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Ford Needs A Vacuum

Q. I have an 89 Ford F150, EFI 300 6 cyl, manual 5 speed with 172K miles. My problem is related to the vacuum switches that are used for the climate control. When it was working properly, you could switch the climate control from the vent to the floor and you could hear it go "shhhhhhhh". Meaning that the vacuum switch is closing one and open the other. It doesn't do that any more. It just keeps blowing out the defrost vents no matter what the switch is set to. I have checked the cables on the back of the climate control and checked the tubes for clogs.

I want to replace/test the main unit that generates the vacuum for this operation. Is there any way to see if it is generating enough vacuum? And what is it called, so I will know what to tell them at the parts store? Also, is it possible that the switch that opens and closes the duct is bad? Is there any way to test this?

I can manually operate the switch that closes the duct with my hand. And it will blow out of the proper vent for a minute, but then the vent will just open back up and not stay. The Chilton manual does not give any helpful information on this subject.

Scott

A. The vacuum that operates the climate control system comes from the intake manifold so as long as the engine's running, there will be vacuum. Climate control systems always default to the defroster mode so if the system fails the windshield will still be still be able to be kept clear. I would check the vacuum line from the intake to the heater control. It's very possible that it has dried out and cracked in half. If the line is okay to the firewall, then you will need to trace it inside the car.

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