I know this isn't the time of year to be concerned about this, but I would like to fix it before warm weather. When I run the a/c in my 93 F-150 on max, it will blow out of the vents for 1-2 minutes, then it starts blowing out of the defrost ducts. I can turn the a/c to normal & then back to max & it works for about another 1-2 minutes before going to defrost. Normal a/c works fine, as does everything else (vent, floor, mix, defrost - in both warm & cool). Any ideas? Could it be vacuum related? This is a real pain in MS heat. Thanks in advance.

 

It definitely sounds like you are losing vacuum. Where from? I don't know. Sorry I can't help you anymore than that.

 

I have the exact same problem on my 94 Bronco
Max a/c cuts in and out
Normal A/C works fine

Please help

Thanks

 

On older A/C controls, vaseline (or something similar) was used inside the control as a seal within the vacuum switching mechanism. When it dried out, it leaked. You could try to get something back into yours to relube it, but I've never opened up a newer one so I don't know how.

 

I'd be willing to bet the problem is the vacuum actuator motor has a pin hole leak in it. Most of these are in the engine compartment and eventually gather road dust and grit which, in turn ruins the rubber seal for the motor. With a pinhole leak, you have sufficient vacuum to initally operate the motor, but over time, the vacuum cannot overcome the leak and the motor diaphram/lever seeps back out.

 

The vacuum motor (for the ventilation system) in the engine compartment is for the fresh/recirculate door directly below the R hood hinge. The one for floor/dash/defrost is inside the dash centered behind the radio. The only other vacuum motors on gasoline trucks are the cruise servo on ?-92s beside the L hood hinge, and the modulator on C-series transmissions.