kansas123

Before I rent a tool to pull the vacuum on my A/C system, can someone tell me what I am in for? And what else I need, like do I need an air compressor? I envision hooking this device up to either high or low side fitting and sucking the life out of it, but is the device self contained or need something additionally? thanks.

kevinator80

I would suggest a set of gauges but I suppose you could do it without them. I have used two types of pumps. One is a heavy electric vacuum pump that resembled a compressor out of a refrigerator and the other was a device that operated on air pressure from an air compressor. Both worked equally well.

No big deal to do. Hook it up to the low pressure side and let it do its thing. I have always been told to pull the vacuum for about 30 minutes but the gauge is usually pegged after a minute or so. I don't know that there is any benefit to running it that long. Keep in mind, I am only a shade tree ac man at best.

mdj21

running the vacuume for 30 min gives some time for the moisture to boil out of the system.

i'm sure it dosen't take 30 min, but if you go for the full 30 min the moisture is guanenteed to be gone.

mrvette

I tell what I do, and so I made up some fittings from parts and soldered/sweated the brass together to make up my cheater parts....

first off, I use 134 in any system....change the oil and do the same....

I put air pressure right from the compressor into the a/c system...up to say 120 psi, close off the gauges, and read it for a couple hours/afternnon time....any dropping and you got a leak, soap bubbles or whatever, find the oil trace, fix it....

when it holds air pressure, figger it's tight, then just hook up the vacuum pump and draw down for some 30 minits, but many an old AC guys has told me let it set there running on suckdown for at least 3 hours.....I go much longer than 30 minits for sure, but 3 hours, rarely....

it must hit 28 or so on draw down or you got a leak which should have been apparent from the air pressure check.....

when it's ok, purge the CHARGE HOSE into the gauge manifold, and fill up with 1/5 -2 12 oz cans of 134 and you home free.....sometimes I get impatient, and use a propane torch to warm the freon can a bit not blister the paint off it's ***, just warm it up to get the stuff outta there faster....GOD, I"m lazy.....

tonyv123

Gene,

Wouldn't you leave your system under vacuum (vac. source removed, system still under vacuum), and charge? It seems that if you purge the system, you induce moisture back into it. I only seen it done once, and think it was left under vacuum.

speedreed8

i'm sure he meant purge the hoses of the gauge set, with freon, before putting any charge into the system to get rid of the air in the hoses.

mrvette

Tony thanks for catching that, I was thinking one way and typed it RONG i'ts been corrected NOW.....

Jud Chapin

You really need a set of a/c gauges to properly do this job. Further, I don't really think 30 minutes under vacuum is sufficient. I usually pump mine down for several hours (4 or more) with both the low and high sides open, and then close the gauges (system sealed),and leave the system under vacuum for an extra hour or so. While doing so, keep an eye on the gauges to determine if the vacuum is holding. If the vacuum drops, there is a leak somewhere and needs to be repaired before going any further. If there is no leak, the system is ready to be charged with refrigerant. The first can should be installed with the system still under vacuum as you do not want open the system as it will relieve the vacuum and introduce air.

Jud Chapin

One additional note. Be sure to use a good vacuum pump that is capable of evacuating a system to a pressure of 29.76 in. Hg or better at sea level.

tellcom2001

go to autozone you can get an a/c vacuum pump and the manifold gauge set. as a loaner tool you give them $315.00 use the tool..wich has instructions with it. then return the tools and they give you back the $315.00..cost to you free....save the wave

MITGS

I have been preping for the same task and I have seen several info sources say that your compressor must be cycling when you add freon to the system. I dont see any ref to that here.

'75

In this thread the systems were charged while turned off. Referigerant can be added to the low side with the compressor operating, but you must be careful to only charge with gas and not liquid. If your compressor is fed liquid while running it will probably bend the valves and ruin the compressor. Keep the can upright when charging with the system on.

TimAT

It's a safe bet to say that you won't get the system full without it running. Put a can or two in the system and then start it. Make sure you charge from the LOW side only. When it's full, the high side pressure will be about 2-1/2 times the outside air temp.

Be very careful heating the can with a torch as was suggested- much safer to have a bucket of hot water and use that.

If the compressor won't cycle on, you can jumper the LP cycle switch on the dryer.

kansas123

[QUOTE=TimAT;1565660150]It's a safe bet to say that you won't get the system full without it running. Put a can or two in the system and then start it. Make sure you charge from the LOW side only. When it's full, the high side pressure will be about 2-1/2 times the outside air temp.

QUOTE]

When I was a teen messing with a/c s on early 60's Chevy's, the correct side to charge was readily known by the size of the fitting; on my '72 I do not know which is the low side, any advice? Thanks!

mdj21

[QUOTE=kansas123;1565666095]the low side would be a port on the reciever/dryer or if the manifold behind the compressor has two ports, it would be the one that's inline with the hose comming from the evaporator.

anywhere from the expansion valve, through the evaporator, and back to the compressor is the low side. the line going from the compresor to the condensor and then from the condensor to the expansion valve is the high side.

hope this helps....

Mike.

68/70Vette

About how long to wait on the vacuum purge..... One thing to think about is that when the AC lines have air in them, the air molecules are all moving, bouncing around off of each other and the interior of the AC lines. This is called Brownian motion. The only way they have to get out is through the little oriface where you have your vacuum line attached too. When the air in the lines is at 14.7 psi, the molecules just woosh out of the opening when there is a vacuum present. Eventually you get down to where there are very few molecules left. An air molecule in the evaporator, for example, has to bounce around many random times if its' going to get to the vacuum line oriface. In fact it'll probably miss the little hole and keep on bouncing around. They longer you wait, the more likely that the remaining air molecules will eventually exit through the oriface.

Another interesting thing. Water molecules act as little magnets. The two hydrogen molecules at attached to the oxygen molecule and the two hydrogen molecules are about 105 degrees apart. This means that the end of the molecule where the hydrogen molecules are have a negative charge and the oxygen end has a positive charge. A water molecule will attach itself to a steel (ferro magnetic) surface and it will be very difficult to remove it. For a steel surface, you have to heat it to about 600 degrees F in a vacuum to remove all of the water molecules. Obviously for an AC system you don't have to be this pristine about getting rid of absolutely all water molecules, especially since they probably won't get loose anyway. It's one reason you want to make an AC system that maximizes the use of non-magnetic metals.