I've got a 2000 Lariat 4x4 5.4 liter. Everytime I accelerate the defroster comes on and it doesn't matter what position the heater selector switch is in. My fellow firefighter friend who is a bit of a backyard mechanic say's he's never seen anything like it and thinks it is a computer problem. Has anyone had this same problem or any ideas on what is causing this Thanks

 

It is a simple vacuum leak problem.

Under acceleration, manifold vacuum drops sharply. As the motors that direct the airflow within the air plenums (except for the blend door) are all run off vacuum, this lowered vacuum supply is insufficient to overcome the springs that force the vacuum motors and doors to their default positions. The system is designed to default to defrost mode in the event of vacuum loss (safety reasons). That is why you are experiencing your symptoms.

Steve

 

projectSHO89 is correct, you have a vacuum leak. Dont look for a computer problem, look or listen for a vacuum leak. I would start under the dash around the vacuum servos and then the vacuum connector behind the control panel. If you still havent found the leak, check under the hood.

 

Thanks grayflare and projectSHO89. My mechanic buddy listened and found the vacuum leak and replaced the line. No more defrost unless I select it.

 

Glad we could help.