billyflubber

Help! I want to be a cool Jeep guy but I can't even get it out of the garage! Was given a 1995 Cherokee that a family member bought and it wouldn't stay running.

Here's the scoop:
Starts and runs just fine and if it's idling it will run for 15 minutes or so and then dies and won't restart. If you are driving it sometimes it will run just fine for miles but then it will die and not restart. After enough time sitting it will fire up again just fine.

When it dies, I've been able to confirm the fuel pump runs but there is no spark. I read through the forums here and started with the basics. Cleaned the grounds, cleaned the fuses, verified good batt and alternator voltage. Swapped out the relays to make eliminate the ASD. When that didn't do it I replaced the CPS with a new factory Mopar one as well as the distributor pick up coil and the coil. Cap and rotor appear new.

Reading through the forums lots of people seem to have similar problems that ended up being the CPS or loose fuse connections. I feel like I've eliminated those as possibilities and I've also tried wiggling whatever wiring I can find.

I'm not real familiar with Jeeps, anyone have any suggestions? Overall this rig seems pretty stock and unmolested other than a crappy lift job and some fool drilling holes in the air cleaner.

Help!

tech

OK just to test it run it till it dies then take some water and cool down the crank sensor and see if it will start back up if it does then change the sensor .

agreen

Here's the short/skinny on how ignition works in this vehicle.

It starts at the crank sensor. A tiny voltage is made into a square wave. Sends a bunch of square waves to the ECU. The ecu takes into consideration a bunch of other things, and sends a ground pulse to the ignition coil. The ignition coil gets power to it through the ASD relay and a 20A fuse (#20, so the FSM says).

Knowing what you know now, I'd get a logic probe (it's an LED test light that has a high input impedance that doesn't draw any current from the test circuit) and test for ground pulse and +12v at the coil. If you're not getting that, use the same logic probe at the ECU at pin A8 on connector C1 (the wire is grey/blk) and check for the crank trigger pulse. Do a visual inspection of wiring, look for signs of chaffed or rubbed wires.

It sounds like you've done some extensive testing and have already changed lots of possible culprits. I hate saying it, but I think it's looking like it's the ECU at this point.

Out of curiosity, you DID check fuel PRESSURE, right?

billyflubber

-Thanks, however crank sensor is new OEM sensor and it's still dying

billyflubber

Awesome info, thank you! I didn't check fuel pressure but since I know it's not getting spark and it runs fine when it is running wouldn't that eliminate the pump as the culprit? I can throw a fuel pressure gauge on it though.

Dave51

OBD I error codes?

Dave51

It sounds like a CPS type of thing. Check for signal at the ECU to make sure it isn't a wire. If you think it's the ECU I'd take it apart and look for broken solder connections at the incoming plugs (I don't think the ECU is dead per se as you can get it to periodically run).

tech

Sorry I missed that you changed the crank sensor . does it run about the same time from cold each time ? because if it does then it could be heat related . maybe the cam sensor or the coil is bad .

billyflubber

So today some new developments today:

1) Now it just cranks, no start and no spark
2) Fuel pressure is 35PSI
3) Pulled the under hood fuse panel apart and checked and wiggled everything. Looks clean and tight. Fuses still good.
4) Finding some discrepancies online when looking at 1995 Cherokee issues. For one, the PCM is under the dash and not under the hood. The crank sensor is the 2 hole mount rather than the single one which is what all the online applications says it should use. PCM is a single connector old school metal box about the size about 1.5" x 4" Maybe that's just the transmission ecu?
5) Coil wires are green/orange and black instead of the grey and dark green/black as listed in the schematics.
6) I discovered engine will only crank in park, not in neutral. Could neutral switch have anything to do with ignition? From the schematics it doesn't look like it.

Argh!

exasemech

Just a couple thoughts . Like others have said sounds heat related but based on all you have said:

1. "and unmolested other than a crappy lift job and some fool drilling holes in the air cleaner." The thing has been hacked on. I would look at every bit of the wiring harness, particularly around any add on's (lights radio,gauges) for any wiring mods, crimp connectors etc that may be intermittent.


2. Re the wire colors. Make sure you are getting the info from the correct year factory shop manual. Haynes etc is a waste of paper.

3 if you have no spark that's the first thing to deal with and I don't know the spec for a 95 but fuel pressure sounds a bit low.

lawsoncl

Fuel pressure per the FSM should be around 39 psi with no vacuum to the regulator and 39ish at idle with the vacuum connected, so I think your 35 is close enough. All the 4.0 Cherokee crank sensors have two mounting holes. The GRAND Cherokee sensor has a single mounting hole. If you're looking at the box under the passenger side below the glove box, that would be the trans computer. The engine computer should be under the hood between the airbox and the fender. My manual shows the ignition coil wires being DarkGreen/Orange for power and black/grey to the ECM. No, the neutral switch won't affect park, just whether it will crank.

We are talking about a regular Cherokee (XJ) and not a Grand Cherokee (ZJ) right? Two different animals.

billyflubber

I believe it's just a regular Cherokee. Yeah, I'm dumb and missed the giant PCM on the inside of the fender well. Someone has obviously been messing with it because it was loose and missing screws. I took it out, inspected under the cover and cleaned the contacts. Reinstalled it and the engine fired right up. It's been running about 30 minutes now which is usually about when it would shut down. I've wiggled the wires and banged on the PCM and it's still running. Stay tuned.....

OutbackOz

bad ground of the pcm?

billyflubber

Does the PCM have a ground near it? I looked around it and didn't see one but it was dark out by that time and I might have missed it.

OutbackOz

no if the pcm itself was loose then perhaps it grounds the shell through the bolts to the body? just a guess other wise how come you tighten it and now it runs fine? XJ electrical stuff can get weird happy it's going