I have yet to buy the cd/manual and I need to know if there's a sensor that tells the PCM to provide a ground for the glow plug relay. I've already troubleshot to a lack of ground at the relay. I jumpered a ground to the relay and I get the voltage to the glow plugs. Don't want to spend the money on a PCM??? I'll probably go ahead and install a momentary switch for the ground side of the relay for "manual" control. I just want to make sure a sensor is not bad and probably hurting me else where. Thanks in advance.

 

You are correct, in that the PCM completes the circuit to ground for the GPR. It does this by sinking the current through a solidstate device in the PCM. As you suggested, an easy fix is to install a momentary switch that you have to hold down to activate the GPR.

 

I copied and pasted from my manual.

Glow Plug System Components:
The 7.3L diesel engine uses a glow plug system that preheats air in the combustion chamber to improve cold engine starting.

The system consists of:

l intake manifold glow plugs (12A342).

l glow plug relay (12A343).

l powertrain control module (PCM) (12A650).

l engine oil temperature (EOT) sensor.

l barometric pressure sensor (BARO sensor) (12A644).

The glow plug system is electronically controlled by the powertrain control module. If the temperature is below 55ºC (131ºF) the powertrain control module will energize the glow plugs immediately after the key is placed in the ON position. Then, depending on the readings from the engine oil temperature (EOT) sensor and the barometric pressure (BARO) sensor, the powertrain control module determines how long the glow plugs will be on.


Glow Plugs — PCM-Controlled

The intake manifold glow plugs are located in the cylinder heads (6049), under the valve covers (6582).

The glow plugs are self-regulating. If the engine oil temperature is above 55ºC (131ºF), the powertrain control module will bulb-check the WAIT TO START lamp but not energize the glow plug relay.

The powertrain control module protects the glow plugs by energizing them for short durations if the battery voltage is abnormally high.

The glow plug ON time varies from 0-120 seconds depending on battery voltage, engine oil temperature and barometric pressure.

On California vehicles the glow plug monitor system is part of the On-Board Diagnostics II (OBD II) System.

The glow plug monitor (GPM) system is designed to find failed glow plugs or failed wiring in the glow plug system.

The GPM system uses a low resistance shunt to conduct current to the left and right bank glow plugs.

Three sensing wires measure the voltage drops across the shunt when the glow plugs are operating. A failure in the glow plug system will set a diagnostic trouble code (DTC).

 

Hey Kwik, this is a little vague....can you provide the theory down to the molecular level??? This is great, thank you very much as it was exactly what I was hoping to find. Looks like I'll just wire in a switch for now. But I'll have the truck scanned to see if there's something else wrong with the PCM.

 

Molecular level??? Sorry I'm not that educated. Hmmm Just make sure that there is enough heat in the combustion chamber to fire the fuel being injected.