Need help ID'ing ZJ Electrical Problem

jeepinmatt

jeepinmatt

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Mar 24, 2005
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Stanley, NC
Back in january, I was driving home in my 95ZJ when I noticed the wipers were really slow, the the guages started acting up and the headlights were dim. Made it home, but the battery was so low that the ignition wouldn't power the jeep over 2500rpm. Clearly (I thought) the alternator died. So I pulled it, went to Advanced, and tested it. It tested good. Took it home, put everything back together, and it worked fine

Then last night, the same thing happened again. Once I got to the garage, I measured voltage with the engine running on the battery, about 8 volts. Voltage from the output of the alternator to the ground terminal on the battery also about 8 volts. Revved it up to about 3k rpm and no change in voltage. I unplugged every fuse, visually checked it, and ohm'd it, everything was good. Battery connections were ok, but I cleaned them anyway. I pulled the negative terminal so the PCM would reset, and put the battery on the charger overnight.

This morning when I went out, the battery was at 12.2 volts with the charger connected. That was enough to make it to the shop, so I was going to go for it. Cranked the jeep and headed out. The voltmeter on the gauges was showing about 10-11 volts, charging system wasn't working. About 2 miles down the road, I put it in 2nd gear and stomp it. About the time the rpms start climbing over 4k, the voltmeter jumps up to just over 14v. I continue on my way, it charges fine for another 12 miles, then drops down again. Im running about 55, so I pull it back into 2nd gear, and immediately the voltage jumps back up to 14v and stays there until I get where I'm going.

Everything I've read says the voltage regulation occurs via the PCM, but I'm suspect of the little black box that mounts on the alternator. I'm just not sure what to do next.
 
Regulator is in the pcm. How many miles on it. If it does it again tap on the alt and see if it starts to charge if it does the brushes in the alt are worn out.
 
About 220k miles. Alternator was replaced 5 years ago with a new OEM alternator. When it was running yesterday, I tapped on it, and nothing changed.
 
Man to be honest with you I'd replace that alternator. It sounds to me like a heat related intermittent issue. Everything I've ever known or read about the alternator is that the actual voltage regulator is in or on the alternator. The PCM may read data and/or send data to it but the actual regulation has to occur there.

If you'd like to replace the alternator and not have to worry about being able to return it if that is not the problem let me know. I can pick the part up and drive out to your place. I won't have any issues returning it even after it has been used. I'll even take your core back if that is indeed the problem.

EDIT: Same thing goes for the PCM, by the way...
 
I know for a fact that most all chryslers have the voltage reg built in to the pcm since 1984. They usualy work or dont work. When it isn't charging see if u have power on both field wires on the alt if u do may be pcm or wiring related. If u only have power on one field wire the alt is bad.
 
Well I guess I dunno crap about the alternator vs PCM voltage regulator bit, but either way I'll help you with the parts if you want to try them. :p
 
cltdba said:
If you'd like to replace the alternator and not have to worry about being able to return it if that is not the problem let me know. I can pick the part up and drive out to your place. I won't have any issues returning it even after it has been used. I'll even take your core back if that is indeed the problem.

EDIT: Same thing goes for the PCM, by the way...
Click to expand...

Thanks for the offer. My short term solution of drive the piss out of it is probably why it has taken so long to manifest again (I have a tendency to do that). So hopefully I can get something figured out.

sinbad said:
I know for a fact that most all chryslers have the voltage reg built in to the pcm since 1984. They usualy work or dont work. When it isn't charging see if u have power on both field wires on the alt if u do may be pcm or wiring related. If u only have power on one field wire the alt is bad.
Click to expand...
There are 4 terminals/studs on the back of the alternator. Which do I check and how?
 
(1) large red goes to battery positive
(2) large black goes to ground on alt case
(3) & (4) are field wires, should be small and in a holder or in a connector that plugs into the alt.

1 should have batt voltage on it with ignition off
with eng running 13.3 to 14.4 volts
2 no voltage only ground
3&4 as I said, only one with power alt bad, power on both, pcm or wirring problems
 
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