How to Diagnose the Ford Glow PlugPosted 7/17/2006
By Jeff Boskowitz
Let's get right to the down and dirty of the powerstroke glow plug. This system consists of a glow plug control relay (GPC) for 49 states or glow plug control module (GPCM) on 1999 and newer California emissions vehicles, eight glow plugs, a "wait to start" light, related wiring and the powertrain control module (PCM) to operate the system.
The "wait to start" light is independent of the actual glow plug on time. The PCM will turn the "wait to start" light on just for three to five seconds to allow sufficient cylinder temperatures to be reached for ease in starting. However, even though the light is off, the PCM will keep the glow plugs energized for a much longer time to reduce smoke and to improve cold driveability. The amount of on time can be as long as 180 seconds, and is determined by the PCM based on inputs from the barometric pressure sensor, the engine oil temperature sensor and available battery voltage.
Symptom: Hard start caused by the glow plug. To diagnose (the following applies to 49 states only):
Testing the actual glow plug operation requires more than a simple resistance test and presence of B+ at the components.
Note: Testing should be done using any device containing a high current inductive amp clamp.
California vehicles operate identically to the 49-state systems. Instead of a 4-pole relay controlled by the PCM, the glow plug system contains an actual module. This module is in the same location but has more wires. The output terminal of the 49-state relay has one large wire for each bank that branches off to individual cylinders. The GPCM has one output wire for each cylinder. As with its 49-state cousin, it still contains the B+ wire, the vehicle power wire and the pink/orange to energize it. It also contains a communication line to the PCM. This module "monitors" individual glow plug draw and reports any errors to the PCM. It can flag a code P0670 if there is a control line fault, a P0683 if the diagnostic circuit is faulty and any combination of P0671 through P0678 for a glow plug fault in cylinders 1-8.
The reason behind the minimum spec mentioned above is that since this module monitors actual current to each cylinder, the malfunction indicator light will illuminate and a code will be generated for any glow plug that draws less than 20 amps, even though no starting issues are present.
I hope this takes some confusion out of this system. If you have any questions, just give us a call.
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