C 3500 Truck 2WD V8-454 7.4L (1990)
CAUTION: Do not use a meter with an impedance of less than 10 Meg ohms.
1. Insert the positive probe into terminal H (circuit 903) and the negative probe into terminal 4 (circuit 904). Set the control to HEATER, pause
briefly.
2. SET THE CONTROL TO DEFROST. The meter should read -7.0 to -7.5 volts. This voltage will only last about 5 seconds after the control is
moved, after that it will return to 0 volts.
3. SET THE CONTROL TO HEATER. The meter should read +7.0 to + 7.5 volts for 5 seconds.
4. SET THE CONTROL TO UPPER AIR. The meter should read +7.0 to +7.5 volts for 5 seconds.
CAUTION: The controller may not deliver the correct voltages to the mode door actuator If the feedback circuit is faulty. Refer to "Mode Door
Feedback Circuit Test"
Mode Door Feedback Circuit