I can't get my truck to switch from defrost. I have no vent and no floor. Which vacuum lines should I check? Does the system operate off engine vacuum? Are there any vacuum pumps that may need replacing? I was in the middle of trying to hook my AC system but I couldn't get any power to the clutch.

PLEASE HELP.....no vent is driving me outta my MIND!

 

The vacuum for the vent control come from the "coffee can", which is located under the hood. If theres a hole in it, and it leaks, this would kkep the vent stuck on defrost, thats its default relaxed position.

You probably don't have power to the AC clutch because it is OUT of refridgerant. Just hook up the hose, and let it take some refridgerant, when it gets enough to satisfy the level switch, it will kick in.

 

Thanks for your help. Now where would the coffee can be located? What does it look like? Thanks again.

 

The vacuum reservoir can is usually located under the hood,possibly under the driver's side Inner fender at the front below where an aux battery would go.


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The only thing I can find that even closely represents a vacuum resvoir is a spherical oblest with only one vacuum line running to it. I'm gonna do some more searching tommarow.

 

The defrost function is the normal position of the door/flapper inside the plenum when no vacuum is applied to the dual-diaphram vacuum motor. That motor, located under the dash, has two vacuum lines running to it. One line is color-coded red....full vacuum to close off the 'defrost' air plenum and send air to the floor vents. The other line is color-coded yellow....partial vacuum when function control in in 'mixed' position(both defrost and floor). Check for vacuum to that vac motor. The vacuum resevoir is located (in my '81) on the passenger side of the engine compartment and resembles a 1lb coffee can cut in half. It has one vacuum line to it. That line comes from a small distribution manifold located on the fire-wall. Engine manifold vacuum comes into this distribution manifold. Between the distribution-manifold and the resevoir there is a check-valve....from that check-valve the line attaches to the funtion control connection block(the HVAC controls). Check for vacuum at the resevoir and after the check valve. If there is vacuum present, check for vacuum at the dual-diaphram motor. The 'weak' points are the lines, vac motor, check-valve. Unless the vac resevoir has somehow gotten holed, it should be ok.

 

On my 77 the vacuum reserve is a round ball on the drivers side inner fender. It has one hose from the engine and one to the HVAV control. It has an internal one way valve to hold vacuum when the engine is off. If you have a leak anywhere along the way the heater default to defrost.