Chances are you have a blend door that is not closing off the outside air when you turn on the heat.
If this system is the same as the ones I've worked on, the blend door actuator in question is the one on the drivers side, under the dash about half-way up.
Remove the actuator motor and close the blend door by hand, now with the heat on you shoudl only get heat. If this cures the problem, then you know that either the actuator isn't working or the blend door is failing to close completely.
You can see the actuator move when you turn the selector to heat and then to the other settings.
Tuesday, December 24th, 2013 AT 7:14 AM