Blower control switch

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.

Thank you. Recently I discovered there may have been a mouse or other small animal digging through the firewall behind the glove box. Every time I put the speed of the fan to 3 or 4, it would vibrate very badly. So I took down the blower and found installation and leaves piled in it. Runs fine now, but I bet it sounds like there could be some debris in the vacuum line.

Nothing IN the vacuum line, just a crack or leak in it somewhere. The rat could have chewed through it too.