Verified (GPCM)
So it turns out that I found an old TSB off of another forum and it looks very legitimate to me. Here it is.
TSB
03-21-36 ENGINE - 7.3L DIESEL - GLOW PLUG CONTROL MODULE - VEHICLES WITH CALIFORNIA EMISSIONS - SERVICE TIP
Publication Date: October 7, 2003
FORD: 2000 ECONOLINE, EXCURSION, SUPER DUTY F SERIES
Article 99-25-10 is being republished in its entirety as part of a consolidated 7.3L diagnostic aid booklet.
ISSUE:
California emissions vehicles built after 10/24/1999 have a new Glow Plug Monitor system which incorporates a solid state Glow Plug Control Module (GPCM). This system replaces the relay and shunt assembly previously used.
ACTION:
Refer to the following Diagnostic Procedure for new GPCM system pinpoint test.
DIAGNOSTIC PROCEDURE
The GPCM supplies power to the glow plugs through solid state drivers and performs diagnostics internally to detect and report system failures. The Powertrain Control Module (PCM) still controls glow plug on-time through a control circuit between the two modules. On-time is a function of engine oil temperature, barometric pressure, and battery voltage. This is the same as the previous system.
There are two circuits connecting the PCM and the GPCM. PCM Pin 101 is still used as the control circuit which activates the GPCM to enable glow plugs. PCM Pin 8 is used as the serial communication circuit which is used for the GPCM to report failures to the PCM. The PCM monitors these circuits for failures. The GPCM internally performs diagnostics for individual glow plug/glow plug circuit failures. When a failure is detected, the GPCM will transmit that failure to the PCM over the communication circuit. If a fault is detected, it will be stored in PCM continuous memory. New fault codes have been added and descriptions are as follows:
P0670 - Glow plug (GP) control circuit malfunction
