Low Pressure A/C reading

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21 Jul 2013 21:44 #15387 by mm928
Low Pressure A/C reading was created by mm928
Hey all,

After the engine pull my freon was a little low, it could only keep up if garaged first. If it got hot after driving or was outside in the sun, it could not cool it down. At least not in a reasonable amount of time. So today I put one can of 134a in.

It seems to be working just fine afterwards, cold now but my pressure reading on the low side is now 40 PSI when I turn everything on full blast. I had read that 30 was normal so is 40 a problem? Or do I need to relieve some of that pressure?

Everything was working ok before, and it never got worse, it was just weak after we got everything back together.

Any comments welcome!

1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -

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22 Jul 2013 12:03 #15398 by wallyp
Replied by wallyp on topic Low Pressure A/C reading
The temperature of the evaporator (and thus the lowest possible temp for the cold air) is determined by the low-side pressure. Approx 30 psig is the usual target, but for R-134a, 28 psig is about 32 deg F, 30 psig is about 35 deg F, and 40 psig is about 45 deg F. Goals include having the desired low pressure, but still having enough refrigerant and refrigerant flow to keep the return line to the compressor cold.

Having non-condensibles (air, water vapor, etc.) in the system lowers efficiency, as does too much oil.

Wally Plumley
928 Specialists
www.928gt.com

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22 Jul 2013 12:39 #15400 by mm928
Replied by mm928 on topic Low Pressure A/C reading
Cool Wally, as usual you provide more than just a yes or no...I appreciate it. I have always had others look at A/C issues, so I wanted to get my feet wet on something simple.

If desired, can I just release a little to reduce the pressure?. From Low or high side?

By the way I am going to get a gauge manifold so I can see both pressures etc. this weekend, and then go from there.

1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -

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26 Jul 2013 14:32 #15431 by wallyp
Replied by wallyp on topic Low Pressure A/C reading
It is illegal and a terrible environmental boo-boo to release refrigerants into the air.

If desired - sure.

Wally Plumley
928 Specialists
www.928gt.com

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26 Jul 2013 21:10 #15439 by mm928
Replied by mm928 on topic Low Pressure A/C reading
Really, I figured there was a way to vent into something from a gauge set.....you mean there isn't? hhhhmmmm seems like there should be since I am sure this is not the first time it has happened to someone. I bet they do exactly what we are not supposed to do. ....sounds like a invention waiting to happen? I assume there are professional ways to do it, but why not something for the DIY? Probably won't get Gov approval though....but if they really cared... :)

1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -

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27 Jul 2013 18:00 #15451 by mm928
Replied by mm928 on topic Low Pressure A/C reading
Finally here are the numbers
Ambient temp 80
Humidity 80
Low side 25
High side 175
Engine warmed up from idle
Temp at vents 60+

Lost some pressure fiddling with the hi side connector, realized it has the r12 fitting still! So I had to go buys conversion connector. It is also by the receiver/dryer up front and not easy t get to for me.

Anyway it looks a little low now, the generic tables say at 80 it should be 40-50 low and 175 to 210 on high. But I thought 28 to 30 on r134 was good?
Yes, I am also assuming there is ware vapor or air in there making things less efficient. Next step will be to get it properly evacuated and recharged. But I still would like to come up with good target numbers (ignoring somewhat the temp it is blowing out the vents for now.)

Ps I deleted my previous post because it sounded more catty than funny sarcasm and did not add much value... Sorry about that.

1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -

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28 Jul 2013 00:21 #15455 by 928mac
Replied by 928mac on topic Low Pressure A/C reading
Here is how I performance tune the AC.
temp gauge in center vent all others closed.
fan setting on low (1-2)
engine RPM 1800-2200
From where you are now, add 1/4 lb and watch the temp inside (should go down)
Add another 1/4 same as before and continue this method until the temp does not go down. Then its full and as cold as it will get which should be less the 50F and close to 40F
If you add some and it goes up, you may have something wrong because you pressures are not that high yet.

BTW Make Sure Your Water valve is not allowing hot water in. I know mine does which is something i need to fix

Brad

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28 Jul 2013 19:58 #15462 by mm928
Replied by mm928 on topic Low Pressure A/C reading
Thanks Brad, yes, that H2O valve was taken care last year by a manual screw down! So no more wondering for me. I close it in April and open it in November.....all cold or all hot, it gave me a good peace of mind and things worked like they were supposed to. And if they do not, at least you do not have to worry about the H2O valve!

I have no issues with temp regulation as it actually does some regulation but does not use the hot water of course. I also usually turn it on or off and keep it all the way vold in the summer and at about 72 or 73 in the winter. Our cars heat up fast!

1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -

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