Bad PCM? Alternator isn't charging

f22beaver

f22beaver

TJ Enthusiast
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Three days ago alternator voltage output was same as V between + and - terminals on battery. With car off, I have the same voltage from negative terminal to alternator post. It's an original alternator, so I change it, start it back up.....and no change. Still not putting out voltage above battery voltage. Voltage does not change with engine load.

So I check PCM to alternator pin continuity. I have continuity from field to PCM and source to PCM. While there I also verify I have continuity from both PCM grounds to the passenger side engine block grounds.

I've been reading a lot of threads on this, I think I have a bad PCM by virtue of a bad voltage regulator? I want to make sure I very clearly understand it, but I think if I jumper 12V onto my generator field pin and it starts charging then that's confirmation of a bad PCM. Or do I jumper onto source? Does that make sense, am I getting that right?

Any help is appreciated. Thanks.
 
I read more of the FSM. Source is the 12V, thus why it's called source. :rolleyes:

But anyway. Engine on, I have 12V at that generator source. So if grounding the field wire kicks the generator voltage up, that's a bad PCM, right? Or I was shipped a bad alternator?

Holy hell am I learning a lot. How can I test if the PCM is grounding the field? If it's grounding, but voltage doesn't go up, then bad alternator? But if it's never telling it to ground then what?
 
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Jerry Bransford said:
Exactly what voltage are you measuring on the battery while the engine is running, and then exactly what is the voltage without the engine running?
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Off - 12.34V
Running - 11.78V

Alternator output post running - 11.71V

Also, I went ahead and grabbed my field alternator voltage at the alternator terminal. It was jumping around, but it was basically hovering around .4V. The oscillations were roughly from .15V up to .45V but the number I saw the most was a .4V.

I'll add, it's a 3-4 month old battery and the battery/charging system has had no indication something would be off. I didn't do anything, touch anything, screw with anything. Obviously that doesn't mean something can't happen, but I'm just surprised.
 
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Something is definitely up. Charging voltage should be at least 13 volts at a minimum, ideally closer to 14-ish depending upon battery type.

The first test I would do is to short the field terminal to the nearest ground. Put a multimeter on the battery or alternator, somewhere you can set it up hands-free. Unplug the field connector terminal on the alternator. Find a short jumper wire, and short it to ground. If your alternator is functioning, it will produce 100% power when the field terminal is grounded, and you will see a large voltage spike. (Do not disconnect the alternator positive or ground from the battery - it needs to dump the current somewhere.)

Do not hold this for any significant period of time, especially if the voltage at the battery spikes above 14.6V. A brief excursion in voltage is unlikely to harm anything, but an extended one will damage the battery and potentially other electronics.

If that test passes, your alternator is probably fine; it’s just not receiving signal from the PCM to charge. You can either replace the PCM (expensive, but best fix) or install an external voltage regulator (much cheaper and adjustable, but not quite as accurate).

I’ve used the external regulators in the past with no issues on the TJ. Since a regulator is maybe $100 and a PCM $300+, I would try a regulator first unless you also have any other PCM issues.
 
Measure resistance between the ground terminal at the alternator, and battery ground. I’d say the ground is bad. Most of these Jeep’s are grounded on the side of the block. If resistance is high, run a new ground.
 
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Albertabackroad said:
Measure resistance between the ground terminal at the alternator, and battery ground. I’d say the ground is bad. Most of these Jeep’s are grounded on the side of the block. If resistance is high, run a new ground.
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This is indeed another possibility. The ground between the engine block and the battery can indeed go bad.

Disassembling both ends and cleaning both contact points, both bolts/studs, and the respective mounting surfaces with emery cloth or steel wool can often solve a wide variety of otherwise confusing electrical issues. In addition, similarly cleaning the positive terminals at the battery and alternator can also solve some issues.
 
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Steel City 06 said:
This is indeed another possibility. The ground between the engine block and the battery can indeed go bad.

Disassembling both ends and cleaning both contact points, both bolts/studs, and the respective mounting surfaces with emery cloth or steel wool can often solve a wide variety of otherwise confusing electrical issues. In addition, similarly cleaning the positive terminals at the battery and alternator can also solve some issues.
Click to expand...
I apologize, I should have specified- I was referring to measuring pin 2 on the alternator (ground circuit) and battery ground for resistance. The alternator pin 2 is chassis grounded to the side of the engine block, and that wire has a tendency to break. So if it measures high, ground the wire at pin 2 and see if voltage increases. If so, run a new ground and bobs your uncle. I’ll attach the wiring diagram. PCMs get misdiagnosed as a result of over thinking very frequently.

5A57E746-090D-4E10-9AE9-E477EA949170.jpeg


A8A7C48D-ABE4-4172-9598-84EE79D268F0.jpeg
 
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Albertabackroad said:
I apologize, I should have specified- I was referring to measuring pin 2 on the alternator (ground circuit) and battery ground for resistance. The alternator pin 2 is chassis grounded to the side of the engine block, and that wire has a tendency to break. So if it measures high, ground the wire at pin 2 and see if voltage increases. If so, run a new ground and bobs your uncle. I’ll attach the wiring diagram. PCMs get misdiagnosed as a result of over thinking very frequently.
Click to expand...
Ok. I want to make sure I understand the assignment and I'm not confusing pin vs terminal. While you were adding that other update, here's what I did.

When you say terminal I think you're saying the physical plug (female) that has the wires attached to it. And pin is the male part on the alternator. Yeah?

So, the generator plug (the female part). Both of those wires back to their PCM pin have 0 Ohms, so they're good. And both of my PCM grounds have 0 Ohms so the PCM grounds are fine.

Now, I also went from generator field (location 2) back to the engine ground and THAT guy is something like 800 Ohms. So that's bad, IF that's the measurement you were asking me for. But were you asking me for the female plug or were you asking me to do the male pin that's physically in/on the generator? On my 2004 FSM it says that location 2 goes to PCM pin 10. But your picture shows terminal location 2 going to engine block ground. So where does location 2 actually terminate?

And yes, I still need to go jumper pin 2 to ground while running to see if voltage kicks up on the alternator.

Adding the pic below, this is why I'm confused about how pin 2 goes to ground, it looks like it goes to the PCM?
1638801209028.png
 
Albertabackroad said:
I apologize, I should have specified- I was referring to measuring pin 2 on the alternator (ground circuit) and battery ground for resistance. The alternator pin 2 is chassis grounded to the side of the engine block, and that wire has a tendency to break. So if it measures high, ground the wire at pin 2 and see if voltage increases. If so, run a new ground and bobs your uncle. I’ll attach the wiring diagram. PCMs get misdiagnosed as a result of over thinking very frequently.

View attachment 294778

View attachment 294779
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That makes a lot of sense. Good idea - very few would think to check that.
 
I'm so confused. Is this picture telling me generator ground is just through the physical case as it's mounted to the engine? Pins 1 and 2 don't directly go to ground according to this?
1638802838225.png
 
@Albertabackroad so long as you meant female plug 2 to battery ground then the resistance on that is 870 Ohms. It's also 870 to the engine block ground. Sounds like I need a new ground from the engine block to the plug. You know how to open up that type of plug so I can pull out the existing green wire?

I got the wire pulled. I'm still looking to understand the difference in diagrams. Mine says the field goes to the PCM, @Albertabackroad diagram says it goes to ground.
 
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Steel City 06 said:
Do not hold this for any significant period of time, especially if the voltage at the battery spikes above 14.6V. A brief excursion in voltage is unlikely to harm anything, but an extended one will damage the battery and potentially other electronics.
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@Steel City 06 now I understand your point about excursion. By grounding out to the engine block I'm making the alternator think it needs a 100% duty cycle all the time, which is why it could run away on voltage. So my green wire does go to the PCM, and if this works then that means that the PCM isn't correctly giving that duty cycle to the alternator? And thus either an external regulator or a bad PCM is your recommendation.
 
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f22beaver said:
@Steel City 06 now I understand your point about excursion. By grounding out to the engine block I'm making the alternator think it needs a 100% duty cycle all the time, which is why it could run away on voltage. So my green wire does go to the PCM, and if this works then that means that the PCM isn't correctly giving that duty cycle to the alternator? And thus either an external regulator or a bad PCM is your recommendation.
Click to expand...
Talk with Mark @Wranglerfix at www.wranglerfix.com who can probably repair the charging circuit in your PCM.
 
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Well. Shit. Here's my setup. I stuck this wire from engine ground into pin 2 on the alternator. I had the green wire out, then with my DMM on the output post I watched the volts. When I stuck the green wire into pin 2 it did....nothing. Voltage didn't move. And actually, not nothing. When I put in the wire the fan blew considerably less air, then when I took it out it picked up again. Plugging in ground lowered engine idle but voltage output didn't move, what? Did I get shipped a bad alternator?

@Steel City 06 is this how you intended me to run this test?

BTW, I do still have voltage on the dark green field wire from the PCM. Ranges from .3-.4V.

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Well. F it. I don't have P0622 but I followed the FSM diagnostics for the field not switching properly. Each yes or no lead me to 6, which is a bad PCM. I didn't have a test light nor a DRBIII but I'm assuming "actuate the field driver" is the same mechanism as the PCM telling the alternator to charge the battery, but I've stuck a DMM on the field wire and only got .3-.4V so a 12V test light would NOT have lit.

Any other ideas here guys? I still have really high resistance from my field wire to ground, but my field wire goes through the PCM so I'm not sure that's a valid resistance test. If my VR is bad then I suppose it stands to reason a high resistance will show.

Absent some fresh ideas I'm probably shipping this things down to Mark to take a look at it.

FWIW, my resistance from the battery and the PCM (black pins 32 and 33) all the way to the engine block ground plus both firewall grounds plus the hood ground are all 0 Ohms.

Any other takers?? It's pissing me off that grounding out the field didn't do jack to my generator output.

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