I have a 98 Expedition w/ front & rear A/C. If I turn the temp control **** to full cold or full hot I get a repetitive thumping noise from under the dash. My wife finds it extremely anoying. The only way to make it go away is to turn the heat-A/C off and put the temp control **** in the middle, then turn it back on and slowly adj. the temp. If I stop to within a hair of full cold or full hot, it's fine, but if you accidentally turn it that little extra until the **** hits the stop, the thumping starts all over again. Any ideas?

 

Probably the blend door actuator assembly.

It contains a small motor and a set of gears that actuate the lever for the temperature blend door. It also has an actuator position sensor that provides lint information to the control mechanism.

My guess is that the travel limit sensing portion is probably defective. It is also possible that there is a cracked gear or that a gear is missing a toooth, however, these usually cause clicking sound or no control over the temperature.

Steve

 

I've seen the temperature blend door itself break more often on Expygators than the motor itself failing.

 

Is the door and/or motor visible from under the dash without having to take anything apart?

 

You can pull the black trim cover off on the passenger's side of the center console and look at the door with a penlight.