Charging issue

is there a way to tell if the regulator in the computer is going bad? I've had low voltage ever since I've had this thing, big cables did not make a difference. It runs 12.5 or so normally but if I turn the ac on high it goes down to just between the red and that hash mark on the gage. I finally had time to take it off and drive to town to have it tested, but of course both testing machines in town are down! Had to put it back on because I may have to work tomorrow. Too darn expensive to just put a new one on to try it and no junk yard units available. This is my 95 RHD, my old 01 always sat at 14 no matter what. It is a 90 amp alt that had been replaced in 14.

Or is there an internally regulated bolt on unit? It has to be the same size, RHD alternators set higher and with a little age on the motor mounts they even rub the battery tray!

Without any testing or diagnostics, why do you assume the alternator is good, or that the PCM is bad ?

Put your volts/ohms multi-meter on the alternator output, and prove or disprove the dashboard gauge readings. Inspect the brushes. Make sure the fan belt is as tight as required.

If we are going to assume anything, the logical root cause is a faulty alternator, probably worn or sticky brushes. A guy in my local Jeep Club works at one of the big name auto parts stores and says up 20% of rebuilt in China alternators are faulty right out of the box. I have my alternators rebuilt locally with Japanese parts.

Thats why I was asking if there was a way to test the pcm, lacking the ability to have the alternator tested i figured maybe I could rule out the computer before I dropped the cash to get a new alternator. Rhds have a spring tensioner, seems to be tight, no belt squeal. Kinda in the middle of nowhere, options are limited. I'll see if I can get my multi meter working again, but I imagine the gage is right, fans and lights come up with rpms.

Get the voltmeter on it, make sure the battery in the volt meter is fresh, else meter may give erroneous reading. You may have a fa,se low reading on the dash gage, so let us eliminate that possibility first.



The fact that your lights get brighter as rpm increases indicates the alternator is outputting, a good sign. also note a bad car battery can cause low voltage problems, a shorted cell may cause low voltage even if alternator is ok.

Shorted cell was my first thought too, so I put the practically new 1000cca marine battery out of the pick up in it, same result.
I know its charging, I drive it 150 miles almost every day, I'm starting to get a misfire and hesitation when taking off occasionally. I just tuned it up in the last 10k so I figured I better take care of the low voltage before I start trying to test other things like the coil or tps.

Alternator failure is common, PCM failure is very uncommon. With the 8 XJ Cherokees I have owned, and the 15-20 in my local Jeep club, zero PCM failures.

I would take a close look at the "field" circuit wiring, corrosion/resistance can effect the charging. Unless you have a very good DVM, it's hard to read 0 ohms.

13.3v , 12.8 all accessories running at idle, is that sufficient?

Unclewolverine said:

13.3v , 12.8 all accessories running at idle, is that sufficient?

Click to expand...

Most batteries prefer to be charged between 14.0 and 14.3 volts. I aim for at least 14 volts battery hot and all accessories off.

The blower fan seem to cause a drop in the dash display. I assume the drop is a result of the wire size under the dash. My XJ does the same thing. For that reason I do not pay it any mind when the AC is on high. Get a volt meter and check it at the battery terminals.

There is a way to accurately isolate the alternator from the PCM charging circuit which I have done in the past. I think most here are afraid to use that procedure.

That is the volt meter reading at the batt.

think ill just go until it gets worse. Found my hesitation, my tps went completely haywire today.

Did this ever get figured out? I have a Renix jeep with a 100 amp alternator and it does the same thing. Runs at 12 V on the gauge and drops to the red as more items are turned on. It did it with the old 65 amp alternator and I thought switching it to a 100 amp would do better. It has probably never read 14 volts at the gauge since I've owned it. The battery and alternator read 14 volts when the jeep is running.

The dash guage isn't that accurate and as you already did, you need to measure directly at the battery. Go visit cruiser54.com and do all the tips relating to cleaning connectors and grounds, especially adding the ground under the dash to the frame. That will help. Dropping voltage on the gauge as you crank up thing like the blower is also somewhat normal on an 89 as the wiring and switch are barely big enough. If you haven't done it already, I also highly recommend doing an aftermarket headlight harness.