Alternator overcharging problem.

I'm working on a 93 XJ 4.0L. Not mine though. Its have an overcharging issue. Battery is good. I had cleaned the grounds and it seemed like it was fixed. However, today I was charging the AC system and it started doing it again. It is throwing codes 41 and 46. 41 is for alternator field issue, and 46 is for overcharging. It was charging around 16 volts. I know the PCM controls the charging, and does so by PWM on one the negative side of teh connection on the alternator. The other is for a positive from the ASD relay. The third connection is of course for the positive charge side.

I have performed the following diagnostics:
a)With the plug disconnected the alternator does not charge, as expected.
b)With only the + side of the field energize (ASD relay) connected it is charging 16+
c)With both connected it behaves the same way as in b.

From the way I understand the system is that both of the field wires need to be connected in order for it to charge, not just the positive side from the ASD.

I'm thinking the alternator is bad but I would like other opinions.

Also, anyone know how to test the PCM side? A simple voltage test isn't good enough as it pulses the - side. I can measure Hertz, but I have no data on what it should be when it telling the alternator to put out more, or less. Any numbers on that?

-Chris

Easiest way to find out is take it to autozone and put it on their tester, and also put a voltmeter on the field terminals to see the voltage output. That removes the ECM from the equation

Re-check/re-do ALL your grounds.

This is a topic 5-90 could provide info on...

Its not the grounds. They are solid. I cleaned them up and put some die-electric grease on them. I also checked the cables and wires for continuity, resistance and voltage drop(where applicable) and they all passed. As for Autozone/Advance their tests are pretty much useless. I had a known good alternator and a known bad one. The bad one tested good at one Advance and one Autozone, failed at the last Advanced... same thing with the good one. Either there is a problem with the machines they use or the user, but I wouldn't trust them to test a light bulb.

If you put a voltmeter on the field terminals while disconnected you will get nothing and you can't test the PCM output that way as it is a pulsed output. In test B that I did the PCM was removed with only the positive from the ASD and at that point the Alt was overcharging. And I do hope 5-90 replies.. maybe he can provide some insight into testing the PCM output.

If this was mine, based on the result from the B test, I would replace the alternator. But it isn't my money and I rather try some more diagnosis and get a second opinion before replacing the alt.

Ignore their tests, the thing you want is for their machine to spin up the alternator and make it produce output, while you use the multimeter to measure the output voltage. You can't replicate the test in the questionable vehicle

Undercharging is most often the alternator.

"Overcharging" though can be a bad voltage regulator. And that is in the PCM.

Talyn said:

Its not the grounds. They are solid. I cleaned them up and put some die-electric grease on them. I also checked the cables and wires for continuity, resistance and voltage drop(where applicable) and they all passed. As for Autozone/Advance their tests are pretty much useless. I had a known good alternator and a known bad one. The bad one tested good at one Advance and one Autozone, failed at the last Advanced... same thing with the good one. Either there is a problem with the machines they use or the user, but I wouldn't trust them to test a light bulb.

If you put a voltmeter on the field terminals while disconnected you will get nothing and you can't test the PCM output that way as it is a pulsed output. In test B that I did the PCM was removed with only the positive from the ASD and at that point the Alt was overcharging. And I do hope 5-90 replies.. maybe he can provide some insight into testing the PCM output.

If this was mine, based on the result from the B test, I would replace the alternator. But it isn't my money and I rather try some more diagnosis and get a second opinion before replacing the alt.

Click to expand...

I can attest first hand to the "lack of skill" at most of the local parts stores in testing alternators, but the "newer" equipment in use at both Zone and Advance has taken most of the thinking out of equation (sadly, that's a good thing)

If you can visit the store first, look for a tester that reports it's results on a LCD screen. These units are designed to be "idiot proof" (sadly, another necessity) and will tell the employee every step of the testing procedure from what plug to use, to where to hook each color coded wire.

If in doubt about the results, ask if you can follow along with the employee and hook it up yourself-- the tester does know what to look for to test an externally regulated alternator. Hook it up correctly and tell it what kind of alternator it is, and you'll get a true answer.

best of luck

birchlakeXJ said:

"Overcharging" though can be a bad voltage regulator. And that is in the PCM.

Click to expand...

Really? Good job on posting w/o even reading the original post.

Thanks for posting this. I will follow. I'm electrically handicapped. Since new, my XJ has always blown out cell phones or whatever else I put into the cigarette lighter. No body parts yet. I'm curious to see where this goes and hope you discover the cause.

What is yours charging at? Did you put a multimeter on it? You may just have a problem with your accessory jack.

Talyn said:

What is yours charging at? Did you put a multimeter on it? You may just have a problem with your accessory jack.

Click to expand...

Thanks but like I said I'm electrically challenged. Schematics make my eyes glaze over and I have only used a multimeter several times in my life. Pretty pathetic. I would end up electrocuting myself or blowing up the car battery (been there done that) before discovering the problem. All I can say is that when I took it to Sears once to have the battery replaced, the know-less-than-I-do technician stuck a piece of test equipment into the lighter outlet and said it didn't work. Really? While it would be nice to fix it, its not my daily driver and certainly not worth the expense of having it looked at by a mechanic.

But I appreciate the gesture and was not trying to hijack this thread.

At times some of the replies here on are a bit questionable.

Talyn, the CD based work shop manual have an indepth testing for that based on OHMS. Unfortunately, I do not have my manual with me. I would have been happy to provide you with a step-by-step procedure to diagnosing the over charging issue.

yardape said:

Thanks but like I said I'm electrically challenged. Schematics make my eyes glaze over and I have only used a multimeter several times in my life. Pretty pathetic. I would end up electrocuting myself or blowing up the car battery (been there done that) before discovering the problem. All I can say is that when I took it to Sears once to have the battery replaced, the know-less-than-I-do technician stuck a piece of test equipment into the lighter outlet and said it didn't work. Really? While it would be nice to fix it, its not my daily driver and certainly not worth the expense of having it looked at by a mechanic.

But I appreciate the gesture and was not trying to hijack this thread.

Click to expand...

I don't think you have an overcharging issue. The dash gauge (if you have one) would indicate so and you would get a check engine light. You might just have a faulty cig lighter socket.

techno1154 said:

At times some of the replies here on are a bit questionable.

Talyn, the CD based work shop manual have an indepth testing for that based on OHMS. Unfortunately, I do not have my manual with me. I would have been happy to provide you with a step-by-step procedure to diagnosing the over charging issue.

Click to expand...

That would be great. I couldn't find anything in the FSM relating to ohms testing. They had a few tests to test the output amperage, but I don't think that would help me.

I'm a bit at sea here - this is a case where I'd defer to someone who knows more than me (and I don't have Rod handy - he's shut for the day, and I don't bother him at home.)

Even with the automated tester that most chain stores use (usually the bear ARBST - not a bad unit, but it's still a "basic" tester,) and it can't really wring out the system.

Y'ask me, I'd end up going to my local alternator/starter shop (in this case, San Jose Generator) - take in the vehicle so he can test it on-car, and it's easy enough to drop in short order so he can throw it on the bench (and alternator/starter house benches can usually really wring out an alternator - plus they usually have the kit to properly evaluate the regulator circuit.)

As far as measuring PWM duty cycling, I'd think that the duty cycle would be higher as the required output increases - vice the old steady-state applied voltage approach, where you'd see a fairly consistent 0.5-4.0VDC at the field input terminals (it was a steady-state DC voltage, varying in level.)

However, per my 1994SM (I don't have a 1993 yet, but the 1994 should be similar...) here's what I can find:

OVERCHARGING ->
1) Grounded alternator field wire, field terminal, or connexions
2) Refer to using OBD system
3) Faulty Generator

OBD-I DTC #46 shows as "Charging System Voltage Too High," or battery voltage sense input above target charging voltage during engine operation - but you already knew that.

The DkGRN/ORG wire on the back of the alternator runs to the ignition coil, the ASD relay, and PCM #57 (all the same colour and the DkGRN wire is the feed from PCM #20.

PCM #20 is the voltage regulator (probably the output/control signal.)
PCM #57 is the ASD Sense (which is probably the signal that the PCM uses to control PWM regulation of alternator output.)

Knowing that, I'd probably also check for a wiring fault in the ASD sense line - if the regulator is sensing a "low voltage condition," it will swing to higher output at the alternator, and the circuit is unfortunately not self-correcting. I've found a wiring fault in the regulator circuit sense line on various vehicles often enough that it's a useful thing to check for an unknown overcharging condition. (While OBD can tell you what isn't working - and sometimes how it's not working - it doesn't always tell you where the fault is, specifically. This is why the current generation of mechs being taught strictly on OBD isn't necessarily a good thing - before OBD, we did have to do our own thinking. I've run into cases where OBD was wrong, and it's often not specific enough to isolate the actual fault - especially on vehicles that are run hard in difficult environments (like ours...) I tend to view DTCs as more along the lines of "hints" than actual trouble indicators.

The DG/O wire from the ASD supplies a constant 12v to the alternator field driver. The DG wire from the PCM supplies the PWM to the field driver. IIRC having the multimeter set on Hz is showed around 400 when the alternator was connected and putting out 16 volts and around 100 when it was disconnected and the engine was running on the battery. So, the PCM is still working that line, but is it doing it correctly? Lower Hz means longer pulses to up the output maybe?

I don't think it is a wiring fault as all the wires check out good ohm wise. However when checking over the alternator disconnected and outside the Jeep the field driver terminals the multimeter showed continuity with the alternator case. Ohming it out showed 9.6 ohms on the PCM side (DG terminal) and 12 ohms on the ASD side (Dg/O) terminal. Should the field driver in the alt have any connection to the alt case?

Also, if the PCM PWM wire was disconnected from the alt wouldn't alternator output drop to 0? In this case the alt puts out 16v if the wire is connected or not (DG/O ASD 12+ side connected of course.

Talyn said:

The DG/O wire from the ASD supplies a constant 12v to the alternator field driver. The DG wire from the PCM supplies the PWM to the field driver. IIRC having the multimeter set on Hz is showed around 400 when the alternator was connected and putting out 16 volts and around 100 when it was disconnected and the engine was running on the battery. So, the PCM is still working that line, but is it doing it correctly? Lower Hz means longer pulses to up the output maybe?

I don't think it is a wiring fault as all the wires check out good ohm wise. However when checking over the alternator disconnected and outside the Jeep the field driver terminals the multimeter showed continuity with the alternator case. Ohming it out showed 9.6 ohms on the PCM side (DG terminal) and 12 ohms on the ASD side (Dg/O) terminal. Should the field driver in the alt have any connection to the alt case?

Also, if the PCM PWM wire was disconnected from the alt wouldn't alternator output drop to 0? In this case the alt puts out 16v if the wire is connected or not (DG/O ASD 12+ side connected of course.

Click to expand...

Hm. This is the point where I'd definitely want to go talk to Rod (or whoever you've got locally,) vice the droids at the local parts house. Rod has better gear - your local alternator/starter shop probably does as well. This is rapidly getting away from the sort of thing I can troubleshoot by remote control - the free thought process is lost in cases like this. Something that turns up in testing could point indirectly to the problem, but if I'm not there to see everything that's going on, I'm likely to miss that (and important data can be lost when relaying it back and forth, just through simple omission.)

But there is no one local.

I did a few more test on it today
With only the ASD relay output (+12 constant part of the field) connected to the alt via a remote start switch I then used a simple incandescent trouble light with one end connected to the PCM PWM output for the field and the other to the battery +. Nothing connected to the PCM field terminal on the alt. So, the only wire hooked to the field was the +12 volt. Using the remote start switch I open and closed the 12+ field circuit. When the circuit was open (no alt output) the light was lit. With the circuit closed (16+ output) the light went out. This told me that the PCM was seeing voltage and was trying to modulate it. I am also sure that the alt shouldn't be putting out anything with only one of the field wires connected.

I also went back and measure Hz and duty cycle. When the circuit was open(no charging) the Hz were 32 and a 99% duty cycle. With the circuit closed(16+ full field charging) the Hz were 415 and 97% duty cycle. I think the Hz may work the opposite of what we thing. I think more Hz= shorter pulses, less = longer pulses and more output requested. As for the duty cycle I am unsure what that is indicating or in fact measuring in this case.

Looks like there is a short to ground in the alt's field circuit. So time for a new alternator.

And just for shits and giggles I took the alt to Advance Auto, Autozone and O'Rilleys. Advance tested it as bad and Autozone and O'Rilleys tested it as good.