Chevrolet Repair: vacuum lines, vacuum problem, vacuum lines

Question
the heater/air conditioner fan goes off and on at highway speeds in my 97 astro van.  this problem does not exist at low speeds.  Is this a vacuum problem? The air conditioning system was worked on recently.  

Answer
Hi Ron,
I don't believe vacuum has anything to do on your A/C system on that.
There is a relay that is supposed to turn a fan on at the radiator area, and it would possibly shut off while driving down the highway. it could be that it was connected to the wrong fan.
Another possibility is if the freon charge is low, the high rpm could change the pressures enough that the system is shutting off to protect itself. If that is the case, actual vehicle speed might not be what does it, but rather the engine RPM speed.
A loose wire connection could also be the culprit, and the driving just be shaking the wire.

Van