Charging system issue

Tiny
UNORTHODOX91
  • MEMBER
  • 2009 DODGE RAM
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 137,207 MILES
No fluff.
Truck has 3rd generator on it, no charge. Battery voltage present on Gen Bat+, Bat+, and TIPM post.
Checked all wiring at PCM pins to connectors, and did voltage drop test across the power supplies.
The only discrepancies i'm seeing are:

1) Gen sense terminal on gen is showing Batt voltage, parallel to Gen Post. But when the 2-pin connector is connected and back probed, voltage is 9v. Confirmed 3v voltage drop across Batt+ and pin-2 (red/grey).
I suspected resistance in wire but continuity checked out. I pull off PCM connector and turned key on and observed the same 9v coming from the PCM male pin corresponding to pin-2. I checked pin-10 and pin-29 as they’re fused bat+ feeds and I’m getting Bat+ voltage at the connector. Are my suspicions of resistance in the PCM causing the 3v drop? Or is it getting 9v from somewhere and spitting it back out? No info online on PCM voltage specs.

2) Pin-1 is the Gen field control. The PCM uses pwm/square waves to activate field. When I tried to measure duty cycle with my dmm, I would get sporadic duty readings, never holding for more than 2 seconds. When I tried to live test the Gen duty cycle via scan tool(key on engine off) I would set duty cycle to 50% and would read sporadic readings across the range. Unsuccessful bypass with pin-1 (br/gy) to bat negative.

Scan tool states duty cycle @ 100% while running, but Gen Post pushing batt+ voltage. Target voltage is at 14.3v.
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Sunday, March 28th, 2021 AT 5:56 AM

7 Replies

Tiny
KASEKENNY1
  • EXPERT
I am not sure I understand where you are finding the 3 volt drop. Is this on the wiring or are you saying you are finding this internal to the PCM?

The reason is, this generator sense circuit is what the generator sends back to the PCM so the PCM can compare that to the B+ it is getting from the TIPM. When they are not the same it commands the regulator to increase the voltage.

Sounds like the PCM (regulator which is internal to the PCM) is not able to command the voltage to increase.

Let me know if I am not understanding where you are checking this voltage. However, the fact that you have 9 volts on the sense circuit is where your issue is.
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Monday, March 29th, 2021 AT 6:47 PM
Tiny
UNORTHODOX91
  • MEMBER
Hello and thank you for your response.

To clarify, I am picking up the 9v in the wiring, but if I recall correctly, I also disconnected connector-1 of the PCM, turned the key to the "on" position, and measured 9v from pin-24, which corresponds to the Gen sense. Which is kind of odd since the B+ connections go thru connector-1.

Anyhow, today I searched for a 5v reference off the throttle body and only picked up mV's. I then proceeded to back probe the connectors and performed a voltage drop test on all B+ and grounds going to the PCM. Nothing but that 9v discrepancy showed.

It's an odd situation since it seems as though the computer is detecting the low voltage and is trying to push 100% duty on the field, but cant signal to the gen put out the target 14.3v.
I'm banging my head trying to avoid making an assumption on the PCM, but it keeps looking like the culprit.

Testing performed:
-Continuity
-Voltage Drop
-Tracer (power probe etc)
-Gen by-passing (ineffective)
-5v reference
-Scan tool (live data & live test)
-Duty cycle measurements on Gen connector
-visually inspected and wiggled Batt cables, grounds, and wiring (accessible ones).
-Powertrain verification test
-Transmission verification test
-Changed pigtail since the prior would lock the red tab.
Had to keep charging battery since it would run down to 7v @ 100% duty cycle. It makes no sense!

I haven't opened the wiring harness to investigate if the suspected wire is causing the issue since I performed a continuity test and power probe etc tracer through it. But I feel like that's the only stone left unturned.
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Monday, March 29th, 2021 AT 11:09 PM
Tiny
KASEKENNY1
  • EXPERT
If all your B+ are 9 volts it sounds like you may have a ground issue. You can check and clean the grounds but to confirm this, you can just back probe the grounds with a safety pin and run them to the closest stud on the engine block just to test this. It will take the easiest path to ground so if there is a ground issue, this will clear it up and then you will be able to track down the issue.
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Wednesday, March 31st, 2021 AT 5:58 PM
Tiny
UNORTHODOX91
  • MEMBER
I went through all 4 connectors and voltage drop tested all the B+ and grounds. All grounds were good and the B+ wires showed battery voltage as intended, but the only 9v is coming from the gen sense. No other circuit is showing 9v. I m also not showing a 5v reference on the throttle body connector. I already ordered the PCM # RL150390AE.
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Thursday, April 1st, 2021 AT 4:08 AM
Tiny
KASEKENNY1
  • EXPERT
I think you nailed it. Let us know what happens after you get the PCM in there. Thanks for the update.
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Thursday, April 1st, 2021 AT 6:59 PM
Tiny
UNORTHODOX91
  • MEMBER
Update:

So after running the aforementioned tests, I concluded that the PCM s voltage regulator had failed, and required replacement.
I ordered a new PCM from mopar (part # RL150390AE). I soon learned that trying to program a pcm via scan tool was a futile venture. Being my first new PCM, it took me a while to determine that it required a flash of the corresponding vehicle file from TechAuthority via WiTech 2.0 (j2534).
After flashing the PCM and performing relearns, the truck started right up and the generator was pushing 14.3v! Success!

Hope this provides insight for anyone else dealing with the same issue.
Thanks for the input and boost in confidence venturing in unknown territory.
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Thursday, April 15th, 2021 AT 8:32 PM
Tiny
KASEKENNY1
  • EXPERT
Not only did you give us good info on the correction but you included all the OEM/aftermarket tool info needed to get it updated. That is a great update.

Thanks for the info and using 2CarPros
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Saturday, April 17th, 2021 AT 7:47 AM

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