I recently went through my a/c system. It hasn't work over the last couple of years. I replaced everything except the evaporator. And converted all to 134. It blows good and cools good... in my shop.Thermometer in the vent shows 47. This weekend I went on a 420 mile trip. It starts out cooling, then falls off. temp. and air flow. Turn it off for a while, it would start cold again. Only for a while. I think the evaporator is freezing over. I just put my gauges on it. ambient air here now 77, low side 25, high side 290. What do I look check next? thanks
1972 coupe 350 auto

 

290 seems pretty high for the hi side at only 77 deg. Indicates an air flow problem across the condenser, or an overcharge. Your other problem sounds like the evaporator is freezing over, how much 134 did you put in it?

 

I think you got it right! The evap will freeze and stop the air. Once it thaws out the air will flow again and cool air.
I am not a system expert but remember this problem. Newer systems have a shut off to keep the evap from freezing. Not A good idea to install if you are running a A6 compressor. Again I am not a system expert, POWER SHIFT is and will help as always.

Dom

 

At 77, your low side should be somewhere in the high 30s and your high side should be somewhere around 160. When the low side is too low in combination with the high side being too high, you have a blockage. It's usually the orifice tube. Sometimes when a compressor burns out, pieces of the pump get in the line and can clog the orifice screen. I see you replaced everything except the evaporator, did that include the lines? If not, did you clean them and the evaporator to get any possible junk out? Also, if your evaporator is dirty between the fins, the air volume flowing over the evaporator is reduced, and this can cause it to ice up. Good luck.

MajD

 

75 I'm not exactly sure on how much I put in. I was watching the gauges more than the amounts. This is my hands on education on A/C. If that is why I'm freezing over, I don't mind extracting and starting over.

MajD I did replace the manifold hose. All others were flushed and blown. I thought the orifice tube was on the later models, not the earlier models. I did not remove the evaporator but looked with a camera and looked clean. If it was dirty, would it blow good in the shop(long periods) but freeze on long runs?

 

In the shop it's at a lower RPM. Not getting as cold. Get it on the highway up around 3,000, you got a lot more cold in that evaporator. Then that nice wet humid air blowing through there it's going to freeze right up on it.

 

olelucky
Assuming you retained the poa valve in your conversion I would suspect that it is not functioning properly. The poa acts to restrict or block refrigerant flow, rather than cycling the compressor on and off, to prevent the evaporator from freezing over. Its a bitch to remove. Is your poa new or rebuilt??
VS

 

 

I apologize, but I believe you are correct that your car does not have an orifice tube.

I still think your evaporator needs blown out. The evaporator in your car was designed to treat a specific volume of air (CFM) passing through. As the air cools, the moisture condenses out and drips from the bottom of the housing. Since heat from the air is transferred to the evaporator, it stands to reason the the refrigerant warms up as a result. This interaction between the warm moist air and the cold evaporator coil is affected by that volumetric flow. When the airflow is too low, it cannot provide enough heat to keep the evaporator above freezing, which causes the condensation to form ice, blocking more air and further aggravating the problem.

Since the evaporator is warmer at low RPM than high, it's possible that the airflow still adds enough heat to keep the evaporator from icing up. It's also possible that at idle, the refrigerant itself doesn't get cold enough to actually freeze the water very well, or at all. However, at high rpm, the evaporator cools even more as the a/c compressor runs faster, but the airflow does not change because it's a product of the electric blower. If the temperature of the evaporator is now too cold for the air flow, then it will start icing up. This isn't instantaneous, and i don't know how long you've let it idle in the shop vs. how long it takes to ice up on the road.

I can think of no other cause for an evaporator to ice up other than insufficient airflow across the coils. Hopefully you found this information helpful, and not something you already knew. Good luck.

MajD

 

I'll go into the evaporator box
and make sure nothing is blocking air flow. thanks MajD

 

Do you have a compressor? Since the evaporator was not replaced, you should blow air through the coil in case dirt has become lodged in the spaces between the fine aluminum fins. Over time these can become clogged, especially since the evaporator can get damp. i hope this fixes your problem.

MajD