When heat is on and mode selector is turned to.

  • 1999 FORD F-150
  • 181,000 MILES
When heat is on and mode selector is turned to floor, heat only comes out of defroster. When turned to other modes on selector, they work, but sometimes they switch back and forth. Example when turned to just Panel mode, heat comes out then switches back to defroster then back to panel. I've changed the mode selector switch and still the same thing happens. Heat comes out the floor a little bit but not much. Would the air flap actuator have anything to do with this problem. I'm really stuck on this problem, i'm not really into takeing the whole dashboard out to fix. Does anyone have a drawing of the vent diagram as to maybe manually move the flap that directs heat to the floor. I live in Wisconsin and the feet get pretty cold in winter time. ANY SUGGESTIONS.
Monday, November 12th, 2012 AT 11:33 PM

1 Reply

The mode selection is done through a series of air doors that are operated by vacuum. When there is either a partial or total loss of vacuum, the system will default to either defrost or on some vehicles, heat. Acceleration can also have an effect on the doors for the same reason.

Look under the hood for a small black plastic hose coming through the firewall. Follow it to the vacuum source looking for any breaks, cracks or leaks. Also follow any other lines that branch off because they may go to a vacuum reservoir (storage tank) and a leak there will cause the same problem. Using a vacuum gauge to determine if you have full vacuum at any given point is the best way to find it. You can cut into the line anywhere and splice it back together with a piece of vacuum hose. Find the broken line, you found your problem.
Was this
Monday, November 12th, 2012 AT 11:37 PM

Please login or register to post a reply.

Recommended Guides