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Old 01-18-2014, 05:47 PM
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Vintage Air specific technical question

Most cars with AC using 134A have a low side pressure of about 30PSI, which would put the evaporator at around 34 degrees (just above freezing)
Why does Vintage air recommend low side pressures of 6-18 PSI ?
I'm very confused by this, because theory would tell me that any moisture would ice up instantly and block my evaporator coil.
Thanks for any reply

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Old 01-18-2014, 09:28 PM
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The outside temp determines what the pressures should read. There is no one size fits all.
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Old 01-18-2014, 10:13 PM
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Quote:
Originally Posted by Denny View Post
The outside temp determines what the pressures should read. There is no one size fits all.
No!! The outside temp will determine the high side pressure. Anyone else??
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Old 01-18-2014, 10:25 PM
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What ever you say



This must be all wrong then !!!
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Old 01-18-2014, 10:34 PM
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Good one, Denny
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Old 01-19-2014, 05:58 AM
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Too bad that chart doesnt go down to freezing. I would be interested if the pressure gets down low enough so the compressor wont kick in, as thats the problem I am having at the moment....
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Old 01-19-2014, 09:25 AM
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Try this out Poncho.
R134a Refrigerant Pressure Temperature Calculator

If you search a little on your system, there should be a low temp switch which will not let the system operate below the normal outside temps of needing any ac. How ever, the newer ones can be used to dehumidify.
And if it's a low pressure switch, then it will not kick in because at the zero temp, the pressure is below the limit do to the outside temp itself.
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Old 01-19-2014, 10:35 AM
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Quote:
Originally Posted by poncho62 View Post
Too bad that chart doesnt go down to freezing. I would be interested if the pressure gets down low enough so the compressor wont kick in, as thats the problem I am having at the moment....
When "off" the pressures equalize, so both the low and high sides are ~ 150#. It should always kick on, it just won't run long before it starts cycling. If yours is not even kicking on, you have other problems...

Russ
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Old 01-19-2014, 01:30 PM
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Originally Posted by S10xGN View Post
When "off" the pressures equalize, so both the low and high sides are ~ 150#. It should always kick on, it just won't run long before it starts cycling. If yours is not even kicking on, you have other problems...

Russ
Mine will kick on if I bypass the pressure switch at the dryer.......I figure either the switch is bad (6 months old) or there isnt enough pressure to trip it. I guess spring will tell.
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Old 01-19-2014, 03:05 PM
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Another reason it won't kick on. If, the system is low lets say, it won't kick on because of that. Reason is, the oil travels with the freon. If you get no oil, you get bad compressor...

Equalized system pressure is determined by outside temp. This varies some, if the engine compartment is cold, or hot. The actual charge is determined by the amount of freon which is in pounds, to determine the proper fill.
If pressure is low to start with, that indicates the charge is low, but, you have to factor in the outside temps.
On newer vehicles, when you go to defrost, it over rides the normal temp sensor, and runs the a/c. But as a safety, there is still a switch that will limit the compressor operation if the system is low.
Here's a link to some more info on r134a.

How to Repair Automotive A/C : Freon check
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Old 01-19-2014, 03:50 PM
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I appreciate the chart you posted, as it has excellent information, but my original post was for someone to answer a specific "Vintage Air" question.
Why does Vintage recommend such low pressures on the suction side?

http://www.vintageair.com/DownloadsS...ng%20guide.pdf
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Old 01-19-2014, 04:29 PM
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Ok. This is not written in stone. And they are really unclear when you don't do this type of work all the time. Bad instructions is what I'm saying.
Now, I will try to explain...
Right here, it says acceptable operating ranges...

ACCEPTABLE OPERATING PRESSURE RANGES FOR VINTAGE AIR
SYSTEMS

A.
R134A TYPE
This means high side pressure, anywhere between 160 to 250psi, depending on the outside temperature. plus an additional 15-20%. That is because one will vary from another, depending on air flow, size, heat, and any slight restrictions in the system. So if it's 80 outside, a 160 psi is normal. However, if the engine compartment is hot, it may be a little higher.
1.
HIGH-SIDE PRESSURES
( 160-250 PSI ) * Note- general rule of thumb is
two times the ambient ( daytime ) temperature, plus 15-20%.

This is also a general pressure range from 06 to 18 psi, usually at a steady rpm, like about 1500rpm's. By raising the rpm's so the system is at a steady rpm, the gauge will read steady and true. Once you return to idle, the pressure will rise some, because we are not at a steady rpm anymore. It can raise to say 30 psi. That is because the freon has slowed down on flow, and the exchange of heat is slower, therefore the pressure will raise. As soon as you increase the rpm's, the psi will then drop.

If it drops below the 06, that means it is pulling into a vacuum. That is another problem that should not happen.
2.
LOW-SIDE PRESSURES
( 06-18 PSI in a steady state)

And at this point, once that checks out, and still having it at a steady rpm's, the center duct, (closest to the evaporator), the air temp should be 36 to 46 degrees coming out. If it drops down to say 32, or stays above 46, you then have a problem, and need to figure out why.

All and all, if you charged that system with the proper amount of freon, the pressures, high side, and low side, should fall within these specs as described.

3.
CENTER DUCT TEMPERATURE
( 36-46 DEGREES F.)

Last edited by Denny; 01-19-2014 at 04:47 PM. Reason: fixing errors, and adding info
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Old 01-19-2014, 04:59 PM
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Thanks for that response.
My car's AC works perfectly, and the pressures do fall within Vintage Air's specification. I have always been curious, and I'm asking because the basic theory that I have in automotive air conditioning tells me that such a low pressure would basically turn my AC into a freezer. My outlet temp when I checked last summer was about 40F.
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Old 01-19-2014, 06:36 PM
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Quote:
Originally Posted by hemi43 View Post
Thanks for that response.
My car's AC works perfectly, and the pressures do fall within Vintage Air's specification. I have always been curious, and I'm asking because the basic theory that I have in automotive air conditioning tells me that such a low pressure would basically turn my AC into a freezer. My outlet temp when I checked last summer was about 40F.
No, the pressure would not turn your AC into a freezer. Air flow across the evaporator keeps this from happening, also, the temp setting for the valve that controls the temp, prevents it from freezing. How ever, on a real humid day, and the fan on low, it could frost up, simply because the fan speed is not moving enough air.

As far as the freon freezing inside. Not going to happen. It boils when submitted to the outside air. freezing can occur, IF, there is any moisture inside the system. Other than that. no.
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Old 01-20-2014, 08:12 AM
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the owner of Vintage Air (Jack Chisenhall) wrote a book about hot rod air,if I recall correctly he stated one cause of low side pressures being too low was the compressor was too big for the system,standard of the aftermarket has been the Sanden SD508 (5cyl 8ci) which maybe the reason we are seeing more Sanden SD7 (7176) being used

Other issues are not enough heat transfer at evap coils,which maybe the result of low ambient temps,restricted blower inlet or evap case outlets,expansion valve cap tube not attach to suction (wide open floods the evap) I'm sure there are more reasons but that is all I can think of right now.

Most hot rod a/c systems are the result of what will fit in the car and not what a engineer has designed to work properly,sometimes it amazes me that auto a/c works as well as it does considering everything it has to overcome,such as engine rpm,ambient temps, humidity,improper refrigerant charge,mismatched components,poor installation technique etc
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