Heater only blowing through defrost.

Schnot · #1 (**permalink**) 02-22-2011, 12:40 PM

My heater is only blowing through defrost and only on high. Is this some sort of vacuum problem or is there some kind of fuse that is blown?

Schnot · #2 (**permalink**) 02-22-2011, 12:41 PM

2000 F550 by the way.

Power Hungry · #3 (**permalink**) 02-23-2011, 05:00 AM

There is a vacuum pump that provides vacuum to the A/C control panel and ultimately the rest of the A/C vent solenoids/diaphragms. Either the pump isn't working, you have a vacuum leak, or the dash control for the vent selection is bad.

When you turn the key on before starting (particularly after sitting overnight) can you hear the vacuum pump running? It makes a hum sort of like a fuel pump, only it comes from under the hood and lasts about 10-15 seconds. If there's no hum, check fuses and then check the pump itself. If the pump is running but doesn't shut off after a few seconds, then you most likely have a vacuum leak or the vent selector is bad.

Here is the service document which should help you get it figured out.

http://www.powerhungry.co/.../Climate Control System - Diagnosis and Testing.pdf

Good luck.

DaDeerHunta · #4 (**permalink**) 02-23-2011, 05:28 AM

I have the same issue but I do hear the hum where could I look to try and find the vacum leak?

DaDeerHunta · #5 (**permalink**) 02-23-2011, 05:29 AM

I have an o4 350

Snake · #6 (**permalink**) 02-23-2011, 06:17 AM

Bill, don't suppose you have one of those guides for a 2005?

Power Hungry · #7 (**permalink**) 02-23-2011, 06:26 AM

Do you electronic A/C (buttons) or manual A/C (dials)? The above document may apply to you as well.

Vacuum leaks are hard to hunt down, and may not always be the problem. A bad solenoid/diaphragm on a vent door could be the issue as well. Do you hear any "hissing" under the dash? You may have to get your head up under there and give it a good listen, and even then it might be difficult to find anything.

In the past, I've found it may be easier to remove the vent control form the dash, disconnect the vacuum hose header from the control, and then check that we're getting vacuum at least that far. Then with a small piece of wiper tubing, we can run a line from the main vacuum input to the different ports on the header to see if the vent registers function correctly. If you have vacuum and everything functions correctly when checking manually, then the control is bad. If any of the registers don't function correctly when checking manually, then the solenoid/diaphragm for that register is bad or the tubing is broken.

Under-dash stuff isn't fun, but take it one logical step at a time and it's not too bad.

Good luck.