Alternator(s) not charging

Last spring we went for a trip to the Badlands in IN, my father-in-law took his XJ into too deep of a water hole there & ever since the alternator has never charged the battery. Jeep never died in the hole, but we assume this was the culprit ( water over hood).

This is a 1991 4.0L HO XJ with an 87 5-speed & 4wd axles. Jeep ran fine till that day, now does not charge. We have replaced the alternator 3 times now (verified last two were good before instaling), and also replaced the wiring harness coming off of the alternator to the ends of those wires.

When read with a multimeter, the two small wires on the back read 1.4v & the large wire reads 10v (same as battery @ that time).

What tells the alternator to charge???
We are tired of stopping wheeling to swap out batteries all the time.

The black wire is ground
The dark green-org is power in with key in the run position (battery voltage)
The dark Green goes to the PCM and is the regulator (actually on the ground side of the circuit)
The black-white is power out and goes to a 60 amp fuse in the under hood fuse box (PDC), I believe it's fuse 8 (but am unsure).

I personally have never had the regulator in the PCM crap out on me, but others have. The normal remedy is to get a junkyard PCM. I'd be tempted to wire in an early Dodge external regulator, bypass the PCM and live with the CEL.

You should be able to retrieve the PCM stored codes, by using the key method and CEL, check for a code 41.

Thank you, I will relay this information.
No CEL displayed, just low volts till it dies.
Pulling a battery cable kills it instantly if that matters.

bump for no response!

What response to what question? Did you check the 60 amp fuse? Did you check the ground, did you check to see if you have power to the green\orange wire with the key on? BTW I would not pull the battery cable off when it is running on ANY engine with a computer.

Post up your results for further help.

First, pulling battery cables with an alternator and computerized car is one fast way to destroy expensive electronics. That old trick went away with generators.

Ok, have you checked things out like 8Mud posted above?

You need to have a fully charged battery, if the voltage is too low the alternator cannot be excited to charge.

So have you charged the battery, cleaned all cable ends and renewed all grounds?

According to 8MUD diagram, if you ground the dark green wire, you should get maximum alternator output. I wouldn't do it for very long or you could overheat the alternator. If you take this wire out of the PCM (maybe push the lead out of the plug at the PCM and ground it, you should get full power. If that works, the voltage regulator circuit in the PCM is dead. You would have to replace the PCM or engineer in an old school voltage regulator. Personally, I would go old school.

jfiscus said:

Thank you, I will relay this information.
No CEL displayed, just low volts till it dies.
Pulling a battery cable kills it instantly if that matters.

Click to expand...

I have had the same problem on other cars and found the battery cable connections were worn/old and had an intermittant or no connection. I would actually start by changing both the positive and negative battery cables first since pulling on them kills the engine immediately and they are cheap and if that didnt fix it I would check the wiring as follows:
I would continuity test the line from the alternator to the positive battery cable if it is good I would continuity test the ground wire (Black) from the alternator to the engine block if it is good I would continuity test the DK GRN/ORG (4.0) From the alternator to the PCM (contact # C12) and the DK GRN from the alternator to the PCM (contact #B10) if all that is OK I would look into a PCM. Good luck :shhh:

You say that you replaced the alternator 3 times. Did you ever have the alternator tested before you replaced it? You say you tested the new ones before putting them on, so I would think you would test the old one before putting on a new one. Is the vehicle blowing the alternators out?

We had the last two alternators tested at autozone (before and after installation) and both read fine, so they are not the problem.

The battery cables should be fine since pulling them kills the engine I would assume that they are both good since the jeep starts fine everytime & does not die unless you do pull one. I have pulled batteries out of running jeeps HUNDREDS of times with no problems - just keep the ends away from grounds, etc. This is how we wheeled it for the past year, pull a good battery from mine running & put in his low battery then swap again a couple hours later once it had a good charge.

Followed the green wire into the fuse bank & everything is fine there, but the other one we could not "definitively" locate on the PCM to check. What year PCMs could we swap into a 1991 XJ with a 4.0L/5-speed?

This may be the limits of my electronical knowledge after this point...

DO NOT DISCONNECT BATTERIES WHILE VEHICLE IS RUNNING! OK, assuming you don't believe this, you need to wire in an non-stock voltage regulator. PCMs cost too much to be replacing all the time. This is the most economical way to fix a non-functioning portion of the PCM charging circuit anyway. Search voltage regulators.

You might have gotten away with pulling the cables - but it's got a no charge problem that isn't getting fixed. There is a possible direct cause there.

Since the introduction of fuel injection and computers, there are now NEW rules. The Old School methods that worked with carbs and mechanical volt regulators no longer apply. 1) Don't pull the cable to check a running alternator. 2) Don't have the other car running when jumping. 3) Don't try to charge a low battery in the car.

Use a battery charger. Low voltage batteries don't supply 12 volts to the electronic regulator - so it never kicks in. If there is enough voltage, the alternator will overheat on an extended recharge because the 100+ amp rating is a max output, not continuous. Alt rebuilders say this is the #1 reason "bad" alternators come back within 24 hours of installation. Charge the battery on a charger.

Ignoring the correct procedure is a vicious cycle of shooting oneself in the foot. The recurring problem never seems solved, and the basic errors keep hiding the actual problem.

The next thing I would check is a good ground path from the alt to the block. Most new alts have instructions to check the volt drop on the ground side. Corrosion on the case, bolt holes and brackets will - will - cause a volt drop to ground. If there is a compromise on the grounding, the system won't see full voltage. Ergo, no charging, good alt or not.

What is the volt reading at the batt terminals running? It should be 13.5 or higher. Any lower, it won't recharge, especially at the 12.4 range, which is just battery voltage - and the system then doesn't have enough electrical pressure to force the chemical reaction of recharging.

All the above is why the primary tool for modern mechanics is a Volt-Ohm meter, not a socket set. Really good digital meters are in parts houses for $20. I just bought another yesterday, it's a lot more fun than the older expensive Radio Shack I used to use, and a heck of a lot more accurate than an old needle meter for $5. It even checks diodes, and gives two digit read out for adjusting the TPS.

I got my Auto Tech degree in the days of drum brakes and carbs, primitive wiring, and muscle cars. I learned about the rest owning late models and having the same issues.

Volt ohm meter. The guys who won't buy one are the same ones who need it the worst.

I can't afford a voltmeter. I have to spend all my money on a battery ($100), alternator ($100) and PCM ($300). I don't have $20 left over. LOL

Parts throwers are all the same, they ask for your advice because they are tired of throwing the same parts at it, and then they won't take an hour to do some simple diagnostic work. Then they come back asking what part to thow next. Lost cause IMO.

88 Wagonman said:

Parts throwers are all the same, they ask for your advice because they are tired of throwing the same parts at it, and then they won't take an hour to do some simple diagnostic work. Then they come back asking what part to thow next. Lost cause IMO.

Click to expand...

Wow, forums have changed since the last time I was here...

such arrogance.

anyway.. bypass the pcm and put in a voltage regulator. or replace the pcm.

this is why i like my 84 with the v6 :looney:

Funny thing, I toasted an Alternator at Badlands about a year and a half ago or so. Replaced it in the CVS parking lot in town at midnight. Then went home an hour and a half!