Hey everyone,

Adding AC back into my car and can't get the clutch to engage. New (not reman) R134 compressor, mounted driver side. New AC hoses. New condenser. New evaporator core.

Added 8-9 oz of PAG oil, some into the back of the compressor some into the lines. Pulled vacuum on it with a vacuum pump for 2 hours. Let it sit for 2 days, and it still had vacuum so it isn't leaking. I removed the ECM long ago, so I don't have an ECM controlled AC clutch. My wiring uses the light green wire from the HVAC harness, goes to the low pressure switch on the accumulator (I bought a new switch, but got a working one from a member on here and both are not engaging the clutch). Charging the system and low side pressure gets to ~40psi, no clutch engagement. I jump the low pressure switch and the clutch engages. Low side pressure drops a bit, I disengage the clutch and continue charging. I do not have a high pressure switch in use on the compressor. Any advice on what I'm doing wrong?


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40 psi in a system w R-134A means either it only has about 2 molecules of refrigerant in it, or it's about -50° out.

The way refrigerant (or any other volatile chemical) works is, if you put some into a closed vessel, it will evaporate and pressurize the vessel until it reaches its "equilibrium vapor pressure" https://en.wikipedia.org/wiki/Vapor_pressure, whatever that is at the temperature in the vessel, as long as there's enough that not all can evaporate (i.e. some evaporates and some remains liquid). It will reach this same pressure from the point where there is just the tiniest bit of liquid and all the rest is gas, or vice-versa. Same pressure no matter how much, over a very wide range, with the pressure dependent SOLELY on ambient temp.

R-134A EVP is between about 60 - 120 psi in the ambient temp range you're likely to be working on an AC unit in. https://www.freon.com/en/-/media/fil...properties.pdf Meaning, if it's above freezing where you are and you have only 40 psi, there's absolutely no liquid refrigerant in the system whatsoever. It is GROSSLY under-charged.

Jump the LP cutoff switch. Leave it jumped. Turn the system on (compressor running). Charge it up. Should take about 3 full cans (something around 40-some-odd ounces). Pressures with the compressor running when properly charged should be around 30 - 35 on the low side, and anywhere from 180 to 350 on the high side depending on ambient temp, with the interior fan on high and the radiator fan running. If your ambient is below about 50° or so, the low side pressure may go lower than that with the switch jumped, and that's because it's in the range where the LP cutoff is supposed to be cutting it off and waiting for awhile until it warms up and enough evaporates to bring the pressure inside the system back up; maybe a 20 - 30 second Off period between compressor operations.

 

Thank you Sofa, makes sense. I should've paid more attention in Chemistry than I did.

Does keeping the clutch jumped damage the compressor? As in, should I jump it every 30 seconds or so to have an on-off cycle to limit low-refrigerant run time?

I kept adding from the point I wrote that post. I got to 15 psi on the low, and almost 225 on the high and the compressor cycle was one for approx 5 seconds then off again. It was only 65 degrees here. Would you be nervous about that high side pressure, or should I keep adding until I see the low side pressure where it needs to be and get the compressor cycling, rather than worry about high side pressure value?

 

I did use the Ford blue orifice tube, by the way

 

No. Not over the time frame of charging it up.

No; sounds completely normal.

This.

Compressor cycling isn't a necessity, in and of itself. Should only happen very much if the interior fan is down low and the ambient is cool.

The low side pressure controls the evaporator temperature. It needs to be what it needs to be, in order to extract heat from the interior air.

The high side pressure is dependent, from there, on the orifice tube. after all, the compressor is just a big dumb pump; it takes in whatever cold juice is available on the one side and spews it out the other. The pressures in the system are dependent on the temperature, the amount of cold juice in the system, and the OT restriction. I'm not sure what the effect of the Frod blue OT might be. I don't generally use that one myself.

 

Thank you for the advice. Jumped the compressor and charged the system, which only took ~20 oz of refrigerant. Ended up at 40 on the low, and 300 on the high. Engine wasn't warmed up so the radiator fan wasn't on.

Do those numbers seem about right to you? Should I release some refrigerant to get back to 35 on the low side? Vent air is pretty cold right now, I'm happy with it just a bit suspicious of the 300 psi at idle on the high side.
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If it works like that, let it ride, and revisit when it gets warm out. All the pressures will come down when the radiator fan runs, and/or go up when the ambient temp rises. After all, an AC is a heat conveyor belt, at its core; it absorbs heat from the interior by letting the cold juice evaporate, which lowers its temp, which allows it to absorb the heat; then by compressing (raising the pressure) the gaseous cold juice and thereby also raising its temperature to well above any likely ambient, allows that heat to be dissipated into the environment, by air flowing over the condenser and making the cold juice condense from gas to liquid. If the rad fan isn't running, then the heat isn't being sent out into The Wide World; instead it's just accumulating in the juice, which raises all the pressures. Without that fan running it's not a true test of the AC system's performance.

 

Thanks Sofa, I'll let it roll as is and if there's any problems I'll update this thread. Good ole refrigeration cycles! Thanks again.
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You should have your radiator fan(s) programmed to come on whenever the AC compressor is running. It doesn't matter whether or not your engine is up to operating temperature. The fact that your fan(s) aren't on is why you're seeing the higher high side pressures.