GPCM to manually switched glow plug relay??

lmorris · April 21, 2016

Customer brought in his 2000 7.3L for hard start cold. First off it's a USA truck with California emissions, second, he doesn't want to spend that much money. P0683, GPCM communication fault and P0603 KAM error. Harness from PCM to GPCM is intact. KAM power to PCM is good as well. I am leaning towards a PCM at this point but want to be sure. Now it's time to dust off the breakout box and determine which module is bad, but the customer doesn't want to spend any more money diagnosing the problem, and for sure doesn't want to put up the money for either a PCM or a GPCM. So I give him the option to run the glow plugs off a standard glow plug relay controlled by a toggle switch, but warned him the Service Engine Soon light will always be on at that point. It should be a pretty simple procedure to modify the harness to connect to the relay. Anyone fore see or know of any issues this may cause? Other than the obvious issue if he forgets to switch them off. That may be cured by a push button switch, but I can't see him sitting there with his finger on the button for over 30 seconds on a cold day.

Ideas, thoughts?

Fredsvt · April 22, 2016

I just looked at the wiring diagrams for both the GPCM and relay controlled glow plugs, they both seem to use pin 101 off the PCM violet/orange wire for the trigger on the relay and I'm assuming a ground signal to the GPCM to turn on the plugs.

If you put the relay in, you can tie into the wastegate solenoid power wire for b + and use the violet/orange wire to trigger the relay on, it should work as normal, cycling the plugs as needed from what I can tell.

I'd test and see if that wire is grounded on the California truck for as long, temp dependent, as the regular relay trucks are.

lmorris · April 22, 2016

Will definitely look into that and post my findings. Wish the guy would leave me his truck to tinker with though. Maybe when it's warm enough for him to get his bike out full time.