Freightliner FB65 with dash light on

This post concerns a dash light problem on a Freightliner FB65 bus.

Hello, all.
I wanted to share a kind of a strange problem I recently ran into on a customer bus. It was an early 2000's model with a C7 and Allison trans.

The vehicle came in for a service, a dash light flashing, and a battery drain issue.
After the service I addressed the light flashing on the dash. At the top of the gauge cluster a light that said "PCM DIGN" was flashing what seemed to be a 46 code. (4 flashes, a pause, then 6 more flashes)

What does a light flashing "PCM DIGN" mean?

After a little thought I figured the acronym meant "Powertrain Control Module D Ignition". A search of Mitchell repair info, then an online search turned up no information on the light or the issue.

As the bus was a 2003 model I doubted that it actually had a powertrain module and figured I was looking at a transmission module power issue. The license on our Allison software had expired but neither the Bosch scanner nor the DG program would show any trans codes, which could be an indication of a problem with the module power.

I decided to start with a visual check of the trans module connection. Often the module is above the transmission, but I had to trace the wire harness from the trans, and finally found the module above the fusebox on the passenger side of the dash.

Check for power at the transmission module and fuse panel.

Batt power to the module was supplied via an inline fuse off a fusebox power stud, and I had power there. On the bottom row of circuit breakers was one marked something like "LOW BM/BK LPS/TRANS MODULE".  That took some head scratching, but I figured it meant low beam and brake lamps and trans module. That's weird. Not only that the module is protected by a breaker instead of a fuse, but that the manufacturer would put lights on the same circuit with module power.

When I turned the headlights on and pulled the breaker the lights went out though, so that circuit seemed OK. Odd thing was when I plugged the breaker back in the flashing PCM light on the dash was out and didn't come back until I cycled the key back off then on. Weird. Maybe unplugging the breaker reset the module somehow.

I decided to leave that problem alone awhile and look into the battery drain issue. All the service writer could tell me was that the customer requested we check for a draw off the batteries, so I surmised they had trouble starting the bus at least once. That was reinforced by the inactive low volt codes I saw when I scanned the truck.

The FB65 bus battery drain turned out to be a failed alternator.

There was no significant draw with the key off, but the batteries were not fully charged. A check of the alternator found it was doing nothing, at least at idle. After charging the batteries passed the load test, so I changed out the alternator.

When I started the engine to check the new alternator. the flashing dash light was out. A test drive confirmed the repair.

So I can kind of understand how the low system voltage due to the failed alternator could cause the PCM light to flash, but I would think that due to the lack of a voltmeter on the bus, there would be a warning light for the charging system. But I guess a manufacturer that would put headlights on a module power circuit would do some other strange things, too.

Bruce