AC Compressor on always/ not cycling

[1983idi]

I decided to try and fix the ac on my truck since from what I've read your mileage drops just about as much having the windows down as running AC on the highway and AC is way cooler air and not as noisy. I pulled a vacuum on my AC system to try and find leaks but strangely the 40 year old AC system held vacuum without issue.

Since it held a vacuum with no problem I figured I'd charge it and see what happened. I emptied 1 12oz can into the system not running and then turned the ac on and added 2 more 12oz cans. The low side pressure got to 30 psi on the second can but didn't go up anymore when I added the 3rd can. AC blows nice and cold in the truck but the compressor clutch is always running and engaged.... On old systems like this is the compressor just always on when the AC is on? Anything else I have worked on the compressor cycles on and off? Is 36 oz of refrigerant not enough and that is the issue or is something else wrong?

 

[BrianX128]

I've never had one that did cycle. Not saying they shouldn't, but my 150 has the coldest ac I've ever owned. I put red tek 12a in it, the evaporator will freeze over and it will quit blowing any air. I have a toggle on the dash I can hit to flip it off for a bit when it freezes over in days that are in the 70s.

 

[ifrythings]

The low side needs to drop down to around 20-25psi before the compressor kicks out, it should cycle on/off when running. What’s your high side pressure?

 

[1983idi]

The low side needs to drop down to around 20-25psi before the compressor kicks out, it should cycle on/off when running. What’s your high side pressure?

I unfortunately don't have a gauge to measure the high side pressure. I'm just charging it with a cheap little hose that has a gauge and connects to the can. So all I can go off of it how much low side pressure there is to see if the system is charged.

My truck is an 83 and the system appears to be a dealer add on. The compressor is a York brand. Not sure if that would be different than a newer truck with how it operates or gets charged. I'm charging with r134 and not R12 in case that makes a difference

 

[david85]

Are you idling while the compressor is always running? Or can you confirm no cycling even when cruising down the highway? On while at idle is normal. On steady at 2000 RPM all day long is not. Sounds like the compressor is working fine, so my guess is that the cycling switch may be faulty.

 

[franklin2]

They are right, the pressure on the low side needs to drop for the clutch to turn off. There are many things that affect this.

If it's a hot day, and you have lots of warm air going over the coil in the firewall, the warm air will warm the refrigerant and make more pressure. On a cooler day in the 50's, you will see it start cycling as the coil gets colder.

You can overfill the system, and artificially make the pressure high all the time. But overfilling usually causes one of three things to happen. Either the high side pop-off valve on the compressor pops off with a "bang", the compressor gets too hard to turn and the belt starts slipping, or the coil in the firewall (evaporator) turns into a block of ice. Or some combination of the 3 happens.

I am guessing you are running a conversion to 134a? You can tweak the pressure switch if you want to change things. If you unplug the wiring plug to the switch, you will find a small screw inbetween the terminals on the switch. Turning this screw adjusts the trigger pressure on the switch. I have found the system works better if I turn the switch adjustment so the compressor runs more. But I only put 2 cans of r134a in a pickup truck.

 

[1983idi]

Are you idling while the compressor is always running? Or can you confirm no cycling even when cruising down the highway? On while at idle is normal. On steady at 2000 RPM all day long is not. Sounds like the compressor is working fine, so my guess is that the cycling switch may be faulty.

I am not able to verify if it is cycling while driving. Just at idle. I may have to pull the hood off to check that or try to splice a wire into the clutch wire and monitor voltage on a multimeter.

They are right, the pressure on the low side needs to drop for the clutch to turn off. There are many things that affect this.

If it's a hot day, and you have lots of warm air going over the coil in the firewall, the warm air will warm the refrigerant and make more pressure. On a cooler day in the 50's, you will see it start cycling as the coil gets colder.

You can overfill the system, and artificially make the pressure high all the time. But overfilling usually causes one of three things to happen. Either the high side pop-off valve on the compressor pops off with a "bang", the compressor gets too hard to turn and the belt starts slipping, or the coil in the firewall (evaporator) turns into a block of ice. Or some combination of the 3 happens.

I am guessing you are running a conversion to 134a? You can tweak the pressure switch if you want to change things. If you unplug the wiring plug to the switch, you will find a small screw inbetween the terminals on the switch. Turning this screw adjusts the trigger pressure on the switch. I have found the system works better if I turn the switch adjustment so the compressor runs more. But I only put 2 cans of r134a in a pickup truck.

I put 3 cans in so maybe I'll take some out and see what happens. All I did for the "conversion" to r134a was added the fitting that changes the lowside valve to a r134a style "fitting"

 

[gnathv]

Your 3 cans should be the right amount for 134. Are you getting good condensation (dripping onto ground)? Is there condensation to the back of the compressor (suction line, the bigger line)? If you want to see it cycle idling runa a water hose over the condenser or a strong fan. How much oil and type is in the system is the mystery for you. What’s the outside temp and what is your gauge on your can reading?

 

[gnathv]

What compressor is on your truck and it sounds like you did not pull the system in a vacuum, is that correct?

 

[gnathv]

This might help you.

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[franklin2]

I am not able to verify if it is cycling while driving. Just at idle. I may have to pull the hood off to check that or try to splice a wire into the clutch wire and monitor voltage on a multimeter.



I put 3 cans in so maybe I'll take some out and see what happens. All I did for the "conversion" to r134a was added the fitting that changes the lowside valve to a r134a style "fitting"

Did you use a conversion kit with the ester oil in the cans? The 134a won't pick up the old r12 mineral oil and circulate it around like it should. The conversion kit comes with r134a with ester oil in the can also. If you had some r12 left in there, it may be picking up the old oil. r12 and r134a all in there together sounds interesting. Not saying it won't work. I am curious if that is what you have, and how it does work. Always open to new ideas.