1979 Mark V AC Questions

Hi Bill –
I have a 1979 Lincoln Mark V with 10K original miles and have a A/C issue.. When I bought the car I found out from my mechanic that the A/C compressor has a leak and would need to be replaced..It had already been converted to R-134 before so I had the compressor replaced and filled with R-134 again..This was done in early spring and it seemed to work fine but what the climate control system does is when you start the car and turn on the A/C it works fine..When you put it in drive and drive slow it still works fine however when you get on the highway and get it up to speed like over 30MPH approx and above with the A/C temperature selector set as low as it will go the climate control system kicks in and slightly warm air starts coming in from the floor heater ducts and a little from the dash and then the A/C will come back on and start blowing cold again and then back to the heat mode..Keep in mind here in Arizona when I drive it is above 100 Degrees F outside. The A/C should be blowing colder for a longer time and the climate control system is kicking the A/C out sooner than it should..I was wondering if it could be the A/C temperature control sensor that is bad and is not recognizing the true inside cabin temp inside the car..What are your thoughts??
Thanks,
Bill
—————————————————————————————————————-
Hello Bill –
The system on your Mark does not cycle the compressor so therefor it is engaged all of the time that the Climate Control is on. The compressor will be off in the Vent position, the Heat position and in the winter when the ambient temp. is under approximately 35 degrees F. Loss of vacuum anywhere in the control system will cause a drift towards blended heat and defrost. You should check the ATC vacuum system as per the factory service manual for a vacuum leak or a vacuum servo internal leak. A system that is undercharged with refrigerant can loose some cooling at higher speeds because of refrigerant shrinkage but will not cause a default to the floor ducts. Did you check to see if any air was blowing out of the defrost ducts when the symptoms appeared? The ATC temperature sensor above the glove box assembly. can also be faulty as you suggest but it also cannot cause a default to heat at the floor ducts. Another possibility is some vacuum lines crossed under the dash or under the hood. Vacuum can leak anywhere that the vacuum is routed including the a/c vacuum reservoir and its Vacuum check valve. Engine vacuum can drop at higher speeds and if there is also a vacuum leak somewhere in the system, the controls cannot and will not maintain maximum cooling. I think that you will find that air is also blowing out of the defrost ducts when this heating occurs. In any case the a/c vacuum system should be tested in a logical sequence ( do not omit any a/c lines or vacuum motors) using the proper vacuum pump with a built in vacuum gauge and shop manual with the vacuum diagrams in order to pinpoint any leaks. Show this reply to your a/c mechanic. He may be experienced and able to locate the problem quickly. Some vacuum leaks can sometimes be spotted easily under the hood. We wish you a speedy repair.
Sincerely,
Bill