B128

Chart:

Schematic:

DESCRIPTION
The incandescent bulb reference circuit is used by the Body Control Module (BCM) to determine when Variable Frequency (VF) and incandescent Instrument Panel (IP) backlighting should be dimmed.

When the park lamps or headlamps are on, the bulb reference value, BD52, should be greater than 8 volts and the park lamp switch input to the BCM, B102, should read "HI". When this condition is present the VF displays and I/P backlighting will dim using the driver controlled dimming switch.


The Code B128 will set if the BCM detects that the headlamp/parklamp switch is on but the bulb reference voltage is less than 8 volts, or if the headlamp/parklamp switch is off but the bulb reference is greater than 8 volts.

TEST DESCRIPTION
The numbers below refer to circled numbers on the diagnostic chart:
1. BCM data value BD52 displays the incandescent bulb reference voltage. The normal reading is 8 volts or greater.
2. Removing the switch assembly and jumping the proper terminals will check to see if the low reading is due to the circuit or the switch assembly.
3. Using the testlight will check to see if the low voltage reading is due to the power circuit (240) or the signal circuit (44).
4. BCM input value B102 displays the voltage state of CKT 308 (parklamp switch) at the BCM.
5. Removing the switch assembly and jumpering the proper terminals will check to see if the low reading is due to the circuit or the switch assembly.

NOTE ON INTERMITTENTS
If an intermittent Code B128 is being set, manipulate circuits 308 and 44. When checking CKT 308 for an open, observe BCM input B102 and watch for the change-of-state indicator to change from an "0" to an "X". When checking CKT 44 for an open, observe BCM data value BD52 and if the failure is induced the reading will jump from its normal reading to zero.