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Re: [GMCnet] A/C vacuum pump [message #326226 is a reply to message #326213] Tue, 21 November 2017 18:09 Go to previous messageGo to previous message
Matt Colie is currently offline  Matt Colie   United States
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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Kingsley Coach wrote on Tue, 21 November 2017 14:22
Tyler

You've just answered a question I was embarrassed to ask !!

I bought one of those several years ago and never tried it..it looked to cheap to effectively do anything. I think the last time I looked, it was
somewhere in the area of where I keep A/C stuff, which is under the trailer wiring stuff I think, which is right under the last box of crap from my long gone and forgotten Airstream....

Thanks

Mike from NS

Mike,

You should be embarrassed to ask only what the rest of us were born knowing. (That is pretty much nothing.) The rest we got to learn from other's wisdom that they were generous enough to pass to us, our own mistakes and other's mistakes when we are real lucky. This could well be one of those last.

Executive summary for Matt's minor rant.
Don't waste your time and effort, buy a decent pump. At what most shops bill, you can recover the cost with the first use.

<rant on>
That "pump" would be junk for getting a real system to working right.
From published specifications: End point vacuum 28.3 at sea level.
While Antigonish is real close to sea level, 28.3 is not real close to 29.92 (760mm) that is the standard sea level barometer. That is more than 1.5" Hg left in the system. If you use a pump this weak, your only hope to survive is to dry or replace the filter dryer every time the system is open. (That is about 40$C shot right there.)

When R-12 was less then 1$us a pound, we often used to bring a system to can pressure with R-12 and the valve things off and use the pump to pull it down. You can't do that in the coach or any other miniature system like that. Your only hope to have a durable recharge is to pull it down hard. Then fill it with refrigerant. This is a case that I have found no different with R-12, 22, 502, 134a or HC-12(a). But back to the case in hand....

The less expensive pump HF offers if ~100$us. Its published end point if 75µHg (absolute pressure). This will do pretty well, it may not be as good as the 22.5µHg that is promised by the pump that is 50$us more, but for a vehicle A/C system, it (IMHO) serve you well.

If you can manage to get the pump out of someone's dead refrigerator. (They can die from either a leak or a pump failure, so be cautious here.) And set it up with the required fittings, that can work pretty well as they usually can pull 29+". From my experience of years ago, 29.6 will usually be successful.

Why does this matter?
Because if there is any moisture left in the refrigerant circuit, it will collect and freeze that the expansion. That will be the TEX (What a GMC has), capillary used in small stationary units or the orifice used in more modern (cheaper) automotive units. If you are not aware of this problem, it can drive you all the way up the wall trying to figure out why the system seems to work well when it is warm, but as soon as it gets really cranking, it crashes.

I am not going to include the process for drying refrigerant filter/dryers tonight. Maybe some other time. It can be done, but it is neither simpler or easy and requires uncommon tools and equipment.
<rant off>

I am still using the vacuum pump I bought when I was doing refrigeration. That and the flaring block will go with my estate. I don't even want to think about the price to replace those in today's money.

Matt





Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

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