4WD light on, 4WD light flashing
Winter is around the corner and this is when owners try out their 4WD drive systems after not using them all summer. If you’re getting a 4WD light on with your Ford vehicle, this article is for you.
Ford used several different control modules over the years to operate the 4WD system, but the starting point is always the same; read the trouble codes first. Do NOT start throwing parts at the system.
Using a quality scan tool, read the codes, write them down and then CLEAR the codes before trying any repairs. Then test drive the vehicle. If you get a 4WD light on again, read the codes a second time and compare. Then clear them again because the module will not respond to any commands while it has active codes stored.
If your scan tool can’t communicate with the module, check the module’s power and ground. The ground can be corroded from no use for months on end. On 1997-2001 4WD systems you’re looking for the generic electronic control module (GEM). That controlled the external relays to run the shift motor. Starting in 2002-2004, Ford used a free standing module. Then, in 2004, Ford moved the control to the PCM.
Inside the shift motor, Ford used four contact plates to report the position shifter. If the system can’t determine the position of the shifter, it will set the 4WD light on and stop shifting. It may also set a P1867 contact plate circuit failure code.
Ford sends a reference voltage (V-REF) to the contact plates and then monitors the voltage. The actual REF voltage will vary depending on the type of module, but the REF
voltage should read the same on all contact plates. Ford also uses a fifth ground wire to ground V-REF depending on the position of the shift motor.
The contact plate parameter identification descriptions (PIDS) are usually labeled A, B, C and D on the scan tool data. But the PIDS are not universal to all Ford vehicles, so check your shop manual for actual PID info.
If the contact plates read correctly while the harness is unplugged but read incorrect with the shift motor plugged in, the shift motor will need to be replaced, as the contact plates are not serviced individually.
©, 2015 Rick Muscoplat
Posted on by Rick Muscoplat