Charging system stuck at 14.9 volts

[J Bravo]

Long time reader, first post. After scouring this forum and others, I can't find the answer - I realize there are lots of charging system questions on this forum but they're not quite what I'm experiencing.

Simply said, my charging system runs at 14.9v at the battery all the time (except one case as explained below). Battery is an Oddyssey AGM.

Here's what I've measured via connector backprobe:
- PCM B+ (Connector C1, pin 29), and ASD Relay Outputs (C3-28,19)
- PCM grounds (C1-9,18)
- +12 at the fuel injectors.
- Alternator ground (Generator connector Pin 2)
- PCM generator field control (oscilloscope on Generator connector Pin 1)
- Voltage at OBDII connector (Pin 16 and 4/5)

Results:
- All the +12 are within ~150mv of the battery positive 14.9v, some are closer,
- All the grounds are within around 30mv from the battery negative.
- The Field Control is showing a normal PWM signal so the field isn't just grounded all the time - it looks like the PCM is actively commanding the charging system.
- ODBII connector reads 14.75v.

What I have tried:
- Swapped alternator with known good unit - no difference
- Running a WranglerFix PCM. Swapped in my old PCM - no difference
- Inspected the famous G105 engine block ground. Clean and tight and measures good under normal running load

Important Details:
- **** The charging voltage sporadically returns to normal (13.6-13.9) after running my air compressor for about 5-10 minutes. Easily the biggest electrical load on board.
- **** An OBDII scanner reports system voltage at 13.9v !!
- My overdrive, rear defrost, and axle lock switches are not installed

Here are my questions:
1) Because an OBDII scanner reports system voltage at 13.9v, what input does the PCM look at to determine the system voltage which is used to control the alternator?
2) I did not measure the TCM stuff on C4. Could that be involved somehow?

I sent Mark at WranglerFix this adat and these questions but have not heard back. @JerryBransford, you're a EE I think, what do you suggest to try next?

 

[macleanflood]

@Wranglerfix

 

[C.J. Roberts]

This might be a simplistic question, but what does the voltage measure at the battery?

 

[freedom_in_4low]

Long time reader, first post. After scouring this forum and others, I can't find the answer - I realize there are lots of charging system questions on this forum but they're not quite what I'm experiencing.

Simply said, my charging system runs at 14.9v at the battery all the time (except one case as explained below). Battery is an Oddyssey AGM.

Here's what I've measured via connector backprobe:
- PCM B+ (Connector C1, pin 29), and ASD Relay Outputs (C3-28,19)
- PCM grounds (C1-9,18)
- +12 at the fuel injectors.
- Alternator ground (Generator connector Pin 2)
- PCM generator field control (oscilloscope on Generator connector Pin 1)
- Voltage at OBDII connector (Pin 16 and 4/5)

Results:
- All the +12 are within ~150mv of the battery positive 14.9v, some are closer,
- All the grounds are within around 30mv from the battery negative.
- The Field Control is showing a normal PWM signal so the field isn't just grounded all the time - it looks like the PCM is actively commanding the charging system.
- ODBII connector reads 14.75v.

What I have tried:
- Swapped alternator with known good unit - no difference
- Running a WranglerFix PCM. Swapped in my old PCM - no difference
- Inspected the famous G105 engine block ground. Clean and tight and measures good under normal running load

Important Details:
- **** The charging voltage sporadically returns to normal (13.6-13.9) after running my air compressor for about 5-10 minutes. Easily the biggest electrical load on board.
- **** An OBDII scanner reports system voltage at 13.9v !!
- My overdrive, rear defrost, and axle lock switches are not installed

Here are my questions:
1) Because an OBDII scanner reports system voltage at 13.9v, what input does the PCM look at to determine the system voltage which is used to control the alternator?
2) I did not measure the TCM stuff on C4. Could that be involved somehow?

I sent Mark at WranglerFix this adat and these questions but have not heard back. @JerryBransford, you're a EE I think, what do you suggest to try next?

My understanding is if you have a battery temp sensor (and the fact that you see anything above 13.6V implies that you do) that the PCM controls the charge rate based on battery temperature. It makes sense that after running a high load your battery might warm up a bit and trigger the reduction in output voltage.

As for what voltage it's supposed to put out, I'll leave that to someone else. It might be in the FSM but I've never gone looking for it, because I'm one without the battery temp sensor so mine charges at a constant 13.6V all the time.

You're obviously pretty sharp on electrical stuff to get this far, but I have to ask what was the original issue you're troubleshooting? Or did you just notice the voltage being higher than you expect?

My OBD2 scan also shows a voltage exactly 1V below what I read with a meter. I always assumed I had a corroded 12V+ pin in a PCM connector but now that I know it's not just mine it makes me wonder if there's something to it.

 

[J Bravo]

This might be a simplistic question, but what does the voltage measure at the battery?

No worries. At the battery, on the actual battery posts, not the connectors, it measures right around 14.89v.

 

[J Bravo]

My understanding is if you have a battery temp sensor (and the fact that you see anything above 13.6V implies that you do) that the PCM controls the charge rate based on battery temperature. It makes sense that after running a high load your battery might warm up a bit and trigger the reduction in output voltage.

As for what voltage it's supposed to put out, I'll leave that to someone else.

You're obviously pretty sharp on electrical stuff to get this far, but I have to ask what was the original issue you're troubleshooting? Or did you just notice the voltage being higher than you expect?

My OBD2 scan also shows a voltage exactly 1V below what I read with a meter. I always assumed I had a corroded 12V+ pin in a PCM connector but now that I know it's not just mine it makes me wonder if there's something to it.

In the later TJ years, the battery temp sensor was eliminated, thus, my 2006 does not have one.

I just noticed the voltage is higher than it should be, or I guess higher than both I and the battery want it to be, according to its label.

OBDII voltage: yes exactly. If the PCM thinks it's maintaining 13.9v but the actual measured system is higher, how is the PCM determining system voltage?

 

[freedom_in_4low]

In the later TJ years, the battery temp sensor was eliminated, thus, my 2006 does not have one.

I just noticed the voltage is higher than it should be, or I guess higher than both I and the battery want it to be, according to its label.

OBDII voltage: yes exactly. If the PCM thinks it's maintaining 13.9v but the actual measured system is higher, how is the PCM determining system voltage?

Interesting. I wasn't aware it was model year based, just that even when it was used, it was seemingly random which vehicles had it.

In the absence of a battery temp sensor, I understood it to just target a constant voltage. Mine (a 2006) is at 13.6, my OBD2 scan reads 12.6. It seemed on the low end to me but I've never had any issues starting so I left it alone.

 

[J Bravo]

@Jerry Bransford, any words of wisdom? I’m pretty stumped.

 

[TEZZA]

2006 NGC3 PCM, Desired battery voltage in relation to Ambient Temperature.

 

[Jerry Bransford]

@Jerry Bransford, any words of wisdom? I’m pretty stumped.

Nothing comes to mind specifically but how old is your Odyssey? At this point after everything you've done I would have it load tested by someone who actually knows how to conduct one properly. Perhaps the voltage regulator is thinking the battery charge is low and has jacked the alternator charge voltage up trying to get it to charge.

 

[J Bravo]

Nothing comes to mind specifically but how old is your Odyssey? At this point after everything you've done I would have it load tested by someone who actually knows how to conduct one properly. Perhaps the voltage regulator is thinking the battery charge is low and has jacked the alternator charge voltage up trying to get it to charge.

Thanks Jerry. The battery is about 2 years old, but the Jeep's charging system has behaved this way at least since I put the battery in, maybe before. I've been tinkering with diagnosis since then and finally gave up and came here to consult the experts! I'm worried that I'm shortening the battery's life hitting it with 14.9v all the time. If I could only figure out what the PCM looks at to decide how to run the alternator.

For reference, the battery is at 12.6v after sitting for four days.

2006 NGC3 PCM, Desired battery voltage in relation to Ambient Temperature.

View attachment 497925

Thanks, Tezza. So this is the stock PCM charge voltage curve? I'll look at the temperature sensor readings again to make sure it isn't pegged cold all the time. My Jeep doesn't have a battery sensor, but has two temp sensors behind the front grill. One is for the thermometer on my rear view mirror, the other is the PCM's ambient air temp. I'll double check the ambient sensor and its connection at the PCM to make sure the signal is good.

 

[TEZZA]

Yes, stock charge curve.
What is the OBD2 voltage when engine is not running with ignition on? Still 1v less than battery?
Check the voltage at Fuse 9 of the PDC or/and at C102 that feeds 29/C1 to the PCM.

 

[J Bravo]

@TEZZA , Thanks for the documentation screen shots. The Power Feeds and Grounds section pretty much says the the PCM looks at B+ C1/29 to determine charge rate. This is what I was wondering most about. Which document was that pulled from?

Per your suggestions, I made some more measurements:

- Ambient Air Temp signal wire at the PCM (C1/22) shows an in-range value and reacts to temp changes when I touch it.
- B+ voltage at C102 and PCM (C1/29) shows ~14.85v and are ~30mv from each other.
- OBDII reported system voltage is pretty much exactly 1v below battery voltage with ignition ON/engine OFF

That last one is the biggest clue. By all measurements, the PCM is getting ~14.8v at B+ (and ASD inputs), yet it apparently sees and seems to be acting upon the idea that system voltage = actual charging system voltage - 1 volt, which would put it right in the charging voltage range in the graph you provided above.

Now the question seems to be, other people's PCMs show charging voltage - 1 volt yet still properly charge at 13.nn volts, why?

Feels like we're getting closer to figuring this out....

 

[TEZZA]

Make sure that the alternator ground and the PCM ground are at the same potential, ie. no voltage difference,
The PCM has multiple grounds.
Manual/Document attached.

 

[J Bravo]

@TEZZA , thank you for the document!

Both the PCM and the alt field connector ground to the same point - G105 - but I measured those points, there is very little potential between them, maybe 3mv, which could down in the noise for my Fluke 117 DC mv setting.

I also measured the sensor ground at the throttle body MAP sensor. It was only 4mv different than the negative battery terminal. I’m convinced the PCM has good ground(s).

 

[TEZZA]

If there was a voltage drop across the PCM reference earth it couild cause extra voltage output from the alternater.
Does the battery voltage match the OBDii reported voltage without the engine running?

 

[J Bravo]

If there was a voltage drop across the PCM reference earth it couild cause extra voltage output from the alternater.
Does the battery voltage match the OBDii reported voltage without the engine running?

The PCM uses a pulse-width modulation signal to control the field in the alternator, which I think is to avoid slight voltage drops like this affecting the PCM’s control over the alternator’s output level. The small voltage drop should not affect the alternator’s output because PWM is used to control the alternator, not DC voltage level.

As I reported above, the OBDII reported system voltage does not match the battery voltage with the ignition ON and engine OFF. It reports the same 1-volt difference. So for example, with the engine OFF but key ON, the battery reports 13.2v and the OBDII system voltage shows 12.2v. The same 1v difference.

Tomorrow I’m going to measure two other 2005 LJs with properly functioning charging systems and see what their important values are. It’s gotta be in there somewhere!!