Code B121 - Twilight Enable Switch Circuit: Notes

The twilight enable switch uses the same physical control as the delay pot. Inside the left switch assembly the enable switch contacts close as soon as the time delay slider is moved off of its extreme min. (off) position. The PC supplies a voltage on circuit 304 to the switch. When the slider is moved from the off position the switch contacts close and the signal voltage is pulled through ground circuit 736.

Code B121 will set if the ignition is on, the twilight delay pot has not failed (Code B120) and the signal voltage is high when the delay slider is moved from the off position (open circuit), or signal voltage is low when the delay is in the off position (shorted circuit.)

1. IPC input I182 displays the voltage state of circuit 304 at the IPC. These conditions can be observed in IPC input display as readings of high or low when delay slider is moved.
2. Check to see if the LO reading is due to the circuit of the switch assembly. If the display changes from "LO" to "HI" when the switch assembly is disconnected, then the IPC is okay,
3. Check to see if the HI reading is due to the circuit or to the switch assembly. If the display changes from "HI" to "LO" when the switch terminals are jumpered together, then the IPC and wiring are okay.