A/C Question
Thread Starter
Safety Car
Joined: Nov 2004
Posts: 4,359
Likes: 57
From: Mundelein Illinois
A/C Question
I have a 1981 and AC system holds 2.75 pounds of R12. What should the pressures be (High & Low) on a full system.
Does anyone have a chart
Does anyone have a chart
Last edited by spedaleden; Jul 19, 2017 at 10:06 PM.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Compressor will cycle on and off around 30 PSI on the low side. Hi side should run about 2-1/2 times the ambient temperature. Pressure readings are as much for diagnosis as anything. I'm sure there are charts somewhere, but that's my input from 30+ years of turning wrenches. And honestly, the system capacity is stated by the manufacturer. On the assembly line, I'd bet they just blasted the stated amount in and let 'er rip.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
My computer has decided that it no longer wants to post pictures here so I have to give you a link to a forum post in which I found the chart I am currently using.
http://www.fordforumsonline.com/thre...failure.14745/
I have found that the recommended pressures vary somewhat from chart to chart, at least on the 134a charts. It could be from the fact that most charts do not take humidity into account.
I've been using this one for a bit and it seems to be pretty accurate, at least on the 134a side, R12 I have not used.
Keep in mind that if you still have the stock fan that the pressure reading will need to be taken with the RPM's above idle. 1100 to 1300 maybe to get an accurate reading. Much depends on the amount of air going through the condenser. Electric fan cars don't have this issue as the fan is running as it needs to to pull air through the condenser.
I just try to get the low side as low as possible and still engage the clutch suffeciently to get the desired cabin air temp. Over fill even by a little and the efficiency of the system drops dramatically.
http://www.fordforumsonline.com/thre...failure.14745/
I have found that the recommended pressures vary somewhat from chart to chart, at least on the 134a charts. It could be from the fact that most charts do not take humidity into account.
I've been using this one for a bit and it seems to be pretty accurate, at least on the 134a side, R12 I have not used.
Keep in mind that if you still have the stock fan that the pressure reading will need to be taken with the RPM's above idle. 1100 to 1300 maybe to get an accurate reading. Much depends on the amount of air going through the condenser. Electric fan cars don't have this issue as the fan is running as it needs to to pull air through the condenser.
I just try to get the low side as low as possible and still engage the clutch suffeciently to get the desired cabin air temp. Over fill even by a little and the efficiency of the system drops dramatically.
Le Mans Master
Joined: Jan 2001
Posts: 6,656
Likes: 1,357
thanks REELAV8R -- I was going to try this but I'm glad to see it in writing. My a/c cools quite well, but then the compressor cuts out and it warms right up. I will try adding just sufficient to keep the compressor engaged and cabin temp low.
[I]"Electric fan cars don't have this issue as the fan is running as it needs to to pull air through the condenser.
I just try to get the low side as low as possible and still engage the clutch sufficiently to get the desired cabin air temp."
[I]"Electric fan cars don't have this issue as the fan is running as it needs to to pull air through the condenser.
I just try to get the low side as low as possible and still engage the clutch sufficiently to get the desired cabin air temp."
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
Do you have a way to read the pressure on the low side? From your description it does sound as if it's just a little low on freon. Although having the high side too high can give similar symptoms, other than I would not expect you to describe it as cold other than when it cycles off, if the pressure was too high.
If you don't have a low side gauge I would get one. Over servicing happens real fast while topping off, especially when using 12oz cans as there is no good way to monitor the pressure and service it simultaneously with those cans. That I know of in any case. For 134a there is.
However if you get a gauge set you can monitor the high side while servicing the low side with the cans. Then unhook the can and hook up the low side and check the pressure.
Something like this;
http://www.ebay.com/itm/R134a-R12-R2...19.m1438.l2649
Keeping the can upright will put the freon in slower vs inverting it (not recommended) or holding it sideways.
If you don't have a low side gauge I would get one. Over servicing happens real fast while topping off, especially when using 12oz cans as there is no good way to monitor the pressure and service it simultaneously with those cans. That I know of in any case. For 134a there is.
However if you get a gauge set you can monitor the high side while servicing the low side with the cans. Then unhook the can and hook up the low side and check the pressure.
Something like this;
http://www.ebay.com/itm/R134a-R12-R2...19.m1438.l2649
Keeping the can upright will put the freon in slower vs inverting it (not recommended) or holding it sideways.
Last edited by REELAV8R; Jul 20, 2017 at 12:41 PM.
Le Mans Master
Joined: Jan 2001
Posts: 6,656
Likes: 1,357
Yes. I have a set of gauges to monitor the HI, LOW, and to allow charging. I was suspecting that I may be overcharged with R-134a so I transferred some to my other vehicle which needed a little R-134a. It is more forgiving since it has larger capacity than my hot rod air capacity of 1.8 lbm.
I will transfer some back very slowly, run my electric fan continuously, monitor vent outlet temp, and monitor compressor engagement.
I will transfer some back very slowly, run my electric fan continuously, monitor vent outlet temp, and monitor compressor engagement.
Do you have a way to read the pressure on the low side? From your description it does sound as if it's just a little low on freon. Although having the high side too high can give similar symptoms, other than I would not expect you to describe it as cold other than when it cycles off, if the pressure was too high.
If you don't have a low side gauge I would get one. Over servicing happens real fast while topping off, especially when using 12oz cans as there is no good way to monitor the pressure and service it simultaneously with those cans. That I know of in any case. For 134a there is.
However if you get a gauge set you can monitor the high side while servicing the low side with the cans. Then unhook the can and hook up the low side and check the pressure.
Something like this;
http://www.ebay.com/itm/R134a-R12-R2...19.m1438.l2649
Keeping the can upright will put the freon in slower vs inverting it (not recommended) or holding it sideways.
If you don't have a low side gauge I would get one. Over servicing happens real fast while topping off, especially when using 12oz cans as there is no good way to monitor the pressure and service it simultaneously with those cans. That I know of in any case. For 134a there is.
However if you get a gauge set you can monitor the high side while servicing the low side with the cans. Then unhook the can and hook up the low side and check the pressure.
Something like this;
http://www.ebay.com/itm/R134a-R12-R2...19.m1438.l2649
Keeping the can upright will put the freon in slower vs inverting it (not recommended) or holding it sideways.
Safety Car
Joined: Jun 2004
Posts: 4,354
Likes: 214
From: Beecher Illinois
The chart I've been using for years.....
outside temp70...low23-28...high135-170
outside temp80...low23-29...high170-205
outside temp90...low26-35...high205-245
outside temp100..low31-40...high245-285
outside temp110..low35-44...high285-320
outside temp70...low23-28...high135-170
outside temp80...low23-29...high170-205
outside temp90...low26-35...high205-245
outside temp100..low31-40...high245-285
outside temp110..low35-44...high285-320
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Thread Starter
Safety Car
Joined: Nov 2004
Posts: 4,359
Likes: 57
From: Mundelein Illinois
Thanks guys for all the answers. I put another pound of R12 in and changed pressure switch. At 80 degrees with 90 percent humidity.
Readings
low 28
high 208
Compressor was turning on and off now just keeps running and vent air is 48 degrees.
Thanks again
Readings
low 28
high 208
Compressor was turning on and off now just keeps running and vent air is 48 degrees.
Thanks again
Last edited by spedaleden; Jul 21, 2017 at 04:27 PM.
Burning Brakes
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From: TX
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