It started last week. My battery light would be dimly illuminated upon start and flicker above 1500 RPMs, then eventually stay off for the rest of the drive. The voltage at the terminals were 12.8 volts standby and 14.4 volts with engine running. Now my CEL is illuminated, so I ran a scan with a Snap-On scanner. The stored codes were..
P0671 – Glow Plug Circuit Failure 1
P0672 – Glow Plug Circuit Failure 2
P0673 – Glow Plug Circuit Failure 3
P0674 – Glow Plug Circuit Failure 4
P0675 – Glow Plug Circuit Failure 5
P0676 – Glow Plug Circuit Failure 6
P0677 – Glow Plug Circuit Failure 7
P0678 – Glow Plug Circuit Failure 8

P0670 – Glow Plug Control Module control line failure

I was expecting the CEL to tell me something about the charging system, but as you can see it did not. I cleared the codes, performed a KOEO, KOER and Cylinder Contribution Test, and only those codes reappeared.

Now the questions.. are the P067(1-8) codes because of the P0670 code? Is the charging system problem caused by the failure of the glow plug system? Does the GPCM need to be replaced?

Any help would be great, thanks!

 

I wouldn't completely rule out the charging system, though it seems unlikely. If you disconnect the EOT sensor to maximize the glow plug on time you can check the battery voltage with the glow plugs running. If the batteries fall below 11v. with the glow plugs on the batteies aren't holding.

Also before replacing the GPCM check the harness connectors at both the control module and the PCM.

 

I will definitely investigate the electrical connections to the GPCM. I will get back to you on this. Thanks.

Although.. the nice big cloud of white smoke in the mornings is funny...

 

UPDATE:
This problem went away a right after I posted it on here (typical). The CEL and Battery light both went away and acted normal. I still inspected the GPCM plugs and wiring. Scanning showed no codes. Last week it came back. First thing I noticed was the battery light not coming on when I first turn on the key. Then after I started the engine it would come on and lightly flicker at idle, and stay off above 1200 RPM. Voltage was 13.9 to 14.1 Vdc. At this time the CEL came back on. I scanned it and the original P067x codes came up. Saturday night my battery light was off but I noticed the voltage needle showing the top of the battery symbol. I checked the volts and it was 11.2 Vdc with the engine running So needing an alternator at 19:45 on a Saturday, I went to Autozone and luckily they had a NEW one (Duralast Gold) in stock. So $189, 3 13mm bolts, 1 10mm nut, a 1/2" breaker bar and 20 minutes later the new alternator is on. Truck was slow turning over (10.9vdc, thankful for 2 good Interstate batteries) but fired up and the new alternator was putting out 13.0vdc (not quite the 14.4 I want, but it'll do for now). 3 drive cycles later the CEL went off. Scanned my codes and it came up clean.

So, obviously I had a failing component in my alternator. Would the PCM or GPCM know this and not light the GP's?

 

If the system voltage falls below 11.5vdc (I think) the PCM will not command the GPCM on.

 

Would the failure of the I circuit cause this as well? I'm just curious now. Perhaps the loss of presence of the I circuit keeps the GPCM off-line, so when the PCM does his startup testing, it fails giving me those codes...