Write-up: PCM alternator voltage regulator bypass - JeepForum.com
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post #1 of 44 Old 09-13-2019, 12:45 PM Thread Starter
Timo_90xj
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Write-up: PCM alternator voltage regulator bypass

As we all know, Mopar truck SBEC/ JTEC/ JTEC+ and probably all the newer engine/ power control modules use an internal voltage regulator located in the control module for the alternator. When it fails, the ECM/PCM should be replaced.


The OEM setup is a good setup in a way; it uses battery temp sensor to increase charging voltage to heat up the battery and keep up the battery charge.
However, when the voltage regulator fails on an otherwise perfectly functioning PCM, IMHO it's pure waste of money to invest in a refurbished - and often crappy - replacement PCM.


I opted for the external voltage regulator, for which you have many options. A lot of people use the Mopar truck- style voltage regulators, which is a cheap and fairly well- performing unit.
You can either go with the Mopar performance or any of the aftermarket versions.
These days they all probably come out of the same manufacturing line, so I opted for Standard motor product VC125. Also bought the S573 pigtail harness.



I cut the green alternator harness wires from the OEM harness connector, about 1.5" ... 2" away from the black rubber thingy.
Then soldered wires to the wires on the connector, and taped off the cut wires that come from the PCM (field source carries +12V so you really do want it taped).




External wiring itself is very easy:
* +12V switched source (ie. from ASD relay) to voltage regulator center pin (blue wire on the pigtail harness) & to generator field source*.
* Ground the voltage regulator case - I installed a ground wire with 8mm ring terminals from alternator ground post to voltage regulator case and used an M8 bolt with washers to secure it.
* Wire from voltage regulator side post (green wire in the pigtail harness) to generator field driver**.


* Generator field source (+12V) on the OEM harness is the green & violet wire; terminal post on the alt is on the drivers' side
** Generator field driver (ground- controlled by PCM) is the green wire on the OEM harness; passenger side terminal on the alt.






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post #2 of 44 Old 09-13-2019, 12:46 PM Thread Starter
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I can add more pictures if needed.


I also need to change my wiring a bit - I currently have the field source/ volt.reg. 12V feed directly from the battery, which is is not optimal. I was in a hurry so it was the fastest way.
I will either use ASD relay as the source, or install a new relay with fused output feed to the PDC, there are a couple empty slots to do that.


Wire harness is not the best out there, I installed some length of wire harness sleeving, but it's not good. Need to make it factory- look

1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...


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1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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post #3 of 44 Old 09-13-2019, 01:55 PM
paulsheer2
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The Mopar charger smoked for me.

I have an aftermarket.

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93 ZJ 5.2, 3.58 stroked. 242 swap. 2.5" lift. dozens of mods.
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post #4 of 44 Old 09-13-2019, 02:27 PM Thread Starter
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Quote:
Originally Posted by paulsheer2 View Post
The Mopar charger smoked for me.

I have an aftermarket.
I'm pretty sure these days they all come from same one or two factories. I've heard these go bad pretty quickly with the 136A, 150A or higher output alternators no matter of brand.
In that sense, the factory PCM- internal regulator is pretty damn good holding up for 20+ years..

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1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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post #5 of 44 Old 09-14-2019, 11:43 AM
sirstvjesse
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Write-up: PCM alternator voltage regulator bypass

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post #6 of 44 Old 09-14-2019, 01:27 PM
StPaul59
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How does the bypass affect charging?
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post #7 of 44 Old 09-14-2019, 01:32 PM
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after the remote alt, did the CEL and associated codes continue? How did you address that?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #8 of 44 Old 09-14-2019, 03:32 PM Thread Starter
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Quote:
Originally Posted by StPaul59 View Post
How does the bypass affect charging?
When the PCM internal voltage regulator fails, you have no charging at all. This is one of the possible fixes. Other two options are to replace the PCM, or to crack open the PCM and try to fix the internal voltage regulator..

Quote:
Originally Posted by jtec View Post
after the remote alt, did the CEL and associated codes continue? How did you address that?
This will cause the CEL to illuminate, I'm looking at a solution to that. As far as I know this should be an easy fix with a resistor between the OEM harness generator field driver and field source wires, but I haven't verified it.
I'm suspecting ~5ohms from looking at the powertrain diagnostic manual procedures. The key thing is PCM should see some kind of a load on that circuit.
Some people have mentioned the voltmeter on gauge cluster go haywire, no issues on my cluster. Shows the correct voltage spot-on.

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1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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post #9 of 44 Old 09-17-2019, 09:48 AM
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There is a kit on ebay that comes with a 10 ohm.

It is a big fat resistor with a heat sync.

It doesnt work: the cel comes on seldom, but it still comes on.

5 ohm is too small --- it will draw 100 watts and get red hot.

10 ohm also gets too hot.

I have 20 ohm now. CEL comes on rarely.



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post #10 of 44 Old 09-18-2019, 05:28 PM
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I installed the same VR in 2015 an never had an issue with it. Higher output alternator should have no effect on the longevity of external VR. I still have my old alt and the EXT VR installed as a backup.

I did go with a high output single wire internal regulator alternator in late 2017 only because I had an opportunity to pick up a 400.00 dollar Powermaster for 160.00.

Sheer is right on the resistor as for the volt gauge if the PCM thinks the alternator is undercharging it will drop it to zero and if the PCM thinks the alternator is overcharging it will max it out.

Mine would max the gauge out in the summer on hot days but after purchasing an AGM battery it has never happened again. I had long thought that the derated charging was due to the battery temp circuit.

Last edited by jrcromwell; 09-18-2019 at 05:29 PM. Reason: Grammer
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post #11 of 44 Old 09-18-2019, 10:20 PM
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Quote:
Originally Posted by Timo_90xj View Post
When the PCM internal voltage regulator fails, you have no charging at all. This is one of the possible fixes. Other two options are to replace the PCM, or to crack open the PCM and try to fix the internal voltage regulator..



This will cause the CEL to illuminate, I'm looking at a solution to that. As far as I know this should be an easy fix with a resistor between the OEM harness generator field driver and field source wires, but I haven't verified it.
I'm suspecting ~5ohms from looking at the powertrain diagnostic manual procedures. The key thing is PCM should see some kind of a load on that circuit.

Some people have mentioned the voltmeter on gauge cluster go haywire, no issues on my cluster. Shows the correct voltage spot-on.
Timo, see posts #16 and 30 of this thread:
https://www.jeepforum.com/forum/f13/...17/index2.html
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post #12 of 44 Old 09-19-2019, 10:04 AM Thread Starter
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Thanks for the info!



Yeah, I've seen both of the symptoms of under- and overcharging where the volt gauge either drops to zero or maxes out. Both cause a "check battery" on the VIC and "Check gauges" in the gauge cluster along with the *bling* - very annoying.
I was sure a suitable resistor can and will cause a load so that the PCM is happy, and volt gauge should show correct reading from the ASD feed. I was just not sure on the correct resitance and I need a high- wattage resistor or not.

I will take my alternator tested, I'm suspecting bad diodes. Slip ring/ rotor is all good, windings look good to me, brushes still have plenty of material left so I don't think any of those are the cause for my dead alternator.
Alternator actually does charge initially, once it gets hotter it stops charging completely.

1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...


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1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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post #13 of 44 Old 02-10-2020, 10:01 PM
Garyellis
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2012 grand cherokee external voltage regulator install is not going well, I am having a hard time with it. When hooked fully up the pcm goes completely nuts. If I disconnect my 12v switch wire the pcm calms down but doesnt charge. I researched several sites and posts and went with a 5 pin relay running one to the battery, one to the fuse box for 12v switch, one to ground and one to the field wire. Can somebody tell me what I'm doing wrong here. Several days of scratching my head. I'm really hating jeep for the internal voltage regulator install.
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post #14 of 44 Old 02-11-2020, 03:23 AM Thread Starter
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This forum section is for '93-'98 ZJ Grand Cherokee.

Your 2012 Grand Cherokee is a WK2, post on that subforum for your issue.

1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...


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1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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post #15 of 44 Old 02-11-2020, 07:54 AM
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I had the drop to zero on the volt meter on mine before I got the random stalling which led to a PCM replacement that fixed both issues, but if all you have is the drop to zero on the voltmeter this is a great option.

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