It was a cold morning at the campground, 24°F, so before starting the truck's engine I flipped the switch to cancel the grid heater bypass and let them do their thing.   After the grid heaters cycled the engine was started just fine with all the gauges coming alive.  Once the oil reached safe operating pressure I flipped the switch which turns on the onboard air compressor for the exhaust brake.  It was now time to get the show on the road and shift to drive; what the, "check engine" light is on. 

 

I pull out my handy dandy code reader an get this:

• P0753 Transmission 3-4 Shift Solenoid / Transmission Relay Circuits 
•  P1765 Transmission 12-Volt Supply Relay Control Circuit 
  
•  P1682 Charging System Voltage Too Low   

 

 The "check gauges" light pops on and the dash volt meter is not reading.  With a multimeter I see that the voltage at the batteries is 12.2V no alternator output..   I'm thinking since I live in warm Southern California and never turn on the grid heaters there and this is the first time I turned on the air compressor with the grid heaters maybe when I shifted the trans to drive there was enough load load increase to blow the 5 amp fuse that I installed in the field lead, blue wire, between the alternator and the PCM. 

 Sure enough that's what it was.  I didn't listen to my own advice when I was testing the amp output to the PCM an suggested a 7.5 amp fuse be used and installed a 5 amp fuse.  During testing I had a reading of 4.53 amps with the engine running and all lights on and heater fan at full speed and thought there should be a little higher amp fuse to withstand a load surge.   Well, problem solved with a 7.5 amp fuse installed.   

 

https://mopar1973man.com/cummins/articles.html/24-valve-2nd-generation_50/51_engine/electrical/alternator-and-pcm-protection-r617/

 

https://mopar1973man.com/cummins/articles.html/24-valve-2nd-generation_50/51_engine/electrical/grid-heater-bypass-r618/

Mine did the same thing after I installed a 5 amp fuse. I was in town though. Dropped in the 7.5 that you recommended. Haven’t had another issue. That was probably a couple years ago. BTW thanks! Glad it was a simple fix. Those are my favorite! 

I'm still using 5amp at the time. Used on board air, winch, grid heaters, not at all at the same time though. I probably came close to blowing it few times, just got lucky maybe. I have a 7.5 in the truck in case I need one. 

Now that you bring this up I'm sure it's a matter of time before it blows 

Still running just a 5 amp fuse in the PCM protection fuse. Never blown that fuse yet but I've managed to shake it out of the clip twice. 

1 hour ago, Mopar1973Man said:

Never blown that fuse yet but I've managed to shake it out of the clip twice. 

JB Weld would fix that.

Not a wonderful idea. I just squeezed the connectors and pushed the fuse back in.