bac0nfat

Hi All,

I have a 1996 Jeep cherokee XJ and I've had persistent problems with the electrical system. I'm on my 4th alternator in 2.5 years.

The alternator it came with stopped working and I replaced it with a remanufactured one. That fixed the problem for about 6 months.

Then that alternator quit working. I was able to replace it under warranty with another rebuilt one. This one lasted about a year.

Frustrated, I decided to take my jeep to an alternator shop instead of buying cheap rebuilt ones from the parts store. I had them remove and test the alternator, and replace it with a newly manufactured (not rebuilt) one. They told me that the one they removed must have been rebuilt wrong, because a wire was physically interfering with the movement of one of the brushes. Hence the premature failure.

Thinking my problems were finally resolved, the jeep stopped charging itself about 11 months later. I assumed it was the alternator again, so I took it back to the alternator shop. They tested the alternator and apparently it works fine. Then they "bypassed the internal voltage regulator" and the jeep started charging again. Then they put everything back to normal, and the problem was gone!

So now I have a jeep that's working, but I can't trust it. Why did it stop working, then suddenly start working again? Does it have an internal voltage regular that's intermittent, and none of these alternators were actually bad? Should I install an external regulator kit? Replace the ECM?

I'm at a loss here, I need this to be a reliable vehicle but I don't know what the problem is. Is there a way to install a switch so that I can bypass the internal regulator next time it acts up? I just don't want to become stranded somewhere.

jordan96xj

It is rare for the PCMs voltage regulator to fail. Though not impossible. It is much more common that the overall condition of your starting and charging system cables and connections needs to be thoroughly checked and cleaned up. Replacing any suspect cabling where necessary. It is also more possible that the battery itself is implicated at this point. Has that been checked/replaced?

Also when wires are disconnected, and then reconnected, and the problem goes away, that is a major clue that it is connection/wire related, and not a failed component such as the PCM.

Have you purchased a $15 digital multimeter so that you can test system voltage for yourself yet? Make that your next step.

bac0nfat

The battery was replaced about 2 months before the last issue. The existing battery was about 3 years old and wasn't holding a charge. Maybe that's related, I'm not quite sure.

I do have a digital multimeter. Anything you suggest that I check?

bac0nfat

Also, I find it interesting that you say it's rare for the PCM voltage regulator to fail. The guy from the alternator shop says that it happens with Chryslers of that era all the time. And there are external regulator kits all over ebay.

The next time it happens, what can I do to test it?

jordan96xj

Don't wait for the next time it happens. There are tests you can do now to prepare.

First test the system voltage across the battery terminals with your meter while the vehicle is running. It should be somewhere between 13.5-14.5 volts. I have a 96 and it runs at about 13.86 volts on mine. (but that can vary a little between vehicles).

Then test the voltage output of the alternator directly by placing the red probe of the meter directly on the large stud on the back of the alternator (with engine running) and the black probe on the battery negative terminal.

See if there is any significant difference between those two readings.

Also test with red probe on the same alternator stud, but the black probe scraped into the casing (the outer shell) of the alternator. Note any significant difference.

Report back what you find.

Be careful when reading the alternator with the motor running. Just be mindful of where your fingers, wires, and probes are.

jordan96xj

The long story short is that the PCM voltage regulator is a relatively simple system that is not delicate and does not have moving parts. So it is not common for it to fail. What is common is for people to see batteries not charging and to tell you that the voltage regulator must be bad (without doing ANY diagnostic work at all to prove it). That is super common. You be the judge.

It is more common that the computer isn't seeing voltage properly (due to connections, bad wires, and corrosion) and is regulating improperly because of it. Or that something else is going on, and a voltage issue is just a symptom of the problem, not the root cause.

bac0nfat

Gotcha. I'll take some measurements and report back. Thanks

bac0nfat

Okay so with the engine running I have 14.02V between the battery terminals, and 14.05V from the negative battery terminal to the connection on the back of the alternator.

bac0nfat

Anyone have a wiring diagram or schematic for the electrical system of a 1996 XJ?

jordan96xj

So with things as they are, if your battery is not charging. It is the battery. Because the system voltage is good, and there is no significant difference between system voltage at the battery, and what the alternator is producing.

What I would do is continue to monitor that system voltage for a while, and see if it is unreliable. But if you get a dead battery at 14v. Look at the battery.

jordan96xj

An accurate wiring reference for the 1996 XJ is elusive. Like unicorns, some claim to have encountered or own one, but they are rarely seen. If you get a 95/96/97 FSM from pacific coast manuals, unfortunately, none will accurately describe the 1996. Because their 1996 manual is actually just a 1997 manual (don't know why they think that is ok).

Wiring between the 95 and 96 is similar (sometimes), but not the same. Wiring between the 96 and 97 is quite different.

I finally buckled down and bought a hard copy from a automotive literature company. It wasn't cheap. But I use the FSM about every week, so I suppose for me it was a good investment. Plus I'm starting to get into the electrical side of things more now, and without good reference material, that gets a lot more difficult.

I purchased the FSM and FSM supplement from here:
http://www.faxonautoliterature.com/

bac0nfat

Maybe I didn't explain the issue well.

The last time I experienced the issue was last week. My wife was driving the Jeep, and she noticed that the voltage gauge was reading lower than usual when she started the vehicle. She was an hour from home, so she attempted to drive the Jeep home. As she drove, the voltage dropped lower and lower, to the point that the speedometer stopped working and the radio shut off.

We took it to the shop, and the technician confirmed that the alternator was indeed not charging the battery. He bypassed the voltage regulator in the PCM (presumably by installing an external regulator) and immediately the alternator started charging the battery.

Then he put it back to the way it way, but the problem didn't come back. His final verdict was that the internal regulator in the PCM was intermittent.

jordan96xj

You explained it well, and I understand. However, even if the regulator was bypassed, which on a 96 can be achieved by simply attaching one of the smaller studs to a ground, it doesn't show that the regulator in the PCM is the culprit. It could simply mean that the ground being manipulated by the PCM that ultimately controls the amount of voltage produced by the alternator is questionable. It is much more likely that a ground connection is dirty, loose, or corroded. Because those things are inevitable if they are not maintained. Could also be the PCM/regulator, but that would be less likely.

Also, I thought about your problem on my drive home, and for a 96, a system voltage as high as yours at idle, makes me think it is actively charging the battery. Usually, once the battery is topped off, the voltage reduces some what (at least according to the FSM it does). I'm curious if yours ever gets to that point, or if it is constantly thinking the battery needs more.

Keep in mind, the alternator provides the voltage, but it is delivered to the battery first, and -then- it is provided from the battery to the PDC via the fuseable link (the little plastic box covers the fuseable link on the front of the under hood fuse box, or Power Distribution Center).

So the PCM has not been ruled out. But it certainly has not been definitively identified as the problem. I wouldn't go swapping a PCM unless I knew for sure it was the culprit, or unless I had a known-good spare sitting on the shelf.

For example, on a 96 XJ with a battery temperature sensor (if yours has one, it will be under the battery). That sensor is also involved in the regulated voltage level. It will be a little higher when the battery is cooler, and the voltage will be reduced when the battery is getting warmer. Was that ruled in/out? Did the technician even know that it is part of the regulator system? If so, did he test it?

For example, sensor says its hot (whether it is or not doesn't matter), so voltage is dropped to a point where the battery doesn't charge. Then the tech bypasses the regulated voltage and provides full voltage to the battery...then it charges....then after 10-15 minutes he is hooking the alternator back up normally, sensor is cooler (or thinks it is), voltage is increased by the regulator, and the battery is charging again. Why couldn't the evidence just as likely implicate the battery temperature sensor?

Usually temperature sensors are thermistors, which means they resist voltage by varying amounts based on their temperature (no moving parts). Then the computer puts voltage through it and measures the voltage on the circuit, and by that can deduce a temperature (certain voltage = certain temperature) So a bad or corroded connection on one can cause resistance which causes the computer to make faulty decisions in voltage regulation.

Remove the battery, see if you have one under it, and see what its condition is like and the condition of any connections it has.

ALSO:

Are there any other components that could be implicated like the heater/AC blower fan? When your wife detected the problem with voltage on the internal gauge, was the fan on full blast? The AC system can put a heavy load on the available voltage (between the ac clutch, the aux cooling fan, and the blower motor) especially if any of those items are worn out and pulling more amperage then they should. On a fresh/good fully charged battery, you might not notice it. But on a marginal battery, the voltage drop can be more immediate across the whole system.

bac0nfat

Ahh, interesting. Perhaps this could be used in an emergency situation to get the vehicle back home?


All of the connections that I've inspected have looked good, but I will check again. Perhaps there were some that I missed.

Agreed, I'm more inclined to fully understand the issue than take the shotgun approach. If I understand the issue, I can be more certain that it's actually fixed, and better educated to fix future issues with this vehicle or others.


I did not know there was a temperature sensor. I will check that out.


It's possible, but I'm pretty sure all of that stuff is in good working condition. When I had the issue with the battery a few months ago, I thoroughly checked the current draw of all the accessories and didn't find any issues.

Anyway, thanks for all of the advice, you've been extremely helpful. I will do some more diagnostics and report back.

jordan96xj

Sorry I was so long winded. Definitely not trying to be difficult. I am troubleshooting a similar but unrelated problem on my 96, and was trying to provide as much info as I could.

Good luck!