Could be your controller or temp mix 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. In some cases, the air temperature door doesn't 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.
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