AC blows hot on long acceleration

[LaOtto70Charger]

So yesterday I had my AC system switched over to R134a.  It blows very nice cold air when the car is at idle and cruising.  From a stop and accelerating a decent clip up to 60 though it starts blowing warm air.  When I get to speed and go to cruise the air gets cold again. 1970 with 318.

I checked the fan belts and there wasn't much play in them.  (Didn't measure though.)  No odd noises from the engine bay.  So thinking fan clutch slipping.  Is the air gap adjustable at all?  Any idea what the specs are?

[Sixt8Chrgr]

Sounds like you may still need to add freon. I had the same issue on another car and was low on freon.

[PlainfieldCharger]

I would take a IR temp gun and measure the temp on the in-coming and out going heater hoses with the car idling with AC on. The control valve on the firewall or the valve in the AC heater box could be leaking enough fluid under high revs to heat up the heater core a bit... the 134A change over is less efficient than the R12. You could even "pinch" the incoming heater hose to rule the valves out in the cooling system. Just a thought....

[Mike DC]

     
Definitely look for leaks.  R-134a is smaller in particle size.  It may leak out of gaps that were too small for R-12 freon to squeak through.

The two freons also call for different charge pressures IIRC.

[LaOtto70Charger]

I will check the temps and look for leaks.  They had dye also just in case so will drive by and have them take a look.

Wasn't thinking it could be speed dependent in this direction.  Thank you for the thoughts.

[c00nhunterjoe]

Could also be overcharged and the high rpm is causing an overpressure.  5 minutes with a set of gauges and rpm changes should give you your answer.

[John_Kunkel]

When accelerating, the engine vacuum drops off; the HVAC doors are controlled by vacuum so the A/C blend door goes to Heat...fairly common complaint. Adding a vacuum reservoir between the engine and the control panel will help.

[LaOtto70Charger]

Vacuum falling off is a good one.  I will look into that one first probably, and make sure the original vacuum can is still good.  Not to mention all the little hoses. Will also have the pressures checked.  Again thanks for the feedback guys!