A/C system pressure - compressor turning on and off

[2017GBGTPP]

I'm having an issue with the A/C Compressor working erratically. Here is a video of it with a gauge connected to the low service port, where it is 30psi when the compressor runs, then the compressor stops and pressure rises until it gets to about 45 psi and switches on again. Common knowledge says 30+ psi is good, but I noticed in the Workshop Manual diagnostics section that pressure should be 42 psi, is this the issue? Do I just need more refrigerant? 42 psi seems high, especially at current cool temps and I don't want to overcharge it.

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[SheepDog]

Compressor quickly cycling on and off is generally a sign of a low refrigerant charge. Just add a little bit until it comes up to that 42 PSI value.

 

[2017GBGTPP]

Compressor quickly cycling on and off is generally a sign of a low refrigerant charge. Just add a little bit until it comes up to that 42 PSI value.

That's kind of what I'm thinking. The thing is, I just ran a test getting the same results initially. I let it run with full A/C on for about 15 minutes and the compressor started running consistently, no stops, pressure steady at 30psi. I then took it out for a short drive, turning off the A/C at one point and then turning it back on and the compressor didn't engage. Checked the low side pressure and it's steady at just below 60 psi.

 

[Cobra Jet]

Aside from the psi testing, I would black light it.

Most auto parts store sell the A/C leak detector kits that includes the black light. With hood open and in a preferably low light area, use the black light and search every hose and fitting going to/from the A/C compressor, dryer and around the A/C condenser. The black light will reveal any leakage from hoses, fittings, or breached areas of the A/C condenser.

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It could also be something very simple like a failing A/C pressure switch. On older Mustangs, one could pull the connector off the A/C pressure switch, jump/bridge the connector with a paper clip or U-shaped wire. Start vehicle and If the compressor kicks on and stays on as it should in Defroster or regular A/C mode, then it’s a bad pressure switch. Usually A/C psi switches are no more than a $30 part and easily swapped out.

 

[2017GBGTPP]

So I borrowed some good gauges and got some UV leak detector. Didn't find any noticeable leaks throughout the system, but did notice my condenser was pretty dirty so I washed it off. There are some bent fins, but this isn't too bad it's it?

When the compressor is running, these are the pressures

And when turned off and allowed to equalize I get this

The high and static pressures seem high, yea? Or is this normal for this system? The good news is that the compressor runs consistently now, but is still noisy.

 

[2017GBGTPP]

So I found this in the Workshop Manual, which answers some of my questions about pressures.

In this cool weather, the high pressure should be between 150 and 230. The table below the graphs also gives some direction on what the problem could be based on the pressure readings. Out of the three causes I think air in refrigerant is the most likely, and since the compressor is whining, I should probably just replace it and flush out the system.

 

[2017GBGTPP]

After reading through many of the A/C threads, I found one commonality between every possible solution. Evacuating, vacuuming, and recharging the system. I opted to try that out and I think it worked! The compressor runs properly, pressures are correct when running (~30 psi low, ~200 psi high), and the A/C blows cold. Fingers crossed!

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