OK... Here's my problem: I have a 1985 Mercedes 500SEL that I bought last year. The AC was blowing hot. The low pressure switch was activated. Since it is a R12 system, I bought some Freeze 12 to top it off. I pulled a vacuum overnight, and then put in Freeze 12 (a R12 substitute). The compressor then kicked in, after the low pressure switch closed. The outside temperature was 83 degrees Fahrenheit. The low side gauge read 50, the high side read 250. The air was cool, not cold. I drove the car a few days. This morning, I decided to add a little more Freeze 12. As I was adding the Freeze 12, it seemed like a safety valve on the AC compressor started releasing (a popping noise). The low side pressure never exceeded 65 pounds. I stopped adding Freeze 12, the popping noise stopped (maybe the pressure equalized inside the system). It cools a little better, but not like it should. Any suggestions on what I should do next?

I also should say, my 1985 380SL was also blowing cool air. I added some Freeze 12 to that system. It now blows cold, like it should. (low side – 40 lbs, high side 250 lbs, outside temperature was 95 degrees Fahrenheit).


David Kelley

 

Is their oil in your system? When inserting the freon was it liquid or gas? The can upright is gas. The cooler in front of the radiator if it is dirty/oxidized it can prevent cold air. Good doushing of cleaner and pressure wash off helps, not to close to bend the fins with the pressure washer.

 

I added Freeze 12 from a 30 lb jug, (upright) so it went in as a gas. The condensor looks clean. This past winter I replaced the heater monovalve, as it was leaking. I need to make sure that it is working and shuting off the flow of hot water to the heater core. On another forum, someone answered: make sure the expansion valve was working. The low side pressure should be around 34 lbs. (mine was at 50 lbs.) although the high side pressure at 250 lbs. was in the correct range. I guess I'll have to read up on how to check the expansion valve.....

 

Expansion valve is kind of a misnomer. It's essentially little more than a venturi. Basically it can be open or blocked. To check it, the line going into it should be cold, the line coming out even colder.