Hey folks,

I'm narrowing in on a very frustrating engine cutout issue and I could use a little help.

My XJ (96 with a 97 4.0L motor) has been intermittently cutting out while I drive. This causes her to buck, and most times stall leaving me dead in the water.

When I try to restart it always cranks, but won't start, like I'm not getting any spark though I've not been able to verify this. It happened to me this morning and would not continue to run after several restarts so I had AAA tow it to a shop to work on it during the day while I was at work.

I just picked it up and here was their conclusion. They chased it for a while thinking that it was the crank sensor, which had been replaced not too long ago and eventually found that if they "play with" the part of the wiring harness that goes to the alternator they could get the engine to cut out. They showed me this happening and it started right up afterward.

I crawled under so they should show me the wire(s) since the alt is on the bottom of the engine.

The wires going in to the alternator first go into a little plastic box. I don't know what this box does, and neither did they. It would seem easy enough to just splice in a new section of wire, but I don't know which wire is causing the issue, or what to do about the box.

Can anyone shed some light on this? Is it an RF interference canceler, some sort of diode, or something different?

If I can figure out the function of this box I'll at least have a better idea of how to fix it.

This still doesn't explain why I have such a hard time starting the thing when it does on the road, but I'm thinking that suddenly loosing all the spark is probably creating a bit of a flooded engine, which is contributing to the starting woes.

I'm open for suggestions and if there are any fixes I can try this weekend I'm all for it. I'd like to drive a worry free vehicle next week lol

Thanks in advance,

Hotrod

 

Pics would help.

You could be talking about the fuse box, but the voltage regulator is in the ECU, so you could be talking about the wires going to the ECU for voltage regulation as well.

 

I'll see what I can do about pics.

It's not a fuse box. This is actually touching the alternator.

 

I was gonna go look at mine, but I've got an 87, I doubt they'd be the same.

 

I have a couple pictures, but it's tough getting something that's very clear without laying on my back in the snow... Totally not going to happen today lol.



I circled the black box in question.

 

Can anyone id that? Does anyone have the same mysterious box on their alternators?

 

It looks like an exciter box. But I haven't seen those since my fullsize blazer days. Perhaps I am wrong, just goin on previous experience.

 

After goin to my neighbors and checking his, It appears as if that is a junction block. my suggestion to you is to check all of your grounds. especially the ones on the block itself. Bad ground=bad alternator performance. I'll check his jeep again in the morning to see if it's anything else.

 

I can try checking the grounds, but it doesn't explain the shop's ability to stall the motor when "playing with" those wires.

What exactly does an exciter box do? this is new to me, but I can't imagine that my Jeep has a freakishly weird charging system for some random reason so one would assume that this is standard on either 96' or 97' 4L motors...

 

An exciter is what older vehicles used to "kick on" the alternator. 99% of alternators now are self exciting. I would open the junction block if possible and see if the junction plat is cracked or corroded.

 

Why would my '97 motor have something that is that out dated? I realize that was quite a few years ago, but this shouldn't be a one off Jeep Engine I have. I would assume that it's a pretty standard 97+ motor. Unless folks with proper motors can tell me different.

If is is an exciter box is this a part that would come with a new alternator, or part of the vehicle's wiring harness?

 

Don't misunderstand me. I said it looked like an exciter. Upon inspecting my neighbors 96 I verified it was a junction block.

 

Oh, my bad. So it's simply a place to transfer power from the harness to the alternator circuit?

One would presume that I could easily bypass the box then and simply splice new wire in right?

I will attempt to open the box tomorrow and see if i can make any progress and if it is in fact openable.

I still don't understand why there would be a junction box there, and for what reason.

 

So I went to autozone and had a look at a replacement alternator. The replacements didn't have the same box as mine does. I'm starting to think that this is a part of the alternator itself, and if i bypass it I should be fine.

I just can't figure out why the box is there if I can simply bypass it.

 

If you're talking about the moulded "rubber" block directly on the back of the alternator, that's all it is - just a moulded rubber block that locates the wiring for the convenience of the shop monkey. No electronics - just wire.

You may have a loose connexion that is giving you fits. Suggest you start with either disconnecting the subharness, putting it on the bench, and doing continuity checks while you're fiddling the wires - or a detailed visual inspection to see if anything is amiss (before you end up putting the wiring on the bench, &c &c.)

Diagnosing intermittent electrical trouble is a real bear. I know - I've done it. Dumped eighteen hours into tracking a floating ground in the fuel injector subharness on my 87 years ago (started with wiring in the whole engine bay - since it showed non-specific Sx. Then kept narrowing it down until I finally found it. Fixed it, then never had that problem again...)