on the back of the ignition module so wondering if maybe something shorted or surged and took them both out. I didn't even realize this model had a computer until looking closely at the Haynes manual diagrams.

Have 6v to coil when ignition on, 10v when cranking, should be enough to make a spark. Something must not be letting the ground circuit break, and it looks as if the pickup is wired to the computer and not directly to the ignition module so maybe that is where the problem is.

Had this XJ since April and really want to drive it on the highway at least once then maybe off road

 

That's a Motorcraft ignition, burning a hole in the back of the control box is SOP.
Did you change this:
http://www.rockauto.com/catalog/more...716&cc=1179558

 

Yes, changed that, the pickup coil in the distributor. Changed that first when I found the no spark condition because I have had that go bad on a 79 F150
before. After that didn't work I found the bad ignition control module.

Have been looking at the wiring schematic some more. The violet and orange wire coming from the pickup goes into the ECU, then a wire from the ECU goes to the ignition control module. I believe that may be the missing link, either a bad computer took out the control box or vice versa, or some combination of all the parts killing each other. I worked for a while with the battery trying to find what was draining it, taking the cables on and off several times, maybe that surged something in the process.

I think I found the computer, mounted under the dash above the steering column where it goes through the firewall. Looks like a CD player from a computer with a big connector full of wires attached to it. If that is it, it is going to be a bear to get to, unless you are a contortionist lol. I don't bend that way anymore. Probably the easy way will be to remove the drivers seat and lay on the floor. That will be about $150 after the core rebate, which is why I want to rule out everything else before I go that route.

 

If you are considering spending that kind of money you might want to think about a Weber carb from Redline and doing the "Nutter bypass" to completely take the computer out of the ignition circuit. I did this with my '85 shortly before getting rid of it after some 20 years of playing with the YFA. I don't think you would be sorry if you went this route.

In those 20 years I never touched the computer and it really has very little to do with the basic operation of the 2.5, but I can't personally say that this is not your problem.

 

It's been decades since I worked at the Ford shop, IIRC that's probably (if there is a PCM connection) an EEC-3 system, basically just a big amplifier to switch the coil to ground when the PCM tells it to. The earlier systems didn't need a PCM at all.
Anyway,the module grounds inside the distributor, there's a black wire attached under a screw. If that's fouled up it won't spark. Generally either the module would overheat and melt or that black ground wire would work loose.
If the module and pickup coil are good, the wiring is intact and the coil has power to it, the only thing left is the PCM.
Now that I think about it, that 6V with the key on might be an issue. Ford used a resistor wire from the module to power the coil, if the coil voltage dropped below 9-10V with the key in "on" the resistor wire was bad. That wire was bypassed in "start", putting full battery voltage to the coil.
I don't think Jeep used the resistor wire but a ballast resistor instead. If you can find one, try jumpering it.

 

http://www.autozone.com/autozone/rep...00c152800a9de9

Figure 11 there is what I have as my system.

I know that pre-EEC Fords the wires went straight from the distributor to the ICM. I think Jeep used a hybrid system with the ECU in between.

Would the "nutter bypass" be something I could to do test the system to see if the ECU is bad, in that if it runs it would mean the ECU is bad?

 

Have you changed the cps?

 

^ lol

 

A-ha. That is the older Duraspark and you are correct. What Jeep has done is to intercept the distributor pickup signal with the PCM, then use the PCM to trigger the ignition module. Probably so they can do some rudimentary spark control with the PCM.

That orange wire going into the module- disconnect it from the PCM and run a jumper wire to it. Momentarily tap the positive battery terminal to pull it up off ground, then ground it. When you ground it you should get a spark.

In the Ford system that orange wire went directly to the distributor pickup then returned on a violet wire.
If the violet wire exists on the module, you should be able to connect the orange pickup wire to the orange module wire, and the violet to the violet and bypass the PCM entirely.
eta: I see that's what a Nutter bypass is. Cool. Seems the perfect way to see if it's a PCM issue.

 

Radi,

Exactly, my ICM has a violet wire that goes into the connector, but there is nothing connected to it. I will try it later today, hopefully I will finally get it running.

The PO had said the reason he parked it was that when he started up a hill the engine didn't have enough power to keep up speed. I thought maybe it was the distributor timing being off. He had it worked on a few times but the problem was never fixed. I am now wondering if it was a slowly failing ECM that has finally died.

 

Well I thought I had it.

Did the bypass, cranked it and got a spark, nice and blue unlike before. Cranked a second time got a second spark. Then nothing. Disconnected the connector to the ICM which still had the green wire from the coil connected and checked continuity reconnected it and another spark, then nothing.

Hmmm now that I am inside I just remembered I did not disconnect the other leads from the PCM to the ICM. I will do that later and try again, I bet it is getting dual signals and is confused. Gah I can't believe I forgot something that simple.

 

Cutting the wire fixed the no spark! Tested it and the thing began to run roughly with only three plugs installed. But after it died I haven't been able to get it started again, well it will run for a few seconds but I know I have the distributor timing way off. But at least I can work on it now that it has spark again.

Big thanks to everyone who threw out those ideas.

If I get a chance I will post some photos of how I wired it. Looks like the computer was bad after all, but this is a much less expensive fix than replacing it. Eventually I want to do the Weber carb upgrade also so this is a first step in that project too.