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Old 10-08-2018, 02:06 PM   #1
rsgt
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Reading A/C gauge

What scale do I read on this gauge for R12 This is the low side with the truck idling. The compressor and clutch are kicked in.
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Old 10-08-2018, 02:17 PM   #2
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Re: Reading A/C gauge

Black scale, the second one in reads PSI, pounds per square inch pressure on you low side. In the picture it is reading about 73.5. The second scale out from the center that is marked R-12 is telling you what your evaporator temperature is at the indicated psi. Both should be in the low thirties with the right charge and the engine speed up, say around 1500 to 2000 rpm, with the interior cooling down.
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Old 10-08-2018, 03:15 PM   #3
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Re: Reading A/C gauge

You read the outside red numbers on the inside red dial.The gauge face reads starting from the outside kg/cm, PSI, R502, R12, R22. So R12 is using the inner most red dial BUT using the outside row of red numbers for the same dial. (R12 & R22 are using the same dial, R22 uses the inside #s, R12 uses the outside #s) In the pic it is reading 72. Yeah, clear as mud, right?

So, reading from the center of the gauge out: R22=43; R12=72; R502=35; PSI=73.5; Kg/cm=just under 5.2
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Old 10-08-2018, 03:18 PM   #4
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Re: Reading A/C gauge

Thanks for the help. Here is the high side. I am trying to figure out what is wrong with the A/C. It has the right charge, Charged from a scale. Under the hood the suction hose to the compressor is sweating real good. I can feel the expansion valve cycling refrigerant by holding my hand on the evaporator output line. I do not know why the low side is so high. It has a new pro ten compressor. Heater hoses not hooked up. Today the temp is 82 here and my center vent reads 62
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Old 10-08-2018, 04:20 PM   #5
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Re: Reading A/C gauge

Looks like they are close in reading. So either low on charge or the compressor isn't working. Is the compressor cycling on and off?

https://www.aircondition.com/tech/qu...ith-Gauges-FAQ
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Old 10-08-2018, 04:41 PM   #6
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Re: Reading A/C gauge

Does your truck have factory A/C? If so, the correct charge (based on the '71 service manual) is 3 lbs, 4 oz. With engine at 2000, ambient at 80, high side should be around 185 -195 psi, low side approx. 30-35 (POA, based on altitude.) discharge temp @ 38 - 41 at right hand outlet. My personal experience, I can almost always get cooler temps with a good system.

One way to confirm you have the correct charge - if your system is stock you should have a sight glass on the receiver/dryer. When the system is at full charge you should see no or very few bubbles in the glass.
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Old 10-08-2018, 06:30 PM   #7
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Re: Reading A/C gauge

Did you evacuate the system after installing the new compressor? Standard practice is to also replace the receiver/dryer whenever the system has been opened. If you did not evacuate the system, air and moisture may be the culprit. Pressures like this indicate that the flow is too high. (STV and/or POA out of spec). But the question about evacuation of the system is the main issue, for me.
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Old 10-08-2018, 06:52 PM   #8
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Re: Reading A/C gauge

The truck a 72, has factory ac. about 18 months ago I had the system apart and flushed the lines, evaporator and condenser with a/c flush. Installed new compressor and dryer. vacuumed and charged. I had no bubbles in the sight glass. It cooled to 48. Lately it has not been cooling good, no bubbles. I put the gauges on it and was surprised to see over 70 on the low side. I did some research and thought the compressor was going bad. So I put another new one on and dryer. I have tried a new expansion valve with the same results as the original one. Do you guys think that the poa valve would cause such a high low pressure reading?
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Old 10-08-2018, 07:20 PM   #9
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Re: Reading A/C gauge

Well, there are only three moving parts: the compressor, the expansion valve & the POA. if you've changed two that leaves the POA. A bad POA can cause incorrect pressure. Some POA's are adjustable, some are not. There are some good threads on here if you want to search.
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Old 10-08-2018, 07:29 PM   #10
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Re: Reading A/C gauge

Wanted to help but read some confusing stuff here. You are reading psi always. The red r-12 scale tells you what your suction temp “should” be at that specific pressure. You use a scale to check what that should be specific pressure given and specific temperature.

You said your suction line is sweating. That is good. Do you have adequate airflow inside that cab? Do you have adequate airflow across your condensing coil? The high pressure vapor line going into the condenser should be very hot. You shouldn’t be able to keep your hand on it at all. The line coming out of the condenser should be much cooler but still warm. This will let you know if the condenser is adequately removing heat.

Are you sure you aren’t overcharged? Those pressures don’t seem that far apart but usually if a compressor is bad you don’t see much differential between the suction and liquid side. My guess is overcharge. I’d charge it right to the point where your sightglass bubbles disappear. Then your done. This is assuming the expansion valve has had time to equalize.

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Old 10-08-2018, 07:50 PM   #11
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Re: Reading A/C gauge

The poa will stick sometimes. Tap it gently on the side while the AC is running, sometimes that will free it up.
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Old 10-08-2018, 08:06 PM   #12
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Re: Reading A/C gauge

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The poa will stick sometimes. Tap it gently on the side while the AC is running, sometimes that will free it up.
That's right, sometimes it takes the "Fonz move" to get it working right.

You know, where he walks up to the Coke machine and hits it just right to get a free Coke?
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Old 10-08-2018, 08:32 PM   #13
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Re: Reading A/C gauge

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The poa will stick sometimes. Tap it gently on the side while the AC is running, sometimes that will free it up.
I tried that today but it didn't change anything.
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Old 10-08-2018, 09:58 PM   #14
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Re: Reading A/C gauge

Thanks Slowguy for clearing that up. In my long-winded post I never said the inner scales are in fahrenheit. I thought that was what what RSGT was asking but now I see that I'm slow & dull witted!

to back up Slowguy & ZEKE68, you want to read the PSI scale.
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Old 10-09-2018, 06:20 AM   #15
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Get a good fan and place in front of the truck blowing through the radiator. See if the temps drop. One thing a lot of people miss is closing the gaps off around the radiator shroud. You want to force the air through the radiator not around it. Air going through it is what will keep the AC nice and cold even if everything is working as it should if you are letting the incoming air go around the radiator it want never cool good.
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Old 10-09-2018, 09:01 AM   #16
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Re: Reading A/C gauge

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Get a good fan and place in front of the truck blowing through the radiator. See if the temps drop. One thing a lot of people miss is closing the gaps off around the radiator shroud. You want to force the air through the radiator not around it. Air going through it is what will keep the AC nice and cold even if everything is working as it should if you are letting the incoming air go around the radiator it want never cool good.
Andy, Yes I did use a fan, even sprayed water. I have my fan shroud all sealed around with weatherstripping.
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Old 10-09-2018, 09:25 AM   #17
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Re: Reading A/C gauge

After all your testing, I’d say the POA is stuck.
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Old 10-09-2018, 09:31 AM   #18
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Re: Reading A/C gauge

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After all your testing, Id say the POA is stuck.
Clay, I think you are right.
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Old 10-09-2018, 03:44 PM   #19
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Re: Reading A/C gauge

Your POA is not working right for sure. The low pressure should be hooked up directly to the POA valve, right? There is not another connection unless someone added one someplace else on the low side so just checking.
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Old 10-09-2018, 06:32 PM   #20
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Re: Reading A/C gauge

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Your POA is not working right for sure. The low pressure should be hooked up directly to the POA valve, right? There is not another connection unless someone added one someplace else on the low side so just checking.
Yes the low side hose is hooked to the Poa
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Old 10-09-2018, 06:33 PM   #21
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Re: Reading A/C gauge

I have another poa valve. Is there a way to check if it is moving free and not stuck before it is installed?
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Old 10-09-2018, 09:05 PM   #22
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Re: Reading A/C gauge

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I have another poa valve. Is there a way to check if it is moving free and not stuck before it is installed?
Yes they can be tested.
Remove the valve or the discharge line from the valve.
Hook the low side pressure gauge to the POA valve.
Then pressurize the POA valve, I use air regulated to 60 lbs at the evaporator inlet with the expansion valve removed. I use a rubber tip blow gun.
For R12 the pressure should be about 30 lbs at sea level, 27 or so for R134a, add 1 lb for every 1000 ft above sea level.
The POA is adjustable through the outlet, look inside you will see a hex nut. Its a strange size though.
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Old 10-09-2018, 09:20 PM   #23
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Re: Reading A/C gauge

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Yes they can be tested.
Remove the valve or the discharge line from the valve.
Hook the low side pressure gauge to the POA valve.
Then pressurize the POA valve, I use air regulated to 60 lbs at the evaporator inlet with the expansion valve removed. I use a rubber tip blow gun.
For R12 the pressure should be about 30 lbs at sea level, 27 or so for R134a, add 1 lb for every 1000 ft above sea level.
The POA is adjustable through the outlet, look inside you will see a hex nut. Its a strange size though.
Thanks Randy. So if I have a poa valve sitting on the bench I could cap off the discharge hook up my gauge and blow 60 lb on the inlet. Will this let me know if it is free inside?
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Old 10-11-2018, 01:01 PM   #24
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Re: Reading A/C gauge

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Old 10-11-2018, 04:01 PM   #25
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Re: Reading A/C gauge

YOU can test the POA in the truck
TO test it idle at 1500 rpm
Use a fan and put it in front of condenser (radiater)
unplug the blower motor
gauge on low side reading should be between 28-30 psig

the POA factory setting should be 32 .

there are other things to check also , Air in system
make sure your heater core is not bleeding by
also spray water on the condenser and see if the lowside pressure drops.
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