B122 Dimming Pot Failure

BCM Code B122:

Headlight Circuit:

CIRCUIT DESCRIPTION:
The panel dimmer uses a potentiometer to control the signal voltage to the BCM. The BCM supplies voltage on CKT 705 to the resistor in the left switch assembly and returns as a ground to the BCM on CKT 736. The wiper provides the voltage signal to the BCM on CKT 686 indicating position of the slider dim level. When the thumb-wheel is moved toward the max. position (bright), resistance is low, therefore the BCM will see a high signal voltage. When the slider is moved toward, the min. position (dim) resistance becomes higher and the signal voltage to the BCM becomes less.

Code B122 will set if the ignition is "ON" and the signal voltage indicates less than 4% (open or grounded circuit) or over 96% (shorted to voltage). During the time the failure is present a substitute dimming value will be implemented to allow continued operation of interior lighting. The reading from the potentiometer BD42 however, will display the actual sensor reading.

TEST DESCRIPTION: Test numbers refer to circled numbers on the diagnostic chart.

1. BCM data value BD42 displays the amount of panel dimming. The normal range is from 4 to 96%.
2. Check to see if open or grounded circuit reading is due to the circuit or switch assembly. If the open or grounded circuit reading changes to a short to voltage reading after jumpering the switch assembly terminals, the BCM and wiring are OK.
3. Measuring voltage between CKT 705 (5 volts) and 686 will determine if CKT 686 is open or shorted to ground.
4. Check to see if the short to voltage reading is due to the circuit or the switch assembly. If a short to voltage reading changes to an open circuit reading after disconnecting the switch assembly, the BCM and wiring are OK.

NOTES ON INTERMITTENTS:
If an intermittent Code B122 is being set, manipulate the related wiring and switch pot slider while observing BCM data value BD42. If the failure is induced, the reading will jump from its normal value to a reading outside the range of 4 to 96%.