Overcharging issue, several OBDI codes

Volatile · Oct 1, 2018

I apologize in advance for the long story.

I started my 91 the other night, and saw the CEL for the first time since I’ve owned it. Did the key cycle and came back with the following codes that I pulled from fixjeep:

12*: Battery or computer recently disconnected within the last 50 start cycles).

41**: Alternator field control circuit open or shorted.

46**: Battery voltage too high during charging.

47**: Battery voltage sense input below target charging during engine operation. Also, no significant change detected in battery voltage during active test of generator output circuit.

I didn’t notice it running strangely at first, until I gave it some gas and the volt meter spiked to 19. When it came back down to idle, voltage stabilized, but if I touched the throttle it would spike. I verified this reading with a multimeter on the battery. Not seeing any glaring wiring issues, I pulled the alternator, even though it’s barely a year old, and had it and the battery tested at advance auto. Both came back good. The battery is also barely a year old. Cleaned all connections at the alternator and battery terminals, bolted it back in, and now my voltage spikes to 19 the second I start it. I’m kind of at a loss here, is there anything I need to check next that I’m not thinking of? Is it possible the alternator could still be the culprit? Bad cell in the battery? I just want to have a game plan before I start throwing money at it.

Thanks.

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Volatile · Oct 1, 2018

I decided to try some more investigating:

Found a crack in my negative battery terminal, replaced it.

Took off negative battery chassis ground, cleaned.

Took off PCM chassis grounds, cleaned.

I even warrantied out my battery.

Still no dice, exact same behavior. Voltage shoots up to 17-19 at idle when I start it. Verified again by meter. Anything else I need to check before I go investing in a PCM?

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trippled · Oct 3, 2018

Those bench alternator testers are notoriously faulty. I would suspect the alternator failing first before the PCM. What brand was it?

Volatile · Oct 3, 2018

trippled said:
Those bench alternator testers are notoriously faulty. I would suspect the alternator failing first before the PCM. What brand was it?

It was a driveworks, but I’ve done a significant amount more in trying to track down the issue.

I decided to replace the alternator regardless of the bench test with a brand new, not remanufactured Carquest unit with a lifetime warranty. The driveworks unit was out of its 1 year warranty by 2 months. Replacing it didn’t change anything.

I pulled off both engine grounds from the passenger side of the block. The dipstick tube ground was pretty gross, but cleaning all of these to spotless, brand new looking condition didn’t change anything. The only benefit here is that now with all my grounds spotless and the new battery, it fires up faster than it ever has since I’ve owned it.

I pulled the wiring off of the PDC next to the battery and cleaned those terminals and mating surfaces. No change.

I replaced my positive battery terminal because why not at this point? Nothing.

Any ideas? I’m about roll the dice and just buy a PCM from RockAuto. I know all about external regulators, but I’d rather have as close to a factory fix as possible.

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techno1154 · Oct 3, 2018

I think the 1991 XJ have the Denso alternator which is controlled by the PCM. The PCM toggles the ground wire (one of the two small wires on the back of the alternator) on and off to keep the charging voltage within specs. If the PCM is bad or the wire is shorted to ground the alternator will charge to its maximum which is somewhere in the neighborhood of 19 volts +/- . I test the alternator on my XJ for efficiency by temporarily shorting the ground wire to ground and allowing the voltage to climb to 15 + volts. 19 volts could cause serious damage to the electronics in the XJ.

ON the 1996 XJ that I have, there are 2 small wires coupled together,.. Dark Green 18AWG and Dark Green and Orange 16AWG. The dark green + orange come from a relay in the PDM and provides a 12 volt Positive when the ignition is on. That is the ASD (Automatic Shutdown Relay) which I would say is good since the alternator works.

The Dark Green come from the PCM and is toggled on and off depending on the voltage the PCM is reading. What you could do is disconnect that wire from the alternator and the harness on the PCM and check it with an OHM meter for short to ground. I do not have a manual for your year XJ so I do not know the pinouts on the PCM.
Another option would be to remove that wire from the alternator and replace it with a jumper that you touch to a good engine ground while checking the voltage at the battery. One way of doing this is to remove both wires them use a short piece of wire to provide the positive voltage along with the piece that is to be grounded. 1,500 RPM, a good alternator along with my tips should be enough to show you what is going on. Your year wire colors may differ from the 1996 model so double check which is positive by turning on the ignition and testing to see which wire is + 12 volts.

Volatile · Oct 3, 2018

techno1154 said:
I think the 1991 XJ have the Denso alternator which is controlled by the PCM. The PCM toggles the ground wire (one of the two small wires on the back of the alternator) on and off to keep the charging voltage within specs. If the PCM is bad or the wire is shorted to ground the alternator will charge to its maximum which is somewhere in the neighborhood of 19 volts +/- . I test the alternator on my XJ for efficiency by temporarily shorting the ground wire to ground and allowing the voltage to climb to 15 + volts. 19 volts could cause serious damage to the electronics in the XJ.

ON the 1996 XJ that I have, there are 2 small wires coupled together,.. Dark Green 18AWG and Dark Green and Orange 16AWG. The dark green + orange come from a relay in the PDM and provides a 12 volt Positive when the ignition is on. That is the ASD (Automatic Shutdown Relay) which I would say is good since the alternator works.

The Dark Green come from the PCM and is toggled on and off depending on the voltage the PCM is reading. What you could do is disconnect that wire from the alternator and the harness on the PCM and check it with an OHM meter for short to ground. I do not have a manual for your year XJ so I do not know the pinouts on the PCM.
Another option would be to remove that wire from the alternator and replace it with a jumper that you touch to a good engine ground while checking the voltage at the battery. One way of doing this is to remove both wires them use a short piece of wire to provide the positive voltage along with the piece that is to be grounded. 1,500 RPM, a good alternator along with my tips should be enough to show you what is going on. Your year wire colors may differ from the 1996 model so double check which is positive by turning on the ignition and testing to see which wire is + 12 volts.

Thanks for the detailed response, I appreciate it.

I crawled back under again, and found the two wires you're referring to. There is a green wire, and also a green/red wire feeding into a black box that bolts to the back two prongs on the alternator. I looked at the wiring diagram in my Haynes manual and the solid green definitely leads back to the PCM.

How would I pull these wires off and test them individually if they're inside this box?

techno1154 · Oct 3, 2018

Volatile said:
Thanks for the detailed response, I appreciate it.

I crawled back under again, and found the two wires you're referring to. There is a green wire, and also a green/red wire feeding into a black box that bolts to the back two prongs on the alternator. I looked at the wiring diagram in my Haynes manual and the solid green definitely leads back to the PCM.

How would I pull these wires off and test them individually if they're inside this box?

Each wire have a small nut 6mm I think holding it to the alternator. You have to remove both nuts before the connector (black box) could be removed. The wires are molded into that connector.

Volatile · Oct 5, 2018

Well it ended up being the PCM.

I got a deal on a used one I couldn’t pass up that matched my numbers and hooked it up. She runs great again.

Thanks everyone for the help!

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techno1154 · Oct 6, 2018

Volatile said:
Well it ended up being the PCM.

I got a deal on a used one I couldn’t pass up that matched my numbers and hooked it up. She runs great again.

Thanks everyone for the help!

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:cheers: Happy you got it resolved.

Overcharging issue, several OBDI codes

Volatile

NAXJA Forum User

Volatile

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trippled

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Volatile

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techno1154

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Volatile

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techno1154

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Volatile

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techno1154

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