How to Vacuum-Pump the A/C System

Date: Thu, 9 Aug 2001


I was reading your nice website and found it very interesting. I am wondering where I should attach the high vacuum pump to evacuate the system. This is something I can't seem to find out about. I would imagine it goes to the low pressure side, where the refrigerant goes in? Should the car be running for this? I guess the compressor clutch won't engage because of no pressure, so the engine can be left off?

Thanks in advance,

Bill DiCarlo

Madison, NJ

Bill - The engine and a/c system are turned off while vacuum-pumping the system. The vacuum pump is usually attached to the center port of a gauge manifold with a hose. The gauge manifold is attached to the low pressure port on the a/c compressor with another hose. This arrangement makes it easy to measure the vacuum in the system and to add refrigerant.

When the vacuum reaches 27 Hg, the pump is left to run for another 15 to 30 minutes, then the manifold valve between the pump and the low pressure port is closed and the pump turned off. The system is left alone for another 20 minutes to verify that there are no leaks, which would show on the gauge as a loss of vacuum. If no leaks are present, the hose is removed from the vacuum pump and attached to a can of refrigerant. Then the engine is started, the a/c turned on, and the low-pressure manifold valve is opened, allowing refrigerant to flow into the system.

If you don't have a gauge manifold, you could hook up the pump to either port on the compressor, but after you finish pumping out the system, you'd have to figure out a way of closing the port without allowing any air to get in. -Glenn.

Date: Mon, 13 Aug 2001

Hello Glenn,

I have evacuated and recharged my system. I used three 10-ounce cans of R-134a. I really don't know if that is enough. I was told 30 to 40 psi on the low side was good to attain, but is this measured when the a/c is on or off? I still notice my a/c gets cold to around 45 degrees being the lowest, but much of the time gets warmer by itself, cycling up and down even though the compressor clutch stays engaged. I was hearing a little clicking sound coming from the compressor when I was recharging and I added 2 ounces of the recharge oil and the noise went away. I wonder if there is another component which tells the a/c system to get cold possibly faulty? I really appreciate your information and these little quirks are hard to diagnose.


Bill DiCarlo

P.S. The vacuum pump I used was from a vacuum plating system and is very strong.

Bill - If your a/c system had been unused for a long time, or if it had been exposed to the atmosphere for more than a few minutes, you probably should replace the receiver-drier. This is a cylinder about the size of a soda can. It has a sight glass on the top that is used to determine how much refrigerant is in the system. If you're not sure what pressure you should be reading on the gauges while you fill the system, just watch the sight glass. When the refrigerant appears as a liquid with just a few bubbles in it, the system is full.

The main function of the receiver-drier is to remove contaminants, especially small amounts of moisture. Any moisture in the system can freeze in the small orifice of the expansion valve and cause just the symptoms you describe. Probably the easiest way to find a replacement for the receiver-drier is to remove the old one and bring it to a parts house that specializes in air conditioning parts. This means, of course, doing another vacuum and charge, but, hey, now you have experience! -Glenn.

P.S. Before I bought my own a/c vacuum pump, I used to borrow my dad's that he used for lost-wax jewelry casting. That was a real heavy-duty pump.

Date: Thu, 16 Aug 2001

OK, it is working acceptably for now, but I will replace it with the next problem if any. Sometimes it will blow out 66-degree air from the vent. ( I use a remote sensor from a Radio Shack temp unit). By the way, my unit does not have a sight glass on it though.

Regards and keep cool,

Bill DiCarlo

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