Curt

1999 Cherokee Sport, 4L, etc. Last time I posted was due to a failed fuse. This was after changing the alternator. Well...

The XJ ran for maybe a week but started to die while in operation. I was able to start it up again each time but only after changing that fuse. I looked for the cause but was unable to find anything. It finally got to the point that it will blow the fuse every time you attempt to start the engine. So to clarify, this was a gradual onset of the problem. Engine runs then fuse blows (looks to be in the ASD to PCM circuit) with large gaps in between. A week or so later, that gap was down to a few minute until it wouldn't start at all. I have tried testing everything I can think of. I'm out of ideas and my other car was totaled this morning due to bad weather. So I need to get this up and running asap.

The problem, again, is the vehicle won't start as the fuse labeled F21 in the PDC blows when the key is turned to start the engine. NOT when it's one-click before. Full start commit, it blows instantly. I'm unable to find any exposed cables, cracked, stripped, burnt, melted or otherwise exposed to cause a short.

As of July, the following was replaced with new:
Distributor Cap,
Plugs/Wires
Ignition Coil
IAC,
MAP,
CPS,
Alternator,
Battery,
Power Steering Pump
Various cables
All grounds.
Suspension parts.
CV Drive Shaft
Rebuilt Transfer Case
I'm definitely forgetting something.

Any help at all is greatly appreciated and I would happily return the favor.

Oldsrip

I think you need to look at a schematic to see what is on that circuit.

then you need to disconnect each component one at a time to see if the fuse stops blowing.

Then you need to inspect that component and it’s wiring.

that’s where I would probably start.

I have seen a shorted starter blow fuses before. You can disconnect the wiring at the starter and turn the ignition on and see if it still blows the fuse.
its free and easy to do

Dave51

Got any engine codes?

Are you SURE there are no other fuses blown and all the relays are good?

Disconnect the oxygen sensors and see if that fixes it. Check all the wiring for them as well.

Disconnect the alternator and see if that fixes it.

Consider coil and fuel pump wiring.

dave1123

Because of the slow progression of the symptoms, I suspect corrosion. Maybe on the underside of the PDC. Fuse 21 supplies the PCM with power. It's supposed to be 15 amp, but apparently the PO had this problem and he cured it with a larger fuse.

Actually fuse 21 supplies the PCM with the feed from the ASD relay. The ASD relay doesn't pull in until the engine is actually cranking, so this is why I suspect corrosion under the PDC. OR a shorted ASD relay, although that wouldn't have been a progressive fault.

Curt

Ok, here's an update. I'm a bit behind on the thread as I've been busy trying to get this running. I found (from several service manuals) that three things are fed off the dark green/orange wire: PCM, Ignition Coil & all 6 injectors. PCM seems to be in series if I'm reading the diagram correctly. It's likely I am not. So I unplug the PCM, try to crank the engine and the fuse did not blow. After rooting through much documentation, the idea that the PCM is bad has a high probability of being an issue. However, something in that chain also was just as likely to cause it. I did replace the ignition coil in the summer and the alternator just prior to these issues. Since the alternator can hit the ignition coil during installation and removal, this seems like it could be something I need to look at more closely.

So, PCM disconnected and the fuse did not blow on the attempt to start. Has anyone run into this in the past?

oldsrip - I did go through your suggestions last night and this morning. I couldn't find anything definitive.

Dave51 - I spent last night with the battery in the house testing out the 5 pin relays. I verified that when power is applied to 85, 86 & 30, the circuit opens, has no resistance and has a 12v output.

dave1123, I am pulling the PDC off to check the underside and look for any exposed wires since I'm there anyway. I don't disagree but a resistance test didn't give me any big numbers. But that test is only reliable if I was reading the schematics correctly and I'm not an EE.

So unless anyone thinks it's a horrible idea, I'll be looking at getting a new PCM, ignition coil and rewiring the IC grounds. I'll inspect the injectors but I haven't heard of them causing a PCM to go bad. At least not yet.

Dave51

In surfing the net I found 2 instances where disconnecting the oxygen sensors in one and the alternator in the other dx'ed the issue.

Hey, it's free...

Clayzer

ever get this fixed?