B121

Code B121:

Twilight Photocell Circuit:

CIRCUIT DESCRIPTION:

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 BCM supplies a voltage on CKT 304 to the switch. When the slider is moved from the "OFF" position, the switch contacts close and the signal voltage is pulled low through ground CKT 804.

Code B121 will set if the vehicle is equipped with twilight sentinel, 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 slider is in the "OFF" position (shorted CKT). During the time the failure is present, the twilight delay pot slider position will be used to determine when twilight is "ON/OFF" to allow continued operation of twilight sentinel.

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

1. BCM input value BI82 displays the voltage state of the circuit at the BCM. These conditions can be observed in BCM input display BI82 as readings of "HI" or "LO," when delay slider is moved.

2. Checks to see if the "HI" reading is due to the circuit or the switch assembly. If the display changes from "HI" to "LO," when the switch terminals are jumpered together, the BCM and wiring are OK.

3. Checks to see if the "HI" reading is due to an open in CKT 304 or CKT 804. If the display changes from "HI" to "LO" when a known good GRD is provided, CKT 804 is open.

4. Checks to see if the "HI" reading is due to the circuit or the BCM. If the display changes from "HI" to "LO," when the BCM terminal cavity is jumpered to ground, the BCM is OK.

5. Checks to see if the "LO" reading is due to the circuit or the switch assembly. If the display changes from "LO" to "HI" when the switch assembly is disconnected, then the BCM and wiring are OK.

NOTE ON INTERMITTENTS:

If an intermittent Code B121 is being set, manipulate the related wiring. When BCM BI82 input change-of-state indicator displays an "X," check the circuit for an intermittent open or short.