I have a 1991 Jeep Cherokee XJ 4.0L 5 speed tranny, with A/C. I have noticed that when I go to work in the morning and the air outside is at a nice temp. the voltage meter on my dash is up at almost 14 volts. with the A/C on high and radio on and even when I turn on the lights.....but when I come home in the afternoon, and the air is hot outside, with the A/C on and radio, the voltage is down to the first tic mark just above the red part. It seems like it is not regulating properly. I changed the Alt. and even the PCM since I read that the voltage regulation is done in the PCM, but it is still the same. I know when the voltage meter is low it is actually low, because the blinkers are slow and sometimes at night the lights even dim . but in the morning, all the way to work ( about a 45 min. drive) it is up at 14....any ideas ? or has anyone else seen this ? Thanks in advance for any help you might give.

 

Alternator's output goes down the hotter it is, but I am not sure how much load (A/C, lights...) vs alternator output there is.
I used to do final testing for alternators going to HMVWW (the Humvee, not H1) and it was one of the tests to get it hot to make sure voltage doesn't drop below certain value under certain load.

 

Look for a slipping belt.
Look for a loose/warm connection inside the power distribution box
Look for a bad ground - engine to battery.

 

I did freshen up the grounds and checked everything. I have noticed that when I run the jeep w/o the A/c the voltage stays up at 14, when I turn on the A/C it drops down a little , like half way to the line that is just above the red area, and when I turn on the parking lights it goes down even more, at idle with a full load it has 13 volts at the battery, and when I run the motor up to about 2000 the voltage goes up to 13.8. but the meter on the dash still reads low. I haven't had a dead battery so it must be charging, but when the lights and A/C are on and I use the turn signals , they are VERY slow, so it acts like the voltage is very low....real strange deal....any thoughts ?

 

^^I would start with this.^^
How old is your battery? It is possible that you may have a cell bad in the battery. I would have it checked. Make sure that they do a load test. Just because it has 12 volts doesn't mean it's good. It's the amps that need to be check to see if you have enough to start your Jeep.
Good Luck and keep us updated.

 

the battery is less than 6 months old and it cranks the engine over just fine. I have not had a dead battery, just low regulation when under a full load. I have freshed up the grounds and I am currently replacing the engine to firewall ground just be sure it is a good clean connection.

 

have same issue over 4 years now never caused any problems i did upgrade battery and ground cable size stopped slow turn signal issue lights got brighter but still low volt issue remains ....if theres a cure i cant find it

 

thanks for the info. I will also get new mattery cables and maybe even another battery...worth a try for sure.