heater controll switch vacuum help
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New User
Joined: Feb 2003
Posts: 12
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From: milton usa
heater controll switch vacuum help
ok my defroster dosent work but i get good heat through my vents when i switch from vents to defrost nothing happens does anybody know how the vacuum runs on this system any help would be greatly appretiacated
1980 model f-150 4x4
stock 351m
1980 model f-150 4x4
stock 351m
Posting Guru
Joined: Dec 2002
Posts: 1,246
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From: Princeton, MN
heater controll switch vacuum help
Every system I have ever seen (which is just a few, but consistent across several brands) uses lower vacuum to switch from vents to defrost. Try really revving the engine up and see if it switches over. That's what I have to do in the 2000 Blazer that I have to drive for work in order to get it to switch over. I can set the selector to defrost and drive for miles with air coming out the vents until I pop it in neutral and rev the engine to about 5500 RPM's. (higher engine speed = lower vacuum).
Senior User
Joined: Aug 1999
Posts: 488
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From: Bel Air USA
heater controll switch vacuum help
Do you hear a vacuum? I had a 78 Cougar in which the vacuum selector valve broke. The replacement valve didn't cost much. After replacing the valve I decided that I could have repaired the old one.
