Hi All, 1990 Ram Van, why does the heater / AC blower fan shut down at full throttle then come back on when you let off the throttle? Getting ready to do a complete AC overhaul, but trying to understand the system. Thanks

 

I'll bet the fan blows, you just don't get any air out the dash vents.Defrost is the default setting to keep the windshield clear. You probably have a small vacuum leak somewhere or a bad check valve. Look around for cracked vacuum lines. Usually it's one on the side of the engine. On V configuration Magnum engines there is one on the passenger side of the intake plenum. It's a little black piece with a flt part in the middle. There are two but I don't have a van and can't take a picture of your setup.

Now, if the fan is actually shutting off, it may be a cut out for the a/c system. In the 1970's and 80's, there was a switch to disengage the system under WOT for performance. (Wide Open Throttle) Emissions controls were notorious for sucking performance out of engines. That is just the system doing what it was designed to do.