I think the first thing people are going to question is your pressures at cruise. Its a little risky, but you could jump your pressure switch temporarily and if the problem goes away you know its the thermostat switch in AC unit.

 

That is backwards from what I consider normal. Do you have the thermostat turned to full cool or somewhere in-between? Maybe there is something wrong with that switch.

Does it have a high or low pressure switch? I have a similar setup and mine doesn't. If yours does maybe it's slightly over charged.

It could be freezing up on the highway. Our old 85 F150 would occasionally do that.

 

Could it also be high pressure and the system is protecting itself ?
May be the precharge is too high, ok at idle but goes dangerously high when working ? Switch might be ok but indicating a problem ?

 

I did install a pressure switch but I don’t think that’s the issue since I can actually hear the thermo switch in the under dash unit clicking on and off.

I did a little test in the driveway. At max cooling and low fan the thermo switch seemed to trip off 35-40 deg and on at about 65 so I assume the thermo switch is working correctly.

My theory which may be out to lunch, is that at idle the evaporator takes a long time to get cold enough to flip the switch so I get plenty of cooling time. At higher rpm the evaporator gets very cold quickly even with the fan on high so it trips the switch off leaving me with no real cooling until the evaporator warms up to trip the switch back on. But, again it gets cold fast and trips off again giving only a short time of cooling.

I was considering maybe pulling the thermo switch sensor out from between the evaporator fins and just have it sitting in front of it.

 

You may also want to check the evaporator for frosting up.

 

Are you certain that you have the correct amount of refrigerant in the system?

Is the new evaporator matched in size to the condenser? A balanced system is important.

 

I believe the evaporator is the correct one for this kit. My ac guy put in 20 oz of 134 which I believe it correct for the Old Air system.

 

I'd give the folks at Old Air Products a call. My experience with their customer service was very good.

 

Are you using a stock style or high efficiency heat exchanger in your stock under dash evaporator? I wonder if the probe on the evaporator switch is properly seated? Could there be an issue with the expansion valve since the rpms increase, but you are stating that the system becomes less cool?

 

The evaporator was from NPD and had more tubes than the original but was simply sold as a replacement.

It seems the thermo switch has the compressor off more than on when cruising but allows it to stay on for longer periods when at idle or in traffic. The net effect is the air coming out of the ac is colder at low rpm.

I’m not sure how to seat the thermo switch probe. I simply just pushed between fins.

My under dash unit has a screen in front of the evaporator. I assumed it was to protect the evaporator so I but it back in but maybe it’s impeding air flow and causing my issue.

Is there supposed to be a screen there ?

 

The location of the temp sensing probe could play a big part because with it stuck in the fins it's going to be sensing some cold temps ,way colder than the cabin temperature.
Seems to me it would work more accurate senseing air temp being blown over the evaporator coil .

 

So I pulled this screen out of the under dash unit. I think it’s someone’s idea of evaporator protection but my guess is it’s restricting air movement and causing my poor ac performance.

Will see soon.

 

Last week I had my AC charged for the first time. I have a Vintage Air Front Runner on the engine and an Old Air handler inside. When I first when ran the system the evaporator would ice up at highway speeds. I lowered the thermostat setting to about half way from near maximum and it works great now. Old Air's evaporator is quite small but that's why I bought it.

 

I have the CAA system. I had the pressure switch go bad, when about 50 miles form home and 100+ F. I hot wired that puppy until I got home and ordered a new one.