1973 Mark IV AC Blower Motor Questions

Bill,
On my 73 MK IV the AC blower motor decreases when I accelerate or encounter hills out on the highway. It spent the day at a reputable AC shop in Tyler. The check valve at the brake booster was replaced along with some hoses under the hood. Still the same problem. I note that this only occurs when the car is in gear. This will not happen with it
in park while revving the motor, AC works great in park and neutral while racing the engine. Drop it in gear and accelerate and the blower cuts out completely. What is causing this?
Pat
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Hi Pat –
That condition as you describe is a common problem and I believe that we have addressed it before on previous posts. The Climate Control system on your car is designed to revert towards Defrost and High heat output in the event of a drop in the vacuum supply to the controls. When you are operating uphill or accelerating etc. the engine vacuum drops and if there is a leak in the a/c system it will begin to adjust to the defrost/heat position until the vacuum returns when you are de-accelerating or traveling downhill. This is a thoughtful safety design that will activate for you in the event of loss of vacuum and is very welcome in freezing ambient conditions. The a/c system has its own vacuum reservoir and check valve. This valve can leak internally or there could be a leak anywhere that the vacuum is routed. This includes any hoses,vacuum motors or controls etc. If the person who is working on the car is not completely familiar or skilled with this design a vacuum schematic is a must in order to locate and test the correct components. I have serviced many of these cars since they were introduced and I have always had the service manual close at hand. Hope that the above helps.
Sincerely,
Bill