My '90 has never had any R-12 added and it blows nice and cold. On a recent trip home from CA (through the desert at 105+ degrees) the evaporator froze one time. The humidity was high too. As soon as I ran the 'vent only' for about 5 minutes the A/C cooled normally. It only happened once but I wanted to check for a low charge.

Today (85 degrees 16% humidity) I checked the pressures. I ran the system on MAX recirculate for about 15 minutes at 1800RPM, the low side was about 28psi and high side was 230psi. The low side pipes frosted badly during this time and the center vent was putting out 35 degree air.

Next, I put the system (electronic system) to normal bi-level and set the temp to 72 degrees. I had 58 degree air out of the center vent but the compressor never turned off. The lowest pressure I saw on the low side was about 25psi.

Do I need to replace the low side cycling switch? I checked it with the engine off and it has continuity. I did not adjust the switch with the center screw but I suppose I could, but, if I never see the low side go below 24.5psi, it will never cycle(per the FSM). Why would my low side stay above this? Over charged? Not sure how that could be unless it self-generates more R-12 after 22 years?????

I appreciate any thoughts on this. I have no fault codes or blinking LED's on the display.

Thanks,
Mick

 

I thought that it would freeze up if the freon is too low not with it too high.

 

Thanks Ben,
That's what I thought too but if the low pressure switch is the only thing that cycles the compressor, it's a possibility too.

 

It shouldn't ice at 28 psi, so your gage may be off. Most can be calibrated, make sure it's at zero before you hook it up. I realize that R12 is 32 degrees at 30 psi, but there's 6 to 8 degrees of superheat meaning the core is well above freezing to about 25 psi which is the designed threshold of the Low Pressure Switch. You can stick your digital thermometer on the Evaporator outlet and compare the temp to the pressure/temp on your gage. The difference is the superheat.

I'd do a couple of things - get your gage calibrated or a new set or have a shop check it. Crank it up to max again and disconnect the blower motor which will drop the low and cycle it at whatever threshold there is in the switch which you can view on the gage. These switches are adjustable with a screw between the terminals which you can turn counterclockwise to lower the threshold and clockwise to raise it. Off hand, a 100 plus day doesn't see anything close to 28 psi on the low, unless it's low on gas, and I'd guess it needs 12 ozs. If you go to a shop, have them leak check it with an electronic sniffer.

 

Thanks Sun,
The gauges are fairly new and the low side gauge agrees with the high side gauge. I will go to max Recirculate and disconnect the blower motor to see what the low side drops to. IF there is a leak, it is VERY tiny since it's held R-12 since 1990 with no service at all. I will
also make a pair of jumper wires so I can adjust the low-side switch with the harness connected. I have two cases of R-12 that I bought a LOOOOONG time ago so I'm good there. I'll add 12oz and see what happens after I do the blower disconnect test.

 

I think it's fixed! I put my jumpers in place between the low pressure switch and the connector. I set the A/C to MAX Recirculate and ran the engine at 2,000RPM for about 10 minutes. The center vent was about 38 degrees and the outlet at the evaporator was 46 degrees. I pulled the blower connection and watched the low side drop to 23psi. It took one full turn clockwise on the switch to get it to stop the compressor at 25psi exactly. My static pressure was 90psi before starting this morning.
I had frosting on the low side all the way from the accumulator to the compressor at this time. I then added about 14 oz of R-12. The frost went away and became condensation. I have 32psi Low and 265psi High with 90 degree ambient and 28% HUMIDITY. Cooling great and no more icing on the low side lines.

Thanks for the help

 

Sounds perfect - congrats! By the way, the replacement Low Pressure Switches I've come across have been set for 22.5 psi which is the threshold for R134 (as it freezes 2 psi below R12). I found that out when I got ice following a rebuild. If anyone is replacing one and sticking with R12, I recommend you check it with the outlined procedure and adjust to 25 psi as needed.

 

Restriction can casue frezzing, block or dirty evap can cause frezzing, low Refrigerant can cause frezzing.

 

on the PT chart for 134a frezzing is 27.8 psi = 32 degrees 25 psi= 22.1 degrees which is below frezzing

 

BWD,
We were working with R12, NOT R134a freezing is 32 degrees. 25psi is the correct setting for the low-side cycling switch if you are using R12.

Mick

 

[QUOTE=QZRBLU;1586486295]BWD,
We were working with R12, NOT R134a freezing is 32 degrees. 25psi is the correct setting for the low-side cycling switch if you are using R12.

Mick[/QUOTE
Actually freezing for R-12 would be pressure of 30.82=32 degrees

 

1, dirty evaporator coil or filter, not enough airflow.
2, Faulty low pressure compressor cycling switch.
3, Radiator Cooling fan on continually causing over condensing (the low pressure switch should cut the compressor off before the coil freezes)

The system is designed to turn the compressor off when the pressure gets low, or when the evaporator is starting to freeze.

This protects the compressor as liquid refrigerant cannot be compressed, bypassing the low pressure switch is not the way to go.

As others have said, the evaporator has a filter screen which can get blocked with leaves etc.

How is the airflow when you have the a/c on, is it blasting air out?

There is also a drain in the evaporator box, can you see water dripping on the ground while a/c is running?

A few things to check

 

Sounds like it needs a orifice tube.

 

Sounds like moisture in the system and the orifice tube is freezing up.

 

Gerard,
Go back and read post #6, my problem was fixed in Sept, 2012.

BWD resurrected this old thread yesterday for some reason. My A/C is still working perfectly since adjusting my low side switch from 23psi to 25psi and adding 14oz of R12. Evaporator is clean, drain hole clear and A/C blasts just great on High or Max A/C. This system has never been opened.

Thanks,
Mick