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1971 Mark III AC Questions

I have recently acquired a 1971 Mark III with 54,000 original miles, car is in very good condition overall. AC was blowing hot air. Compressor wire was disconnected when I got it. I reconnected wiring and compressor engages and is working as it should. I put the system under vacuum and it held vacuum for 90 minutes without dropping. So I went ahead and purged out all old R-12 that may have remained and put adapters on for 134a refrigerant. Added oil and then the proper amount of 134a to the system. Car was then blowing about 42 degrees out of vents with AC on and I was very happy. Two days later, I take it for a ride and notice what looks to be smoke/fog coming out of defroster vents inside car (AC was not on at this time). I pull over and look under the hood and there is no smoke or problems under hood. No smell of smoke in the car either. So, I think back to recent AC work and turn on AC and it is blowing hot. A leak detector was used and did alarm a bit when stuck into the defroster vents inside the car, this was a week after the AC/smoke visible in defroster vent incident.
With the limited information provided, does this seem like an evaporator leak? Is the replacement of the evaporator a tough job? Do you remove the evaporator from under the hood area or do you have to get at it from inside the car as well? Any guidance you can provide would be appreciated.
Thank you
Greetings –
Congratulations on your recent purchase of the 71 Mark III. They are great cars to have and they are well liked by all car collectors. We look forward to helping you as our customer with repair parts as necessary to help you maintain it in good condition.
The symptom that you describe with the smoke and fog emitting from the heat/ac ducting is a common description of a classic heater core leak. Tiny amounts of anti-freeze can leak onto the hot heater core fins and then become steam. A leak in the core can usually be proven by disconnecting the heater hoses at the core and joining them together. If after driving the car a few days with the core bypassed the fog is gone, reconnect it and observe for the fog to reappear. The refrigeration system will not emit steam or fog if it develops a r134 leak. I can’t tell if the charge is actually lost of course because you may not have checked the pressure in the system to find out. Hot air discharge when cool air is needed could also be a control problem.
To sum this up for you it does sound that the heater core is leaking and also that the refrigeration system has lost the charge of r134. If the a/c has indeed lost its charge I would not be at all surprised because when you purchased the car the compressor was disconnected and the system was empty. This is strong evidence of a previous undiagnosed leak. The copper evaporator coils on the Mark IIIs have proven very durable in the past, therefore the whole system should be leak tested carefully to prove a suspected leak or leaks to avoid replacing good components. A system that holds vacuum for a period of time is not absolute proof that a leak does not exist. We test for refrigerant leaks with nitrogen pressure at 120 lbs psi and if there is a drop in pressure after a good period of time, the remaining pressure can then be used to pinpoint the culprit. The heater core and evaporator core are both removed from under the hood as per the Factory Shop Manual. The evaporator will have two fasteners under the dash as well. We have new heater cores available and good used evaporators available for you as required.

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