Code B128 - Bulb Reference Circuit: Notes

The incandescent bulb reference circuit is used by the BCM to determine when VF and incandescent I/P backlighting should be dimmed. When the parklamps or headlamps are on, the bulb reference value, BD52, should be greater than 8 volts and the parklamp switch input to the BCM, BI02 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.

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 jumpering 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 BI02 displays the voltage state of circuit 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 of the switch assembly.