BCM Code B120, Twilight Delay Pot Circuit: Notes

The twilight delay uses a potentiometer to control signal voltage to BCM. The BCM supplies voltage on circuit No. 705 to resistor in left switch assembly and grounds through circuit No. 736 to BCM. The wiper supplies voltage signal to BCM on circuit No. 271 indicating position of slider and delay time. When slider is moved toward maximum position, resistance is low; therefore, BCM will see high signal voltage. When slider is moved toward minimum position, resistance increases and signal voltage decreases. Signal voltage varies between zero volts (open or grounded circuit) and 5 volts (shorted voltage).

Code B120 will set when ignition is on, signal voltage indicates less than 6 percent (open or grounded circuit) or more than 94 percent (shorted to voltage). During a failure, a substitute delay time (minimum delay) is used to allow continued operation of twilight system and reading from BD43 will display actual reading.

1. BCM parameter BD43 displays twilight delay. Normal range is 6-94 percent.
2. Checks to see if open circuit is caused by circuit or switch assembly. If open or grounded circuit reading changes to a short to voltage reading after jumping switch assembly terminals, BCM and wiring are okay.
3. Measuring voltage between circuit No. 705 (5 volts) and circuit No. 271 will determine if circuit No. 271 is open or shorted to ground.
4. Checks to see if short to voltage circuit reading is caused by circuit or switch assembly. If a short to voltage reading changes to an open circuit reading after disconnecting switch assembly, BCM and wiring are okay.