Calling all A/C guru's!!

Ok, I have a 92 pickup with a aftermarket R134 a/c system. The a/c works and gets cold but the compressor won't shut off, the high side will surpass 300psi, I suspect the high pressure switch on the dryer is bad but the dryer is obsolete and I can't find one anywhere.

My question is, if I can find a different dryer with the same fittings and a high pressure switch attached to it, can I use it?

 

The high pressure switch is a safety not what directly controls your compressor
I’m not an expert in car Ac.
What is the reading on your low side???

 

Low side is about 20-30psi.

I suspect the high side switch is stuck, and keeping the compressor on, theres only 3 things that I found on this system that will control the compressor.
1. The A/C on switch, which works
2. The A/C thermostat, not 100% if it works but I know the air isn't cold enough for this switch to trigger anyways.
3. The high side pressure switch, my #1 suspect.

I dont see any other switches or component that would cause the compressor to stay on.

 

Do have a schematic or wiring diagram for this aftermarket system?

 

I wish I did...its got a sticker on the firewall stating it's a R134a system, looks like it was installed in Texas also, that's about it.

 

I suspect the ac on/off switch. Some more details would help. Does the compressor run whenever the truck is on? Have you checked potential or continuity at the on switch when in the "off" position?

 

Not an A/C mechanic, but I know enough to say it's not the dryer. Sounds more like it's the clutch on the compressor. When you turn on the a/c, the electronic clutch engages serpentine belt pulley, causing the compressor to spin. Turn it off, clutch disengages, allowing the pulley to spin but not the compressor. I believe that this is your problem.

 

The switch works, you can clearly see and hear the A/C compressor clutch engaging and disengaging when operating the switch.

I dont suspect the dryer is bad, I suspect the high side pressure switch located on the dryer is bad, but since this is a old system, I cannot find a new pressure switch or even a new dryer with the switch on it. That's why I'm asking if I can use a different dryer/pressure switch. I say dryer because I'm looking for a dryer with a pressure switch attached to it like the one pictured.

 

I am not an automotive AC guy, I work in residential. I just don't see how a high pressure switch could cause you compressor to stay running when you have a call for cooling. The design of high pressure switches in residential split system is to shut the system off when it gets too high. Maybe I'm wrong, but I think that's not your problem. Also, if your low side is 25-30 and the high side is 300, it does sound like you have a restriction, possibly in the filter dryer.

 

The high side switch is a safety cutout, they are typically set for somewhere between 400 & 500 PSI.
The Evaporator Temp Sensor/Thermostatic Switch or Low Pressure Switch (depending on the system) is what's meant to cycle the compressor to prevent freezup.
Typically high head pressure can be from the system being overcharged, low airflow across the condenser, a restriction in the system, or way too much oil in the system.

Ambient and interior temperature will directly affect your pressure readings, higher temps = higher pressure so on a very hot day you can see high side pressures approaching 300 PSI on a good working system.

 

So i removed the lines and blew compress air into the lines and condenser, no blockage there. BUT when I blow air into the evaporator from low side, air will come out the high side no problem, if I blow air from high side it will let alittle air out of the low side then it will plug up, is that normal? It's almost like I'm pushing a valve close with the air.
If I have a blockage it's gotta be that.

 

That is normal, the expansion valve in the line is a controlled orfice. That's how the system works.

Did you pull a vacuum on the system before you charged it?

 

Yes. I have access to a Robinair a/c system. I pulled vacume for 10mins, add 1.5oz of oil then charge.
The sticker on the truck calls for 26oz of freon, which is about 1.60lbs, I charged 1.30lbs to be safe.
I worry of the system over pressurizing because the high side gauge would get to 350psi if I revved the engine, I don't know if it would keep climbing if I keep revving it, I just stop when it got to 350psi.

 

It may just be how it is, I'd imagine a 92 would have originally been R12 and some R12 systems don't perform very well when converted to 134A.

Is your fan clutch working properly? Not enough airflow over the condenser can cause the symptoms you describe as well.

 

I feel this is a aftermarket a/c kit and not a retrofit from how it's layer in the engine bay.
I thought air flow also so I added 2 fans to the front and its still the same.
I thinking that this is just how the kit it made and im just over thinking it...I'm gonna charge it and send it...lol