LMM Duramax fluctuating Alternator voltage

[bluerider]

Hi All,

Have been chasing a issue on a LMM duramax engine I've converted into a Nissan Patrol, that has me really stumped.

I had a fluctuating idle, about +/- 25rpm, that i thought was a fuel issue. I replaced FPR, checked all lines, filters etc and it didn't fix it. Noticed at the same time the voltage on my banks idash was fluctuating anywhere from ~13.8v to 15.4v, so i started looking at the alternator. I noticed after 10min of running at idle it was very hot to the touch and thought something must be up and associated possibly with the high and fluctuating voltage. So i disconnected the 2 pin plug on the back of the Alt and straightaway it pegged the voltage at 13.8v on the idash and the fluctuating idle was gone - one problem found atleast.

When i say the idle/voltage fluctuates, it basically goes in a cycle. It will fluctuate from 13.8 to 15.4v for about 10 to 15 seconds then stabilizes bang on about 14.0v for a few seconds, then off it goes again.

For reference it has a brand new ACDELCO 160a alternator on it.

Things ive tried,

Put the original bosch 125a alt back on it - same problem
voltage checks with a meter at Alt and Bat - reading almost identical
voltage drop checks of positive and negative cables to B+ post on alt to Battery and Alt case to Battery neg terminal and chassis- all within spec.
All ground and pos connections are clean with no corrosion
Brand new tensioner
Ran a dedicated Negative jumper cable from Alt case to neg bat post and chassis - same
Ran a new + cable from Alt to fuse box - same.
Hooked a 2nd battery up in parallel with jumper cables - same
Continuity checks between L and F pins on Alt plug to ECM pinouts - all confirmed wired correctly and minimal resistance.
All connections are tight.
Battery is brand new, holds charge well and starts engine straight away.
No charging related DTC codes

All i can think of now is the ECM or BCM is potentially the problem??

I've discovered after using efilive and tech2 over the last few days, that the conversion package i was supplied has 3 different vin's between the ECM, BCM and TCM. Could this be the problem? A vin search shows the ECM and TCM are both from 4x4 LMM vehicles but the BCM is from a 4x2 LMM vehicle.

This has me stumped and I'm hoping someone has come across this before. With the 2 pin connector unplugged on the back all works perfectly fine albeit just a fixed voltage output @13.8v which as ive read is the set fail safe voltage for the Alt

 

[2004LB7]

what you are seeing may vary well be normal. the ecm controls the duty cycle of the alternator output. the ecm sets the output based on different criteria like lights on or off, rpm, temperature, etc. mine does the same fluctuating, albeit less now with the 2nd alternator installed

 

[bluerider]

what you are seeing may vary well be normal. the ecm controls the duty cycle of the alternator output. the ecm sets the output based on different criteria like lights on or off, rpm, temperature, etc. mine does the same fluctuating, albeit less now with the 2nd alternator installed

This thing just keeps fluctuating like this continually no matter what the load, there is definitely something not normal going on.

 

[2004LB7]

since you say both original and new alternators show the same behavior, the only thing left to do is scope the duty cycle on the two pin connector and see if it matches the voltage fluctuations. then you'll know if it is controlled by the ECM or not. may not be a bad idea to check those wires resistance to see if there is a break in the harness that is making intermittent contact

 

[Ron Nielson]

Same post on another forum, I suggested that maybe the ECM/TCM/BCM all need to have the same VIN# synced. They came from different sources. Anything to this ?

 

[corneredge]

I'm following this thread. My lmm duraburb is showing this behavior after I installed a new mechman alternator and some new ground cables. I'll have the truck in my garage for service in the next few weeks, will try to run some ground cable diags to find source at that time.

 

[2004LB7]

if any of you with the issue have a DC amp clamp, maybe check the current of the intake heater and glow plugs to see if the low voltage coincides with them turning on

 

[corneredge]

Did you ever find the source of this issue?

 

[bluerider]

Did you ever find the source of this issue?

Yes i did. It ended up being a 3rd party CANBUS translator module I'm running alongside the ecm/tcm to convert some signals on the GM CAN to talk to the Nissan CAN so the nissan cluster will display the duramax rpm, coolant temp etc. This module has the ability to output an Alternator charge control signal, it was transmitting this signal on the CAN network and confusing the GM ecm which normally controls the Alternator output voltage. Turned this function 'OFF' in the CAN translator module software so it would no longer transmit and problem solved.
After many many hours of troubleshooting it was as simple as hitting a few keys on the laptop to disable this function....
My fix more than likely doesn't apply to any of you guys though.