Fan will not switch from floor to defroster to regular vent

WILLY420

MEMBER
1999 MERCURY COUGAR
4 CYL
FWD
MANUAL

The switch will turn On and off my air conditioner but when I turn it on to a different positions such as a regular vent position toward the driver and passenger the air continues to blow out of the defroster position. I'm thinking the problem is the actualspot where theair changes positions under the dash. Like I say it works selecting hot and cold air and it works selecting the power of air blowing it just won't select the position to blow the air out of its either or in the defroster position. Thank you for any information

1 Reply

WRENCHTECH

MECHANIC
20,757 POSTS

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.