You have a Climate Control Panel (CCP) that switches the blend door, blower motor ect... to control air, heat and AC in the cab. If this is working properly, the problem is in your blend door. You can usually get access by removing the lower heat core access panel and see if the blend door linkage is off, or the spring is off or broken. Your CCP controls a motor that in turn operates the blend door.

If no mechanical fault is found at the door:

1. At the mode door actuator, check the Brown wire for 12 volts.

2. Check the Black wire for a good ground.

3. Check the White wire voltage from the control head, it should be about 1-volt in defrost, 12 volts in A/C, and 6 volts in heat mode.

4. If voltages check OK, replace the mode actuator. If not OK trace wires to control head, verify powers and grounds, replace control head.