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Discussion Starter · #1 ·
Hello, all! I have a 2003 Grand Marquis that has severed me well, but has been having some AC issues lately. One day, it overheated a little bit, not bad enough to throw a check engine or boil coolant. I turned on the heater for a bit and the temps came back down. The next day, I topped off the coolant (it has a small leak, I'll get around to fixing it some day) I went to drive it and found that my AC wasn't working anymore. I popped the hood, and the AC compressor was cycling on and off rapidly (2-3 seconds between cycles.) I rented a set of AC manifold gauges and bought some refrigerant, and found that the low side read 0 psi. I tried to add refrigerant, but it won't take any. I figured it was because the clutch wasn't engaging long enough to pull refrigerant in, so I pulled the low pressure switch and shorted it with a paperclip. Now the clutch will stay on, but it still won't take refrigerant.

In the process of diagnosing the issue, I reached my arm up into the dash, felt the blend door and messed with the hot/cold heater control, and found that the blend door is in fact moving, so I don't believe it is a blend door issue.

Any ideas what is up with it? Thanks!
 

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I don't think your low pressure actually depressed the stop in the LP port. When the compressor isn't running, the low and high side will be near equal. At 80 degrees, you will have about 86 PSI when the system has been idle.
 

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Discussion Starter · #3 ·
I
I don't think your low pressure actually depressed the stop in the LP port. When the compressor isn't running, the low and high side will be near equal. At 80 degrees, you will have about 86 PSI when the system has been idle.
I just ran outside to test that theory out. I pressed down the Schrader valve on the low side, and refrigerant sprayed out. So I connected the low side gauge up and it read 107 psi. Earlier today when I was trying to charge it, I put 107 psi from a can into the low side, and it looks like it retained that same 107 psi in the line over the last few hours. It also doesn't change at all whether the clutch is engaged or not. If the system were truly at 107, it would be blowing cold and it wouldn't be giving me the AC clutch cycle issue, right? Due to the fact that the compressor being engaged or not doesn't change the low side reading whatsoever, I have a suspicion that something is blocking the low line somewhere. Is there a sensor/valve that may be blocking the low side?

As another note, when I run the car with the (nonfunctional) AC on, the line with the low side valve on it doesn't get cold at all. The line with the high side valve gets warm, and the line directly below the low side valve gets cold. I'm not sure what that means.
 

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It means your system isn't moving refrigerant. I'd be interested in seeing what you have for pressure and temperatures on the lines.
 

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Discussion Starter · #5 ·
It means your system isn't moving refrigerant. I'd be interested in seeing what you have for pressure and temperatures on the lines.
At 97 degrees F ambient temp, the high side reads 125 psi with the compressor off, and bounces to a hair under 150 when the compressor turns on. The low side will hold whatever pressure I feed it from the can, but doesn't change when the compressor turns on.
 

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I think you have a problem connecting to the LP side. On the HP side, the pipe coming off of the compressor should be about 130°F (there's wiggle room here). If it's hotter, you may be low on charge. If it's cooler, you may be flooding back from the evaporator. Operative word "may". There are other factors that can come into play.
 
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