1969 Mark AC Issues

Hi Bill,
I am seeking consultation once again from you wealth of knowledge. I’ve also attached a picture to help describe what I “think” is the issue.
I recently repaired an AC leak which happened to be the large nut on top of the compressor that holds the suction manifold to compressor body. Leak Fixed. I pulled a vacuum on the system for 30 minutes and it held for 15 minutes so I went ahead and charged the system. I put 2-12 ounce cans of R-12 in and with the system set on MAX (ATC lever at HIGH and temp lever at 65) and a thermometer in the register, it was blowing at 40 degrees F. AWESOME!!!
So I shut everything down and put my equipment away. About 10 minutes later I started it up and it was still Ice cold, however after driving it less than a mile it started to get warmer and the air temperature held at a steady 60 degrees.
I figured it was low on charge as the shop manual states the system charge at 3 pounds, 3-12 ounce cans should be just under that requirement. I started with two cans as I didn’t want to overfill, but went ahead and put a third can in with no change in temperature.
My initial thoughts were temp door control servo but then realized it was cold with two cans initially and how can it change so quickly? So I decided to take a peek under the hood and saw the expansion valve (evaporator side) is caked in ice and the hose side is just cold. Does this indicate some sort of obstruction in the valve? I imagine it shouldn’t get that built up?
I should also mention that there is a lot of hot air running through the HVAC system even with it off… I’ll get sudden burst of hot air on my legs when driving once the car is warmed up. What would cause this?
Sorry for the lengthy e-mail but I appreciate any help you can give!
Dan
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Hi Dan –
From the looks of your photo It would appear to me that the expansion valve is indeed shutting down or that your de-icing switch is not cycling as required. To diagnose properly, a knowledgeable a/c technician will attach proper gauges on the High and Low side valves of the system in order to pin the problem down. The de-icing switch will cycle the compressor to avoid “icing up” the evaporator. The cycling times will vary in response to the blower speed and the ambient temperature as the sw. will act as necessary to avoid icing the evaporator. Another item that may contribute to poor cooling and incorrect system pressures can be a plugged Receiver/ Dryer unit. This dryer contains a filter which can plug up or partially plug up. A new expansion valve and receiver/dryer combination repair is not uncommon.
As for the hot air entering the car with the system turned off experience tells me that it can be mainly due to seals and air valves that are made of foam or rubber that have crumbled due to age. I also remember though that we had some complaints regarding warm air entering some vehicles on hot days with the a/c off even when the cars were new. The only way to avoid this may be to use all available ventilation features of the car or use the a/c if it is functional. It seems that the vehicles built with Automatic Climate Control were meant to be used continuously as designed.
So Dan, it would seem that accurate diagnosis is the key for your refrigeration issue. If the expansion valve proves to be the culprit I would recommend replacing the receiver dryer as well as a measure of good practice. If you do need any further advice or any repair parts please do not hesitate to call on us.
Sincerely,
Bill