Loses major voltage

I'm hoping someone can lead me in the right direction. A search turned up similar issues , but not quite what I'm experiancing. I have had to replace the battery and altenator in the last couple weeks, both were toast. The altenator was replaced just 4 months ago. While I'm running the motor with all accessoires on is when it is most noticeable. It runs regular then all of a sudden the volts drop off to zero on the gauge and the head lights dim way down and the blower hardly runs. This only lasts for a few seconds then it boosts back up to 13/14 volts and everything is back to normal. It did it 3 times on my 1.5 mile trip to work today. All the fuses are good and like I said new battery and altenator. What can cause a major drop in voltage like that and then just jump back to normal so fast ?
Thanks for any ideas :compwork:

boise49ers said:

It runs regular then all of a sudden the volts drop off to zero on the gauge and the head lights dim way down and the blower hardly runs. This only lasts for a few seconds then it boosts back up to 13/14 volts and everything is back to normal. It did it 3 times on my 1.5 mile trip to work today.

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I was thinking maybe a shorted rad fan motor, that would be one thing that comes on and off while driving. However, I would think the signal for that fan would stay on longer than just a few seconds. I had another car that had a frayed wire at the starter and when I stomped on the gas the engine would torque just enough that the frayed part would touch a piece of the frame, drove me nuts 'til I found it.

check your grounds, clean your bat cables.

when you installed the new ground cables did that include the one from the block to the firewall? Did you sand/wire wheel the mounting point back o shiny bare metal and then use Ox-Guard or Dielectric grease to protect the connection?
I don't know if you have the c101 connecto in your year. I have it in my 87 and it['s a pain. It's below the brake booster in the firewall. It has a bolt that holds it together. Undo the bolt and take a look there, practicaly all the wireing runs through that. Mine got weird after my clutch master leaked all over it. Taking it apart is a PITA. I can't remember the toque specs on the bolt, but it's not majow, so don't go tightening the bolt down super hard when you put it together.

After thinking about it some more, if it was a shorted fan or some other accesory in the car, it should be blowing fuses, if the proper fuse is in place. So, the only other thing I could think of that would drop down your voltage temporarily like that would be a bad wire coming from the alternator. Maybe check the terminal ends, or see if the wire is crimped or pinched somewhere?

Here is a post from a guy who sounds like he nailed it on I Love Jeeps forum.


Have you modified your grounding and charging circuits? The OEM wires are inadequate to support the full output of the alternator, and there's next to nothing for body grounds (though the vehicle seems to wanna run off of em !)... Net result is that your alternator isn't getting all that it's driving back into the battery or the systems it's supposed to be supporting... What you're describing is the "late stage" failure where the alternator spends a lot of time at full field and heats up causing all kinds of erratic behaviors.

Look at the alternator's output wire. ask yourself if you'd run an audio amp that may draw 30Amps peak on that tiny of a wire -- then try to figure out why Jeep tries to 'charge' from an alternator that can maintain steady output at 2-3 times the current (61 - 105 amp alternators are stock) that through that same wire? (Double it at least - add a larger gauge wire in parallel with the existing crappy wire...)

Dude - seriously -- look at a 100Amp (1200Watt) stereo wiring kit from "WalMart"... and then at the tiny factory charging and grounding stuff in the XJ, and try not to cry...

While you're there look for grounds. In the first couple of years of the 4.0 XJ, the battery wasn't provided with much of a serious body ground and the charging ground (back to the engine) wasn't much to talk about either... Look at where your headlamps ground (and the fan and the stereo and t rear defrost and the computers and ...)... ADD a strap from the battery to a good body point - and also from the battery to the block near the alternator (mounting bolt is top-flight for this)...

Typically, you'll see 1-2 volt improvement in charging (alternator is actually working less and delivering more usable power instead of heating-up crappy little or too long of wires) - the 1-2 volt improvement will be usable throughout your vehicle as well (path's back OUT of the battery are improved , alternator to body is improved, etc)...

If you've NOT modified this charging stuff in your XJ, then get that done before you try to chase ANY gremlins... I've seen XJs of this vintage come in with "electrical issues" to find that they're charging only through the head-strap and the OEM 8awg line. (alternator output reading at 14V with a batter charging voltage of 11.5 then only 8 or so at the headlamps!) Others were chargin only through broken motor mounts (I don't know how those started - the starter draw must'a been interesting!) Anyway -- I'll look for a write-up link on the "how to", but this is the #1 electrical issue for the early 4.0s and it acts EXACTLY like what you're talking about (and dim lights and plenty of other strange issues, including auto-tranny shifting issues)


Yep I have the shifting issues too. You must of nailed it !
I'll keep looking for the link if you can find it. While I'm waiting and hoping you can find it I'll check the stuff you mentioned. I have did the ground things already. The charging wire will probably be a good start.
Thanks a bunch !

Well I added a larger charging wire and drove it. It did it again, then I noticed when it did it that it pops the circuit breaker on my power windows.
I unplugged the power window connection at the fuse box and it doesn't do it anymore. So evidently I have a dead short in the power window lines somewhere. Does any one know where I should start when looking for window shorts ? ? ?

Is the power window motor getting hot when you cycle it up and down? That's a good place to start.

I'd look at where the wires go into the doors. lots of flewxing there, not uncommon for wires to be broken, if it's a broken wire then the flexing may have worked the wire through the insulation.
other than the obvious areas of where wires move, you're taking off door panels and looking under the dash.

Four door? Is so under the carpets is also a good place to look.

Yeah I think I'll unhook the 2 connections on the passenger kick panel in the front. If it still pops then it will be from there to the other side somewhere.
That way I rule out the passenger side and only have to work on the drivers side. If I remember correctly I unhook those 2 connections and the drivers window still worked when I worked on them before.
Thanks !

langer1 said:

Four door? Is so under the carpets is also a good place to look.

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I always forget you XJ guys got more doors.

87manche said:

I always forget you XJ guys got more doors.

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Yeah 5 on mine if you count the hatch. I hate power accessories. Always have.
10 years down the line they all fail. I could careless about the door locks, but windows
are kind of important. $1200 8 years ago for the XJ though, and 15K later I'm not gonna dump it with out a fight.
:twak: