93 XJ no start, no spark

[JeepBox]

Hi everyone, great forum. I’ve used it in the past as a reference but just made and account looking for help.

1993 Cherokee, 4.0, standard transmission, 4x4.

The Jeep was idling outside work for a few minutes and when I went outside it was dead/would not restart. As of right now it is still broken, and I think it needs a PCM, before I buy one I want to run my troubleshooting by some Cherokee experts,


There was pressure at the fuel rail, so I swapped the ASD relay with the AC relay because I know the AC works. Then I bought and installed a crank position sensor because I’ve owned Jeeps in the past. I am not getting any voltage to the ignition coil. I tested continuity from the ignition coil wire grey to PCM 19 grey and it checks out 0.2 ohm. I tested continuity from ignition coil wire green/orange to PCM green/orange and it checks out 0.1 ohm. Key on, engine off - no voltage from PCM 19 grey wire. I visually inspected the engine grounds and 02 sensor wires, no obvious issues.

I know the cam sensor can effect the Jeep starting, the distributor and sensor was replaced about 3 years ago by the previous owner. Also, correct me if I’m wrong, but failed cam/crank sensors will interrupt the green signal wire, the grey wire should have continual voltage ? I pulled the PCM connector off and checked for issues with the pins or corrosion but found no issues.

There are no wiring issues leading to the PCM but the PCM is not outputting any voltage to the grey ignition wire. Should I replace the PCM?

Side note: I’ll add a picture of my XJ, some of you guys might be slightly aroused. It’s a rust free, stock, great condition 2 door stick shift. Cool little Jeep.

Sent from my iPhone using Tapatalk

 

[Rob Mayercik]

All I can think of are the underhood fuses/fuse box and the ignition switch.

As far as you can tell, is everything else that's supposed to come on with the key in "on" coming on? (HVAC blower, lighter socket, etc.?)

 

[JeepBox]

Yes. The radio, hvac, interior dash lights and everything come on with the key. There are no engine codes stored.


Sent from my iPhone using Tapatalk

 

[Gunter]

check your neutral saftey switch. or however the manual trans units do this function

 

[muddypunk]

You can source a Brain at any junk yard unfortunately I carry a spare in my 95 manual trans Cherokee its had issue not unlike your own. I believe it was about $25 I've used an automatic equipped pcm and a manual equipped no change to the regular operation of my rig I've even thought about hanging a Gremlin bell for one of my Harleys on it. Best of luck

Sent from my SM-J337V using Tapatalk

 

[75SV1]

Is it turning over? IE cranking, when key turned or not?

 

[Rob Mayercik]

Had another thought - check all the wires coming out of the "bulkhead connector" on the firewall below the brake booster (connects to the cabin fuse box), see if any are degraded/rotted. Be thorough, actually pull it off and manipulate each wire to be sure - don't just do a visual.

I had issues about 10 years back where my '92 would black competely out until I futzed with that connector to get it going again. The most nerve-wracking occasion came on the way home from the pick-and-pull (where I'd just cut the connector out of a Cash-for-Clunkers Jeep so I could replace mine), on an interstate off-ramp.

Wasn't until I started doing the replacement that I found out just how badly mine had corroded over the years - the 10-gauge wire in mine was completely corroded away, only making connection by laying against the remains of the terminal. I only discovered it when I pulled on it so I could cut it loose and splice the replacement connector's line on.

Maybe you have a degraded connection or two in that connector?

 

[JeepBox]

Yes it cranks.

It’s not a neutral safety switch, it died while running and also I don’t think it would crank if that switch failed.

I touched those wire harnesses mentioned and yes they are brittle, haven’t inspected them yet, here’s what I’ve been up to:

Decided to put a PCM in it, my junkyard has none at all. I asked an employee and it turns out one guy comes and buys every pcm he can for any car every day because the junkyard only charges $28, then he ebays them... nice.

Side note: the part number for the PCM that came out of my Jeep is a mopar performance PCM for a carbureted 4.2 running aftermarket fuel injection. Weird, I’m surprised it worked.

There’s a used jeep specific guy in town so I tried to buy a PCM there, he didn’t have any for a Cherokee. I told him “give me anything with this plug” out of desperation and he sold me one for a 92 YJ.

I installed it and had 7 volts from the constant 12 volt wire (previously had 0). I removed the coil and sanded all that grounding area, no change. I cut the grey wire a few inches after the pcm and tested again, had 12 volts. I figured the wire was rubbed through somewhere in the harness so I made a test wire jumper from the pcm to coil, as soon as I plugged it in it went back to 7 volts. I unplugged the connector and made my own new connector (thinking that the connector internally failed), but my new connector did the same thing. So I am thinking that there is a problem from the green wire “signal voltage” harness (it leads to the alternator).

I took my 12 volt jumper (from the grey PCM wire) and connected it to both terminals of the coil. From my understanding this should work, but you bypass all the safety features of your crank sensors etc. My goal is to hear the Jeep run at this point, but even with 12 volt to both terminals of the coil it will not start.

I no longer smell gas while cranking so I checked the fuel rail, no pressure. I installed my original PCM and fuel rail pressure returned, but the PCM grey wire lost voltage.

This confirms my original diagnosis of the PCM failing. I installed the PCM from the YJ and jumped the ASD relay as well as the fuel pump relay. I had fuel pressure and 12 volts constant to the coil but the Jeep still won’t run.

At this point I am going to buy the correct PCM for it. Even though my Jeep had 12volts at the coil, fuel pressure and still won’t run I am banking that something else with this PCM is preventing it from firing. Either way it needs the correct PCM, I can’t keep my fuel pump jumped.

Where can I get a PCM? eBay? Just any 4.0 HO xj PCM will work ?


Sent from my iPhone using Tapatalk