70 Cutlass Supreme Convertible with factory air - recently services and recharged (R12). When I turn on the A/C it works great and blows really cold. However after about 45 minutes is apparently freezes up and restricts the flow of cold air. Any suggestions or guidance?

 

I didn't know there was such a thing as air conditioning that blew too cold In the summertime in North Carolina, I equate A/C with life support.

 

Let's back up. Is airflow through the evaporator restricted, or is the air just not cold? Those are two different problems.

 

 

Thanks for the reply Joe. The air stays cold all the time but after awhile the air flow is restricted. If I turn it off for 20 minutes or so and then turn it back on the same thing happens.

 

The evap, when the system is working correctly, can get below freezing. So in certain situations the normal water vapor in the air (especially in hot/humid areas) will freeze in the evap, turning it into a large block of ice. As noted, a problem with the expansion valve or POA would cause the refrigerant to not be regulated correctly and make the evap far too cold. In my experience with these old systems the most likely cause is a system problem - incorrect charge, bad POA or bad expansion.

However, there's a simpler possibility.
There's two sets of connectors on the outboard side of the A/C box in the engine compartment. One is a set of resistors for the blower motor. The other is a freeze switch for the A/C. It's inline with the power feed to the compressor clutch. The idea is if the evap freezes, the air on the cold side of the evap gets really cold, and this switch (it's just a resistance wire) opens, thereby disengaging the clutch until the evap opens up.
I've had evaps on newer cars do this, especially when their freeze switches failed. That was a painfully expensive lesson. I'm only hesitant because freezing one of these old systems is pretty hard, so those freeze switches shouldn't get triggered except in very unusual situations. Or Florida.

 

Thank you gentlemen for the guidance - I now have a place to start.

 

Most of the time it is a low charge.

 

From the pictures , I would say that you have a stuck or restricted POA valve . (That's the part that's "frosty" .)
Any time you have a restriction in the system , the parts of the system that are downstream of the restriction will get frosted .

 

x2 on the low charge!