Electrical Article - Alternator and PCM protection

[Mopar1973Man]

Alternator and PCM protection

What I'm going to describe is a rare event. This is possible with any alternator stock ND or Bosch or even the aftermarket alternators that are bigger amperage output. In a nutshell, at any point, the blue field lead happens to short to ground the entire short is routed back through the PCM. Since the PCM is protected by a shared fuse of 20 amps this means the circuit board fails during these events. There have been two cases of this which is @pepsi71ocean and myself with the same failure. The only difference is mine failed so badly that it burned a hole through the PCM circuit board making the PCM unrepairable. I was forced to replace the PCM completely. 

Diagnostics

You want to start the engine and check the blue for +12V. This +12V power should be present with the engine running. It's tested by using a DVM or test light. If power is not present then the PCM is damaged. The green wire will show a good ground more than likely but this will not work is there is no +12V to the field. This is where you make use of the article for ECM /PCM rebuilders. Make sure to test the alternator and replace it as well.

The Modification

This is based on the W-T ground wire mod. Being the field lead is powered from the PCM and the PCM, ECM and VP44 all share the same 20 amp fuse in slot G of the PDC. This fuse is too large to protect the PCM from a field wire short. This means you would have to purchase a fuse holder and solder in a fuse holder on the blue wire. With the PCM connectors removed from the PCM look at the connector towards the fender, this is connector C3. Pin 25 is the blue wire you can ohm test from end to end to be sure the wire is not broken. Then ohm between the blue wire and ground to verify its not shorted to ground still. Now take the tape near the loom and unwind it. This will give you about another 1 inch of wire. About 2 inches from the loom cut the blue wire. This should give you like 6 inches from the plug end and then a kind of stubby 2-inch wire on the loom end. Now solder in the fuse holder and use shrink tubing over your solder joints. 

 

 

With a bit of neat folding of the wire you should be able to loom the fuse holder into the loom again.

 

I know that @IBMobile did the load testing of the field lead and suggests a 7.5 amp fuse. I'm going to do the testing with a 5 amp fuse. 

 

As for protection of the alternator on the charge lead, I opted to purchase a resetable 150 amp breaker. I know the factory alternator fuse was 140 amps. I know that the alternator will not produce more than 135 amps of charging current. I know the breaker is not there for protection the alternator for excessive charge current. The breaker is there just in case there is a dead short on the diode pack. If the short was good enough it could light that 6 gauge cable on fire. So having an exact 140 amp breaker is not required. The other thing I was reading up on was the breakers tend to trip prematurely because of underhood temperatures. I wanted to be just that little bit bigger that it does not trip the breaker because the alternator is already at full current charging on a hot day like jump starting another vehicle. 

 

Fallout, External Regulator users

As for all you as fallout from this problem you typically were doing the external voltage regulator because of the PCM failure. I'm going to suggest that you consider doing the PCM repair and doing this mod and getting away from the external voltage regulator. 

 

 

 

View full Cummins article

[milradioman]

Mike,

When my blue wire shorted to ground, the PCM apparently shut that line down, preventing damage, I had purchase a replacement PCM thinking that was the problem.

 

My engine dipstick tube would rub on the wire harness every time the engine torqued, from starting or hard acceleration. Eventually the insulation rubbed through, but only shorted when I started the engine, talk about a nightmare to track down; When the engine was off, the blue wire not shorted, start engine and no short, but no output.

Since I started and stopped the engine multiple times, my best guess is each time the wire shorted, the PCM shut the output off to protect the circuit.

Although in my case the wire was grounded momentarily, a constant short to ground may have a different result.

 

I have no problem performing the modification, better safe than sorry, and I will add it to my list of must do.

I wanted to share my experience that is similar to yours.

Nice clean job on the mod.

 

Brian

[pepsi71ocean]

I got very luck when mine went out. It was fixable.

 

 

[Leaky88]

I have fused the blue wire at the PCM per Moparman’s example.

Unless I missed it, a 150 A resettable breaker was also installed in series with

the Charger line coming off back of Alternator

Where was the 150A breaker placed?

 

Thanks Leaky

[Mopar1973Man]

150 Amp circuit breaker it is mounted on the firewall side of the battery tray away from the heat of the radiator. This will stop false tripping.

 

Fuse should be installed closest to the PCM plug on the blue wire. 5 amp max fuse.

[Leaky88]

Thanks. Leaky