AC Low side reading 110 psi with compressor off....what?

[Keven]

Just confirmed with a 2nd gauge. My A/C on the low side with the compressor off is reading 110 psi. Isn't this way high? What the heck happened?

 

[roadhard1960]

Pressure will equalize. High side should read the same number. No idea what the number would be for a "normal" working system.

 

[Dirtracr95]

Not sure on vw but most of the cars ive done ac work on equalize around 50-60psi. When running high side is like 150-175psi low side is between 25-35psi

 

[Enabled]

That's normal with the compressor off. Then watch the pressures when you turn it on. The low side will drop to 25-40psi, and high side should go to around 150-200psi.

 

[Keven]

Ah, I see.

What happens when the compressor won't kick on because I suspect a low freon charge? Is there a way to override?

 

[RacerTodd]

There is a relationship between the pressure in a closed AC system (compressor not running) and temperature. It's called "Pressure/temperature saturation"

This chart shows the pressure/temp relationship for R-12 and R-134a. Look up the ambient temp and it will show you the pressure you'll see in the system. Note you can tell what refrigerant is in the system if you know the temp and pressure.

You can't tell the amount of charge just by the static pressure. Any closed system that has liquid in it will be at the pressure listed in the chart, no matter how full the container is provided some of the refrigerant is in liquid form.
If your gauges show less than the pressure in the chart, then that means you've lost enough refrigerant that there is no liquid in the system - only gas.

If you tested your car's fully charged AC system, a full 14oz container, a partially full 14oz container or a partially full or completely full 30lb container - they'd all measure the same pressure assuming they are all at the same temp.

Only when the compressor kicks in do you see the low side and high side deviate from the temp listed in the chart.

 

[Enabled]

Ah, I see.

What happens when the compressor won't kick on because I suspect a low freon charge? Is there a way to override?

Check both sides, but if you have 110psi at system off, then system will turn on for sure. I'm not sure what the low threshold is for the pressure switch (to still allow A/C on), but I would assume much under 70psi.

If it doesn't turn on, then your pressure switch could be bad, or your wire to the compressor is broken, among things.
(I *think* the light on the A/C button will not come on when the system is disabled from the pressure switch, but I have to confirm)

 

[Keven]

Thanks folks, looks like I need to focus my investigation elsewhere.

 

[dogdots]

Not knowing your car...I would guess you have a bad radiator fan (usually the larger of the 2) that resulted in a blown compressor clutch.

Do both radiator fans come on with the a/c switch activated with the key on and engine off?

Ohm out the 2 pin connector at the compressor.

 

[Keven]

Well, swapped out the G65 switch and the FCM and still nothing. Time to take a look at the compressor/clutch?

 

[Keven]

Well, swapped out the G65 switch and the FCM and still nothing. Time to take a look at the compressor/clutch?

Dammit, clutch and coil have been replaced and still nothing. I'm kind of wondering about my system charge again. I only checked the low side due to the gauge I have but now I think it's time to buy a real manifold and check both sides. I can't get the system to engage to check the pressures.

Check both sides, but if you have 110psi at system off, then system will turn on for sure. I'm not sure what the low threshold is for the pressure switch (to still allow A/C on), but I would assume much under 70psi.

If it doesn't turn on, then your pressure switch could be bad, or your wire to the compressor is broken, among things.
(I *think* the light on the A/C button will not come on when the system is disabled from the pressure switch, but I have to confirm)

Are we certain about this?

 

[gforce1108]

Are you following the troubleshooting manuals? (two listed here depending on your build date)
http://forums.tdiclub.com/showthread.php?t=242699

 

[Keven]

I am trying, most everything checks out except anything that requires a duty cycle meter to check.

 

[Keven]

Now I'm convinced it's some sort of rogue broken wire or a low charge.

 

[BobnOH]

Page 3 of this
check control circuit, I think you passed the not-running pressure check

 

[Keven]

That's unfortunate, I still want to check the pressures to be sure though.

 

[BobnOH]

If you're simply unsure of the gas, you could just evacuate and refill with the recommended amount of stuff.
You'll want to follow the recommended procedure.
I would evaluate the controls.

 

[Enabled]

You read 110psi in the circuits with the compressor off. Those pressures are fine to run the system.

"If pressure and duty cycle are high enough to allow running (14.5 psig trip - 68 psig reset/G65, 12.5% trip - 20% reset), and below the High High pressure cutout (450 psig trip – 350 psig reset/G65, 82% trip – 64% reset), but the clutch is not engaging, then go to the AC control circuit troubleshooting instructions below."


This means that if your pressures with system off are between 68psi and 350psi the system will operate from the standpoint of pressures. You are wasting your time if you are still trying to check if you have enough R134a.


That is what I told you earlier.

 

[50harleyrider]

Just confirmed with a 2nd gauge. My A/C on the low side with the compressor off is reading 110 psi. Isn't this way high? What the heck happened?

Can pressure on a warm day will show 110 psi-perfectly normal for a shut off compressor.

 

[Keven]

Alright, you convinced me. Looks like I need to go to town with a multimeter again.

I didn't have a good ground at the T4 - 2 Connector but even when grounded the AC wouldnt kick on.