Code B122, Panel Dimming Switch Pot Circuit: Notes

The panel dimmer uses a potentiometer to control the signal voltage to the BCM. The BCM supplies and monitors voltage on circuit No. 705 to the resistor in the left switch assembly and returns as a ground to the BCM on circuit No. 736. The wiper provides the voltage signal to the BCM on circuit No. 686, indicating position of the slider and dim level. When the slider is moved toward the maximum position (bright), resistance is high, therefore the BCM will see a low monitored voltage. As the slider is moved toward the minimum position (dim), its resistance decreases and monitored voltage increases. This signal voltage will vary between zero volts (open or grounded circuit) and 5 volts (shorted to voltage circuit). BD42 displays a low percentage for dim and a high percentage for bright panel illumination.

Code B122 will set if the ignition is on and the signal voltage indicates less than 2 percent (open or grounded circuit) or greater than 98 percent (shorted to voltage). During the time the failure is present, a substitute dimming value will be used to allow continued operation of interior lighting. The reading from the potentiometer BD42, however, will display the actual reading.

1. BCM data value BD42 displays the amount of panel dimming. The normal range is 2-98 percent.
2. Checks to see if the open or grounded circuit reading is due to the circuit or the switch assembly. If the open or grounded circuit reading changes from a short to voltage reading after jumping the switch assembly terminals, the BCM and wiring are okay.
3. Measuring voltage between circuit No. 705 (5 volts) and circuit No. 686 will determine if circuit No. 686 is open or shorted to ground.
4. Checks to see if the short to voltage circuit 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 okay.