Air Conditioning (mis)behavior

Car: 1973 Mercury Capri 2.0, four speed. Lovingly restored over the past three years. The car has Ford aftermarket air conditioning, recently serviced with R-12 (past two weeks) and I have never used it until this past weekend (I bought the car and then restored it). We used it in 90 degree heat and this little car did great even in the mountains of W.Va (60 mph uphill with A/C!) But I digress. For five minutes after being turned on, the A/C cools as expected - a real champ – no issues. Then, it begins a cycle like this: The air starts to get progressively warmer for about 10 seconds. Then, there is a squeak, a drop in motor rpm as the A/C/ clutch engages, and it has cold air again for 20 seconds. Then the cycle again. This is the behavior whether at idle or at highway speed. It has its own two knob control panel – not connected to the heater controls. The fan speed is on high and the “cold” knob is turned all the way cold. Turned “warmer” it does as expected - very little cold air. It seems like a thermostatic control is telling it to disengage the clutch even though it should not – the cabin is cool but not cool enough to justify that. Any thoughts? Picture of the car attached.

Bowie Maryland USA

1973 Capri
1954 Citroen Traction Avant
1973 Beetle

Yes and no. This type of AC does cycle on and off, unlike newer factory systems that blend hot and cold air.

Your problem is in the evaporator. There is a sensor that is placed on the “downwind” side of the evaporator coil that if it gets out of place, it will do exactly what your AC is doing.

I agree with Keith’s diagnosis. I had a 1971 Ford Maverick with factory air. It would cycle on and off. I could feel the compressor kick on and off as I drove the car. However, I don’t remember it cycling as frequently as your air conditioning unit is cycling. You might want to check the line pressures as well as the sensor at the evaporator to be certain you have a full charge of refrigerant.

Thank you Keith and Triedaq. In search of a new evaporator sensor!

It doesn’t necessarily need to be replaced. They are held in position with a little tar like substance or a clip. All thy have to do is fall out of place due to vibration or shock to cause this problem. This is not a simple sensor to replace, it usually means replacing the evaporator core, but putting the “bulb” as it is sometimes called back in place is not a big deal.