I didn't know how to title this, but what happens, is when I accelerate, the A/C works OK up to about 45 MPH, (I don't have a tach), and then cuts back to a slight flow of cold air at the floor. The Truck is a 1992 F150 Custom Extended Cab 4x4 with a 5.8L Midland (previous owner and repair shop both said this was true) E4OD tranny, with 4 inch lifts, and the A/C has been converted to R134, by an A/C shop. It has over 208,800 miles on it. Front has factory hubs replaced with Warn manual locking hubs.

At first I thought that is probably something that is sensitive to air coming into the engine compartment through the grille, and moves into the way, cutting off the A/C air flow at about 45 MPH. When I have to back off the throttle completely even at 60 MPH, the full A/C air flow starts up, and will continue until I accelerate above about 45 MPH. If I am still near 60 MPH, as soon as I touch the throttle, the A/C air flow is reduced as before.

To beat the heat I have to drive local roads (stay off the freeway) and in the slow lane to stay at or under 45 MPH.

Another question, is there supposed to be some gear whine at speed, and especially when backing off? I asked the repair shop owner, and he checked the rear differential which has some metal particles in the gear lube. I suspect that I will have to have the differential rebuilt.

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Sounds like a vaccuum leak to me.