Can I get by with a HF vacuum pump for my A/C - PeachParts Mercedes-Benz Forum

Scott98 · #1 04-05-2007, 07:44 PM

I'm redoing my A/C this weekend - new compressor, expansion valve and receiver/drier. I plan on flushing the whole system and then pulling a vacuum. My problem is that I don't have a "real" vacuum pump. I just have the little metal box from Harbor Freight that you hook your air compressor to in order to pull a vacuum. Noone around here rents a real vacuum pump and NAPA sells them for $380 which I'm not going to spend. Can I pull a vacuum with my HF pump and let it sit for a couple hours? Will that be good enough for removing the moisture?

Here's a link to the HF vacuum pump I'll be using (I have the R-12 version)

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92475

Thanks,

Scott

tjohn · #2 04-05-2007, 07:49 PM

I suspect that a lot of people will tell you that these things are crap, and that it won't work. However, I've used mine sucessfully on several occasions. It can pull to the limits of my gauge (more than 28") in my applications. The caveats: I do my A/C work at garage where I have access to shop air (some ungodly cfm at 90 psi) and I'm at sea level. If you have a small oil free compressor, its not gonna cut it.

Scott98 · #3 04-05-2007, 07:58 PM

Quote:

Originally Posted by tjohn

I suspect that a lot of people will tell you that these things are crap, and that it won't work. However, I've used mine sucessfully on several occasions. It can pull to the limits of my gauge (more than 28") in my applications. The caveats: I do my A/C work at garage where I have access to shop air (some ungodly cfm at 90 psi) and I'm at sea level. If you have a small oil free compressor, its not gonna cut it.

I've got a 33 gallon 6hp oil free Craftsman compressor and I'm at 1 mile above sea level. What do you think?

300SD81 · #4 04-05-2007, 08:17 PM

It'll should work with a small compressor, just make sure you have a valve inline between the vacuum pump and the car. When the pressure starts to drop, close the valve and allow the compressor to refill. I'm also planning to attach an old water heater to my pathetic little compressor as a huge tank. Will be doing it tomorrow or saturday, depending on how long it takes me to do my ball joint..

Scott98 · #5 04-05-2007, 10:08 PM

I just pulled a "test" vacuum and I can only pull the system down to about 22HG. Is that enough? I thought you really need to be at around 29HG?

300SD81 · #6 04-05-2007, 10:11 PM

Quote:

Originally Posted by Scott98

I just pulled a "test" vacuum and I can only pull the system down to about 22HG. Is that enough? I thought you really need to be at around 29HG?

How many psi are you running through it? I got my pump to do 29hg of vac, but I had to turn the thing up to 100+psi.. 22 is not enough

Heres a calculator for the boiling points of water at different vacuums..

http://www.partyman.se/calculator.html

You have to convert inHg to mmHg and then subtract from 760mmHg (atmospheric pressure) to get the absolute pressure in mmHg

#7 04-05-2007, 10:16 PM

That's all you're going to get at that altitude. 22 is fine.

The higher you go, the lower vacuum (gauge wise) you can get because of the lower atmosphere pressure. The atmosphere is about ~12psi at 5,280' vs 14.7psi at sea level.

300SD81 · #8 04-05-2007, 10:18 PM

Quote:

Originally Posted by ForcedInduction

That's all you're going to get at that altitude. 22 is fine.

The higher you go, the lower vacuum (gauge wise) you can get because of the lower atmosphere pressure. The atmosphere is about ~12psi at 5,280' vs 14.7psi at sea level.

Ah, that makes sense... Need to correct for atmospheric. Any idea what the pressure is in atlanta, ga?

Scott98 · #9 04-05-2007, 10:19 PM

The reading was the same running 90psi or running 120psi. How long should I pull the vacuum for?

#10 04-05-2007, 10:26 PM

Quote:

Originally Posted by 300SD81

Ah, that makes sense... Need to correct for atmospheric. Any idea what the pressure is in atlanta, ga?

Atlanta-
738-1,050 FT
14.2-12.9psi

You should keep it under vacuum for at least 1/2hour.

tangofox007 · #11 04-05-2007, 11:21 PM

Quote:

Originally Posted by 300SD81

Any idea what the pressure is in atlanta, ga?

Subtract one inch Hg from the reported barometric pressure.

300SD81 · #12 04-05-2007, 11:31 PM

Thats strange... Yesterday, I could definitely pull 29hg of vacuum, today, I'm down to a max of 27... I also need to get that old water heater connected to the compressor as an air tank... the hf vac pump empties my compressor in seconds. I'm trying to rig a 1-way valve onto the pump so that it doesn't suck air back in as the vac drops...

300SD81 · #13 04-06-2007, 12:56 AM

Ok.. Wow, I just discovered the ultimate vacuum pump for anyone with an air compressor... Take off the intake filter, and connect a vacuum fitting there... It pulls 27.5"!! Thats more than the HF pump pulls at 120psi, and more than my mityvac can pull... (using mityvac as vac guage)

Just 1 question though... Any way this could damage my compressor? I don't really see how it could, since to it, all its doing is pumping 14.7psi, right?

#14 04-06-2007, 12:58 AM

Um, come again? You block off the entire intake and crank on the engine?

That will pull the oil seal out of the turbo.

300SD81 · #15 04-06-2007, 01:03 AM

Quote:

Originally Posted by ForcedInduction

Um, come again? You block off the entire intake and crank on the engine?

That will pull the oil seal out of the turbo.

Not the car's air intake, the air compressor'...

(yes, thats my POS, 3/4hp compressor, but it was free... old owner gave it away because the pipe fittings were loose! lucky deal)