My 92 Jeep 4.0 was throwing a code that the alt was bad so i replaced it. The battery was weak due to the previous alternator not charging it. Battery is also 6 yrs old.

Anyways, the jeep fires right up after i changed the alt and the CEL went away. However, after driving it for about 5 minutes i lost all electric: lights, radio, turn signals.

Why would that happen? Bad battery? Why wouldn't the new alternator charge the battery? The vehicle is not throwing any codes so i assume the alt is good.

Fuse?

sounds like a bad ignition switch or battery cable is shorting to something

All the fuses look good and i can't find anywhere that the battery cable may be shorting.

I guess it could be a bad ignition switch but that would seem like a big coincidence. Haven't had this issue until I had the alternator issue.

You can always have battery tested for free. Odds of another part failing 5 minutes after you just replaced another part are slim. Check your work, make sure connections are clean/solid and **** like that

Yeah battery is on the charger right now. If it takes a charge I'll see if i still have the same issue.

The battery gauge is kinda jumping around when i turnt he key to "on". Although that could be bc of the charger being on it.

If the ground wire to the alternator is not connected well, it can cause all kinds of weird things.

CactusThomas said:

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If the ground wire to the alternator is not connected well, it can cause all kinds of weird things.

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Alternator bracket is typically made of metal and bolted to the engine, uses the same ground path as the spark plugs. Never seen an alternator with a dedicated ground wire from the factory.

I replaced the battery and checked all of my connections, etc. still having the same issue. The battery is draining after a few minutes of driving.

Any ideas on how to troubleshoot this? I imagine there is something wrong with the wiring somewhere.

Another funny thing is that the electric fans run from the moment the jeep starts. Typically they do t come on till it gets closer to temp. I checked all the wiring for the fans and it looks fine.

Put DVOM on battery and test to see what voltage is when running. should be over 12.6. Ideally 13.5-14.3

Pardon my ignorance, but what is dvom?

And the battery voltage gauge never got over 11, even with the brand new battery.

Digital Volt/Ohm meter.

You need to remove the alternator and go to oreilly and have it tested.

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Anyways, the jeep fires right up after i changed the alt and the CEL went away.

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With all due respect, the battery starts the car. The alternator has nothing to do with it. If I misunderstood your comment, let me know.

While driving, I've seen a really weak battery, eg dropped a cell, cause electrical issues. But a normal battery won't cause electrical issues while driving since the majority of the load is handled by the alternator.

At 1500 rpms at idle, I'd expect 13.5+ volts from the alternator if the battery is good.

Your statement about your fans running constantly is odd. Those are on relays. The only way the switch is closed is for current to be flowing through the coil side. Is your thermostat sending unit defective?

Yeah the fan issue is odd. It is running at half speed all the time. When the jeep reaches temp, it kicks on to full speed like its supposed to. Not sure why its constantly running, though. Maybe I need to adjust the controller.

I am still confused by the alternator though. Its brand new, and I checked all the connections and they look good. Voltmeter gauge never gets above 11, and the electronics drain the battery within a few minutes. So the alternator is not charging properly.

Maybe I got a bad alternator. I will take it in for a test, I guess. Seems unlikely, but there aren't too many things it could be are there?

Gigemags05 said:

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Yeah the fan issue is odd. It is running at half speed all the time. When the jeep reaches temp, it kicks on to full speed like its supposed to. Not sure why its constantly running, though. Maybe I need to adjust the controller.

I am still confused by the alternator though. Its brand new, and I checked all the connections and they look good. Voltmeter gauge never gets above 11, and the electronics drain the battery within a few minutes. So the alternator is not charging properly.

Maybe I got a bad alternator. I will take it in for a test, I guess. Seems unlikely, but there aren't too many things it could be are there?

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I dont think the 92 Jeep PCMs were advanced enough to control the charging system, so I think its a good idea you have the alternator bench tested. A lot of these rebuilders just fix the broken part and "spray and pray" i.e. a new coat of paint and toss it in the box. A "re-manufactured" unit is supposed to be completely torn down and redone, saving only the case.

So My suggestion, take the battery and alternator up to Oreilly, have them test it for you.

Report back with findings.

91 and up Jeep PCM controlled the field circuit

robertf03 said:

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91 and up Jeep PCM controlled the field circuit

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Maybe the PCM is bad then.

A little more info.

This 92 4.0 is in a 1980 CJ7. The wiring harness was all new in 2013. The electric fans are controlled by a painless f5 fan control, also new in 2013.

The jeep is throwing code 41 which is " Alternator field control circuit open or shorted.

Is there any way to test if the PCM is bad?

Ah. Ignore that, you're good

Almost every mpefi jeep conversion throws that code since chassis wiring isnt modified to have the PCM control the alt

Ive even heard of hesco/mopar kits doing this

Gigemags05 said:

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Is there any way to test if the PCM is bad?

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Please do what I suggested first before wasting a bunch more time.

I have the Hesco harness.

Are you saying ignore the code? Or ignore the possibility that it could be the PCM? I am under the impression that the PCM does control the alternator. The wire from the PCM to the alternator is connected, and this code is in line with the symptoms I am having.

1ags, I will for sure get the alt and battery tested before doing anything else.

Gigemags05 said:

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I have the Hesco harness.

Are you saying ignore the code? Or ignore the possibility that it could be the PCM? I am under the impression that the PCM does control the alternator. The wire from the PCM to the alternator is connected, and this code is in line with the symptoms I am having.

1ags, I will for sure get the alt and battery tested before doing anything else.

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If you are sure the alternator is hooked up to the PCM then don't ignore it. I mis read what you posted earlier and assumed a new chassis harness and that a 92 jeep EFI harness was used.

Probably a haul for you, but Richardson Generator on Greenville can test and rebuild the alt. I think the guy is an Ag, or maybe his kid is. Either way, beats the mouth breather Oreilly crowd selling junk rebuilds. Maybe you can find a decent rebuild shop up there.

You know, I swapped this 4.0 into the jeep bc the guy I bought the vehicle from gave it to me. I rebuilt it and installed it. After I drove it around and gave it more thought, I asked myself why I would swap an inline six for a slightly newer inline six. Only real advantage is the fuel injection and a little power. I should've gone with an LS. Although at the time I had no money so it wasn't really an option.

I found some other tests to run to try and pinpoint the issue. As Robert mentioned earlier, there is a possibility that it is an ignition switch. If the alt tests good I'll run these tests.

I am about to take the alternator to Auto zone to have them test it. But in the mean time, maybe y'all can help me figure out how to complete the tests linked below.


I performed steps 1 and 2 of the first test and got over 12 volts for both.


I am a little unclear how to test step three of the first test or how to do the 2nd test.

Not sure how to do this: "check for resistance in the field driver circuit generator harness terminal and cavity # XXXXX on the PCM connector. " (picture of PCM connector is on the link).

or this:
"1. With the ignition off, disconnect the PCM connector and disconnect the harness from the alternator. Measure the resistance across the alternator field terminals. If it is less than 5.0 ohms then go to the the next step. If the resistance is 5.0 ohms or greater then replace the alternator. (which you already did)

2. Check the resistance in the field circuit of the generator harness. If the resistance is less than 5.0 ohms then repair the short to ground in the driver circuit.(Dark green wire)If the resistance is 5.0 ohms or more then replace the PCM."

I understand to turn the multimeter to OHMS and to test resistance, I just don't understand where it is telling me to put the probes.


*EDIT, Forgot to add the link:

https://www.justanswer.com/car/10jco-41-trouble-code-1994-jeep-cherokee-replaced.html

Do not take it to auto zone. They don't have the smart alternator tester like oreilly has. Plus their people don't know wtf they're doing.

Had a texager bring me their 08 TAhoe last month with a random dying issue. They had the alt tested good at Autozone. I got it, charged the battery, started the truck and got 12.5 volts. Rev it up to 2500 rpm and 11.3 volts. New alternator and it was good again.

ok I will take it to Oreilly. But my local Oreilly is usually the one with the idiots who don't have a clue what they're doing. The Auto Zone here is far better as far as the employees go. But if they don't have the test equipment I'll chance the Oreilly. I also bought the alt at Auto zone. It has a lifetime warranty so if it tests bad at Oreilly I will take it back to AZ and swap it out. Then I will take my brand new one back to Oreilly and have them test it again.

Alternator and battery both tested good at oreilly.

Now what should i look for next? I've changed all the connections and checked the wiring for any cuts, etc.

Gigemags05 said:

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Alternator and battery both tested good at oreilly.

Now what should i look for next? I've changed all the connections and checked the wiring for any cuts, etc.

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Then the PCM isnt commanding the alternator to come on...hmmm

ETA, the Hesco website says that the harness has an alternator field wire....is that hooked up?

Yes, that wire is hooked up. The harness is pretty straight forward, all the wires are labelled. It ran perfectly for over 5 years and just started acting up the other day. I don't drive it a whole lot. Probably only have about 5,000 miles on it since the swap. But I have made a few long trips in it.

So are we thinking its a bad PCM? A new one is $175, which isn't awful, but I would like to confirm that's the problem before I buy one.

Hang on, I have an article about Chrysler alternator troubleshooting on my old laptop.

Whats your email address?

Reman alternators don't always work correctly. I had one in my Mustang that decided to start spitting out 17 volts after two weeks. My heated seats got real hot and then never worked again.

I got the email. Thanks!

I'll read through it and let you know how it's going