You will need to check the signal from the control head the mode door.
1. Gain access to the air temperature motor for circuit testing. There are 5 wires at the motor.
2. The Brown wire is ignition power.
3. The Yellow wire is a ground at all times, through the control head.
4. The Light Blue/Black wire carries a 5-volt reference from the control head to the temperature motor.
5. The Light Blue wire is the feedback circuit to the control head and should measure between about 1 volt (cold) and 4 volts (hot). This feedback circuit operates similarly to a Throttle Position Sensor (TPS).
6. The Dark Blue wire is the motor control circuit sent from the control head.
7. When the control head sends out 5 volts on this wire, the motor moves towards hot.
8. When the control head sends out 0 volts, the motor moves towards cold.
9. When the temperature motor reaches the correct position (as measured by the feedback voltage on the Light Blue wire), the control head sends out 2.5 volts. This voltage signals the temperature motor to STOP and hold its present position.
10. Check for correct operation of these voltages.
11. Note that the air temperature motor can also be driven by the output voltage from the Throttle Position Sensor (TPS). Disconnect the Dark Blue wire from the temperature motor. Place the TPS output jumper wire at pin 6. Sweep the TPS, the motor should move, stop, then move the other direction. Do not move the motor past its normal range.
12. In some cases, the air temperature door does not fit correctly in the plenum and will cause the air temperature motor to travel too far. This will cause the control head to become confused and operate erratically.
TO RECALIBRATE - NOTE: When replacing the air temperature motor, do not operate it without installing it first. The control head initially calibrates itself by driving the blend door to full cold and full hot. If the motor is operated before installation, the expected "end of travel" voltage will go beyond recorded limits. This will shut the control head down. To fix this problem, remove the cover from the air temperature motor, and using a 9v battery, drive the motor close to a center position. Then, install the motor and remove all key on and memory power from the control head for about one minute. When memory power is restored, start the engine
, do not change any settings. The control head will automatically recalibrate itself for both cold and hot positions within one minute. Normal operation should occur after this is completed.