Charging system and ECM behavior

[Giovanni Boezio]

First post so Hi, got a 2017 Ram diesel 2500 work horse.
I tought of putting my finding here in case that someone needs it and maybe getting some help if someone else have data on this thing.

Story :
So 6 months ago the alternator quit working, a few days and starts later ( I was brigning it to the dealer) it resumed charging like nothing happened, 3 weeks ago it happened again, 3 starts and boom charging again. So I brought it to the dealer and they said they will check it out, they told me I need a new ECM for 5500$, they told me that the ECM do not respond well and need replacement. They scored a used 900$ ECM, intalled it and it wouldn't reflash or respond well. So they have reinstalled the old ECM and told to try it out.

So 2 ECM and the problem seem to stick, I think we have a wiring issue here, either power or ground even a faulty alternator
I have done some testing on the charging system while it worked :
I have a 100 Hz PWM on the 2 wire plug of the alternator
The amplitude is the full voltage of the truck 13.5V
Period is about 75% ON state 25% off at idle and about 85% ON at 2k rpm
If I disconnect the plug I have nothing on the 2 pins and of course it throws the batt icon
The red wire pin on the alternator is a +Battery level, my guess is that it's a voltage sense for the ECM to regulate the charge and also the reason that there was nothing on the plug while disconnected. I may try to redo a scope test on batt + and unpluged conector to see if the ECM still maintain the PWM on the black grey wire while throwing a code.

I'm not gonna replace a 5500$ ECM for a maybe failed 50cents transistor. I can easily make a 100hz voltage sensitive regulator with a 20 amps driver. But before going there I will check every wire and ground.

And I love the Cummins BRRRRRRRRR specially cold !

 

[csuder99]

Step 1: Find a new dealer. A major dealership group here in the US lists the PCM for $3256 which is around C$ 4000. Remanufactured ones are $2043. Granted the used PCM could have been bad but the fact they couldn't diagnose it does not inspire confidence.

Step 2: As you know the alternator is controlled by the PCM via PWM to the field coil. Don't know specifics on the 4th gens but typically the PCM looks at the voltage it "sees" and adjusts the PWM accordingly. Compare the voltage the PCM reports with the alternator output and look at the PWM ratio while changing the electric load e.g. turning on headlights. Other ideas are check voltage drops on the ground side (battery to engine block, body and PCM grounds).

Edit: There are external voltage regulator conversions for older trucks where the PCM PWM output failed that probably can be adapted.

 

[chri5k]

Find another dealer. Sounds like the one you went to does not have a competent service department.

 

[Giovanni Boezio]

Update : I have decided to drive it untils it fails so we can't miss the problem.
A month ago as I was driving past Quebec city I could smell smoke and the alternator light turned on.
The alternator wire and battery connection was smoking hot.
I have left it at the Lévis dealer, they could witness the PCU not responding (orange screen frame on the tech computer), but they changed the 2 batteries, the alternator wire and the alternator itself. Never had a problem ever since.
So a shorted battery was causing this. And my hometown dealer sucks in electronic troubleshooting lol