Here is a little information from the manual. The headlight are controlled by the Front Control Module (FCM) which is part of the Intergrated Power Module (IPM), The IPM is somewhat known for corrosion on 4wd. The switches only have controling voltage on them as they only send a signal to the FCM to turn on or off the light. This is done via the J1850 multplex wire. I thought I read that when a fault was determined that the fault went to low beams on, but I can't find it now so anyway. Check your connections, reboot the PCM, clear any remaining codes and hopefully the lights will come on. If they do come on and then go back out you know you are still outside the normal operating parameters of the system.
FRONT CONTROL MODULE
DESCRIPTION
The Front Control Module (FCM) is a micro controller
based module located in the left front corner
of the engine compartment. On this model the integrated
power module must be positioned aside in
order to access the front control module. The front
control module mates to the power distribution center
to form the Integrated Power Module (IPM). The
integrated power module connects directly to the battery
and provides the primary means of circuit protection
and power distribution for all vehicle
electrical systems. The front control module controls
power to some of these vehicle systems electrical and
electromechanical loads based on inputs received
from hard wired switch inputs and data received on
the PCI bus circuit (J1850).
For information on the Integrated Power Module
Refer to the Power Distribution Section of
the service manual.
OPERATION
As messages are sent over the PCI bus circuit, the
front control module reads these messages and controls
power to some of the vehicles electrical systems
by completing the circuit to ground (low side driver)
or completing the circuit to 12 volt power (high side
driver). The following functions are Controlled by
the Front Control Module:
² Headlamp Power with Voltage Regulation
² Windshield Wiper “ON/OFF” Relay Actuation
² Windshield Wiper “HI/LO” Relay Actuation
² Windshield Washer Pump Motor
² Fog Lamp Relay Actuation
² Park Lamp Relay Actuation
² Horn Relay Actuation
The following inputs are Received/Monitored by
the Front Control Module:
² B+ Connection Detection
² Power Ground
² Ambient Temperature Sensing
² Ignition Switch Run
² Washer Fluid Level Switch
² Windshield Wiper Park Switch
² PCI Bus Circuit
DIAGNOSIS AND TESTING - FRONT CONTROL
MODULE
The front control module is a printed circuit board
based module with a on-board micro-processor. The
front control module interfaces with other electronic
modules in the vehicle via the Programmable Communications
Interface (PCI) data bus (J1850). In
order to obtain conclusive testing the Programmable
Communications Interface (PCI) data bus network
and all of the electronic modules that provide inputs
to, or receive outputs from the front control module
must be checked. All PCI (J1850) communication
faults must be resolved prior to further diagnosing
any front control module related issues.
The front control module was designed to be diagnosed
with an appropriate diagnostic scan tool, such
as the DRB IIIt. The most reliable, efficient, and
accurate means to diagnose the front control module
requires the use of a DRB IIIt scan tool and the
proper Body Diagnostic Procedures manual.
Before any testing of the front control module is
attempted, the battery should be fully charged and
all wire harness and ground connections inspected
around the affected areas on the vehicle.
