Hello, the engine vacuum is greatest at idle, when you are accelerating engine vacuum reduces and in order to keep the a/c from defaulting to defrost your vehicle utilizes a vacuum reservoir and a check valve to prevent the stored vacuum in the reservoir from being utilized by the engine under load. The vacuum reservoir on your truck is located between the battery and right front fender, the check valve is installed in the vacuum line between the reservoir and engine. I suspect that your check valve has failed. To check this, locate and remove the check valve (it is white on one side and black on the other, has two vacuum nipples and is only the size of a nickel and about 1/8 inch thick) look down in by your a/c accumulator (receiver/dryer). once removed try to blow air one way through the check valve and then try the other way, you should only be able to blow one way through this valve.
No, take the battery and battery tray out and you should have easy access to the reservoir and yes a leaking control head can cause this condition.
Under the passenger side of the dash are two vacuum lines, black one and a white one, there is a vacuum connector in this location, disconnect the connector and attach a vacuum pump to the black line that runs to the control head and plug the port to the white hose, turn the key on and select vent, apply vacuum to the black line and see if it holds and air continues to exit the vents. if it does go to vent your leak is in the dash or switch, if it stays on vent, then your problem is in the engine compartment. I have not had or heard of anyone who has had any luck repairing the control head, if that is the problem.
not necessarily, it just depends on what selection that the switch was on, I believe if you have the mode select in the off position it will fail in the way that you are testing. Are you testing with a vacuum gage?
keep me posted
It sounds like you have found your problem.
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