Questions and Answers

Ford E-150 HVAC Random Changes Q. My 1995 Ford E-150 Van climate system randomly changes from A/C to Vent to Floor to Defrost regardless of where the control lever is positioned. I noticed this when traveling at high speeds with the A/C on. At slow speeds, 45 mph or less, the control lever works as expected. What may be causing the problem? Bill

A. The HVAC system in your vehicle is vacuum operated. When the vacuum supply is interrupted for any reason, the system defaults to DEFROST mode.

Under high engine rpm, such as acceleration or higher speeds, the amount of vacuum available goes down. At low rpms or speed, such as at idle. the vacuum is high. This is what causes the switching back and forth of the HVAC modes.

To prevent this a vacuum storage tank and check valve is used to store vacuum for the HVAC system. At high vacuum the check valve opens. When manifold vacuum drops, the check valve closes and the HVAC uses the stored vacuum to hold the current mode.

Now since you HVAC is switching back and forth between modes, we know it is getting vacuum. So that leaves the problem in the check valve not sealing or the vacuum tank has a leak in it.