I've been combing this thread for a while and haven't really found anyone with this same problem...

I've got a '97 XJ 4.0. Ok, so I started her up tonight and pulled out of the drive and noticed my battery Volts were reading 12v instead of 14v. (Sounds familiar so far...) But no worries, she'll come up. But INSTEAD, she drops down to <9. Again, not unheard of but here is the ODD thing. It DOESN'T die. In fact it runs completely normally. I took it for a few spins around the block at < 9v and she's fine. So I shut it off, start it back up, and everything is fine and it reads 12v again. Then it drops... So I check the battery and it's holding 12v with the truck off. So I start it and check it and it's still holding 12v. When the gauge drops down to < 9v, i check the battery voltage and it says..................... 13.9. :-)

So what the hell is actually happening?

The ECM throws a low voltage code, but the battery is getting 14v of charge from the alternator at the same time the internal voltmeter says 9v. I don't actually know if I have an external voltage regulator (97 was an odd year for the cherokee I'm finding...) but I'm thinking that could be the problem.

What do you guys think? Thanks in advance, because you guys all rock.

 

mine dose the same thing with the heater and the headlights on i would think it might just be a faulty guage or a short someware

 

You need a new PCM, the voltage regulator is located on it.

Or, take the alt out, bring it to a alt repair shop and they can put a external regulator in the alt case. Just dont hook up the field wire to the alt if you get the external regulator.

 

My 88' XJ 4.0 did the same thing. I replaced the 61 amp alternator with 105 amp and the voltage does not drop with the accessories on anymore.

 

Well, slightly uneventful solution:

Turns out it was the alternator. After I let it run with "no" voltage for a while, the Meter started showing the voltage dropping slowly. So it wasn't necessarily dead, it was just dying. Swapped it out and problem solved, and cheaper than a new ECM!

Oh, FYI, the 120 amp alternator (for a Grand Cherokee maybe?) is too big around for the 90 amp alternator mounts, and I didn't look like there was enough material to grind away to make it fit. At least on my XJ, but mine seems to be built differently than others I've worked on. (??) Cheers to all who responded!

 

not the same

 

not the same either.

 

yes there is a thread on the install of a 136 amp gc alt. in the search area on this, thanks though maybe the others will search now when they want to do this.Glad it was a cheaper fix 4 ya.

 

Mine has been doing this on and off for quite awhile. Regardless of what accessory is on or off it will typically read in the red at the gauge, but fine at the battery. Never had a problem starting and it never dies. I believe the problem is more of a loose wire....

 

Where do we get a guide to what vehicles we an get higher voltage alternators to fit our year jeep