B122

Code B122:

Twilight Photocell 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 a ground for the resistor on CKT 736. The wiper provides the voltage signal to the BCM on CKT 686 indicating position of the slider and dim level. When the slider is moved toward the max. position (bright), the voltage signal at BCM is low. As the slider is moved towards the min. (dim position) the resistance is less and the voltage signal at the BCM increases. This signal voltage will vary, between 0 volts (open or grounded circuit) and 5.0 volts (shorted to voltage), on CKT 686. An open or short in CKT 705 will result in 0 volts and an open in CKT 736 will result in 5.0 volts.

Code B122 will set if the signal voltage indicates more than 98% (open or grounded circuit) or less than 2% (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 reading.

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

1. BCM data value displays the amount of panel dimming. The normal range is from 2 to 98%.

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 (5.0 volts) and 736 ground will determine if problem is in CKT 705 or 686.

4. Measuring voltage between CKT 705 (5.0 volts) and 686 will determine if CKT 686 is open or shorted to ground.

4A. Measuring voltage at BCM will determine if CKT 705 is open or shorted to ground including BCM.

5. Checks to see if the short to voltage reading is due to the circuit or an open in CKT 736 and 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.

6. Measuring voltage between CKT 705 (5.0 volts) and 736 ground will determine if problem is in the switch or CKT 736.

NOTE ON INTERMITTENTS:

If an intermittent code B122 is being set, manipulate the related wiring while observing BCM data parameter BD42. If the failure is induced, the reading will jump from its normal value to a reading outside the range of 2 to 98%. Slowly moving the slider over to its full range of travel while looking at BD42 for a reading outside its normal range will help detect an intermittent switch problem.