Dimming Problem

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION

The headlamp switch monitors the park lamp input (CKT 9) to determine if headlamps or parklamps are 'ON' (voltage). If the lights are 'ON', the headlamp switch converts the dimming control input (potentiometer in headlamp switch) to a PWM signal and distributes the signal on CKT 8 (0 duty cycle equalling max dim and full duty cycle equalling full brightness).

Incandescent

Incandescent bulbs are powered directly from the output of the headlamp switch, via CKT 8. This signal powers the dimmable incandescent bulbs in the headlamp switch, radio head, and throughout the vehicle.

VF

The PWM signal also is used by the IPC as a reference for its incandescent lights and VF displays. The IPC monitors the dimming input (ID42) whenever the parklamp input (II94) is HI. The IPC then drives its VF displays to correspond with the PWM input.

The IPC also controls the Radio Heads VF displays. The IPC feeds a constant 16 volt feed to the Radio to power its VF display. When dimming is required, the IPC sends a PWM signal (IS45) via CKT 724 to the radio to control the dimming of the VF display. The radio will only dim when it sees parklamps 'ON' (voltage at pin '7', CKT 9).

TEST DESCRIPTION

Test numbers refer to circled numbers on the diagnostic chart.

1. Since the incandescent signal is used to trigger VF dimming, the first step ensures proper incandescent operation.

2. The IPC will only control VF dimming if it sees that the headlamps are 'ON' (HI reading for II94)

3. IS45 overrides VF dimming output from the IPC. Expected voltages at the radio are close to 0 volt when full bright (IS45=99) ramping up to 5 volts at max dim (IS45=00).

4. The radio will only dim if it sees voltage at pin 7 (lights on).

5. Since the IPC controls the radio dimming, and the radio functions, the IPC is receiving all the correct inputs and the failure must be internal to the IPC.

6. The IPC provides the Radio with a 16 volt feed to power the VF displays. If there is a short to ground on this circuit or if the radio pulls the circuit low, the IPC VF will blank.

1. If incandescent lights are always 'ON', the problem is a short to voltage along CKT 8.

2. If some incandescent bulbs work, while others don't, the problem is either burned out bulbs, loss of ground, or most likely an open in CKT 8 isolating the inoperative bulbs from voltage.

3. The fuse will blow for a short on CKT 9 or a short to ground on CKT 8. Removing CKT 8 and testing isolates one half of the potential shorts.

4. Since no incandescent lights work, the problem may be in CKT 8, headlamp switch, or power feed to the headlamp switch. However, Code I022 would set for all of these conditions except for an open CKT 9 to the IPC and the headlamp switch, therefore look for an open CKT 9 between splice S411 and fuse 'D13'.

5. If no power is present at the fuse, the problem is with the power distribution to the fuse. Refer to DIAGRAMS.