My Max has started blowing cold air at high speed. I have a '96 GLE with the auto climate control. Any time I am driving about about 45 MPH (basically anytime I'm on the freeway), my heater will not put out hot air, even if it is set to 85 degrees and is pumping out nice warm air before getting up to high speeds.

Any ideas?

Thanks.

 

Could be a weak thermostat that isn't closing far enough, so that at higher speeds the engine and coolant temperature lowers due to increased convective heat transfer.

 

My cold AC turns hot when im going about 120

 

Your coolant level might be low. So when the engine heats up at highway speeds, the thermostat opens up and uses up the coolant for the engine, not leaving enough for the heater.

You might also check your thermostat as it might be stuck open.

 

Above a certain throttle position the AC clutch disengages.