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Defogger flaking out...

>>I've got an '85 5000S with a very strange problem. The windshield defogger only
>>seems to work at low RPM's (MPH?). I'm not sure why this happens, but when I
>>get over about 50 MPH the windows start fogging up immediately, and when my
>>speed drops down they magically defog. Because it's acting like this, I can't
>>tell if the compressor is kicking off at a certain RPM, or maybe the A/C clutch
>>is slipping at progressively higher RPM's, or if it's something else entirely.
>>Any ideas - similar experiences?
>>Thanks in advance-
>>Larry Spector

Sound like it might be a vacuum leak which is changing the position of
the flaps which divert air to the windshield.  As you back off the
throttle, you create a greater manifold vacuum which then moves the
flaps back into where they are supposed to be.

If your AC compressor is working properly, speed should not affect its

Query: Is this happening in warm humid days or cold days? (Note that
when the ambient temperature drops below 38 degrees Farenheit, the AC
compressor will not kick in...)