maximusb1984

So it started out with my voltage meter dropping out while I was driving. I went home and first pulled the battery and alternator to have them tested. Both tested out good. I assumed this meant a cable was the problem so I replaced the cables to the starter, alternator, and relay box. This seemed to fix the problem for about 30 min. As soon as I turned the jeep back on the voltage meter is reading 9 again and then dropping out in a min or so.

I know the regulator is built into the ECU. Could this be the problem? If so how do I test that?

Pelican

Since you mention a "relay" box I assume you have a '91+ XJ but you studiously avoided including the year in your profile. All years are not the same, not even close.

Have you checked voltages with a meter?

Roler

Verify the voltage on the battery engine not running
Then with your engine running, check the voltage again
It should be at least> 0.5V higher, preferable higher.
Turn headlights, radio, blinkers, AC etc on (except window defrost) and all the accessories and see if your alternator can keep up the voltage.

The above will give you a rough indication if your charging system is up to scrap.
Make sure your ground connections are in good, clean and tight condition.

maximusb1984

maximusb1984

I know the voltage won't show right. On the Jeep it tests bad, but if I take it off and test it alone it tests good. That is why I thought it was the cables. So I changed all the cables with copper or brass fittings.

My question is, because the voltage regulator is in the computer could my problem be the computer? If so can it be tested?

Pelican

Others will no doubt chime in but I will just say that the regulator circuitry in the pcm can fail but rarely does. Is this the only problem you are having with the XJ and you are saying that you have verified with a multimeter (as suggested by Roler) that voltage at the battery and/or alternator terminals is indeed ~9 volts?

BTW the "relay box" is known as the PDC. Doesn't matter what you call it but this just simplifies communication.

Someone else will have to tell you how to specifically test the PCM circuitry.

maximusb1984

Ok so I put The voltage meter on the Batt. showed good right a like 12.4 started and perfect 14.2. I let it run for a while with my radio and stuff on and checked the meter again, and it had dipped down to 12.4 while running. turned it off, started it back up and showing 10.6.

This would lead me to believe that the alternator is heating up and is losing voltage. Which is the cause of my problem. Am I right?

peep

Perhaps the 10.6 was the batter being drained a little from the accessory load and then the restart.
If you turn off all accessories while running, does the voltage come back up?
I know if running on my strained OEM stock alternator with all stock and exterior accessory lighting on and the Amped CD running loud, my voltage definitely drops off. If I cut back some lights or shut down the tunes, the volts recover. That's running a 1050cca with 120min reserve battery, this battery tests very good on load tester. My alt just can't keep up with a total power demand.
You mentioned your battery tested well? Are you running the OEM alternator?
There are a number of good threads on alt upgrades. If you don't max it all the time, then the OEM will do you well.
If running fresh wire circuits (I think AMC used the lightest possible gauge in mine), always go with a little heavier where you can.

maximusb1984

No the voltage never comes back. Yes I am running the OEM 90 amp alternator. I replaced the cable with twisted copper wire from Lowes. The cable I was told was the best by someone on this board. I even tried to disconnect my Amp just to make sure it wasn't pulling to much of a load. The voltage never came back up.

However, this morning when I checked it, it shows good. so I let it warm up and all voltage looks good. BTW I still have my Amp disconnected and no lights or anything else on. I turned it off and started it back up and the voltage has dipped again.

Sounds to me like the alternator is not producing correct voltage when hot.

Bustedback

I have done this dance before, replace the alternator and be done with it.

maximusb1984

Thank you

1arunem

Alternator..