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ggenovez on Thu July 15, 2010 10:19 PM User is offline

Year: 1990
Make: GMC
Model: Suburban r-1500
Engine Size: 350
Refrigerant Type: R12
Ambient Temp: 95
Pressure Low: 45
Pressure High: TBD
Country of Origin: United States

Hi all,

Sorry about the lack of high side reading but for some reason the shrader valve is really short and my adapter can't get to it.

Here's the problem

Chart shows that at this ambient temp, I'm supposed to be at 22 PSI. She starts at 85 static, and slowly goes down to 45 and stays there. Compressor is always running, and never cuts off.

Lowest vent temp I had was 64.

Do we think she's overcharged?

Any advice?

BTW she only has a front AC unit not a rear/dual...


bohica2xo on Fri July 16, 2010 2:54 AM User is offline

Need a high pressure reading to say much of anything for certain.

The static pressure of 85 psi @ 95 f would indicate a less than optimal charge however.

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HVargas on Fri July 16, 2010 11:17 AM User is offlineView users profile

I agree. The high side reading is going to tell you a lot more about what is going on in the system than the low side. But with an ambient temperature of 95*, assuming the car is outside, and a static pressure of only 85, I would venture a guess that the system is undercharged as well. Check all of your connections and hose crimps to see if you can see any oily residue. If not, I would recommend getting the system professionally reclaimed to see how much refrigerant was in the system and have it leak checked from there.


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mk378 on Fri July 16, 2010 11:35 AM User is offline

If it really is a "deep throat" valve (the pin on the valve core is well below the top of the fitting), you'll need this adapter to connect to the high side.

ggenovez on Sat July 17, 2010 12:05 AM User is offline

ggenovez on Sat July 17, 2010 12:07 AM User is offline

Thx Guys,

Doesn't look like a deep throat. My guess is the shrader was replaced and is a little shorter than the factory one. I'm sure if I can find a way to push the valve another 1/16th I can get a reading...

I checked the static after I had ran her and don't know if I she had time to re balance the pressure. What should the static be at 95?

77shovelhead on Sat July 17, 2010 12:18 AM User is offline

Still learning here and wanted to check my knowledge a little here.

Is static pressure the same as when the engine is off and the system has had time to equalize?

Quality is never an accident it is always the result of intelligent effort.

ggenovez on Sat July 17, 2010 12:29 AM User is offline


Cussboy on Sat July 17, 2010 12:52 AM User is offline

Originally posted by: ggenovez
My guess is the shrader was replaced and is a little shorter than the factory one. I'm sure if I can find a way to push the valve another 1/16th I can get a reading...

Once upon a time, I added a little bit of solder to the depress "button" on my R-12 gauge fitting to get it to depress a "low" schraeder valve.

And on my '94 Suburban (which was factory R134a) I always had to hold down tightly the high side fitting to get a reading. Now I've since replaced the AC manifold lines on that vehicle, and the replacement manifold fittings work fine "as is" with my R134a quick-connects, don't have to press down to take a reading any longer.

I think it's a GM thing, like the twilight zone, bizarro world. Every time I need a part for it there are multiple choices. Took me forever to find that the gaskets above its oil filter could be found under aux. oil cooler adapter, not under engine stuff...

Edited: Sat July 17, 2010 at 12:53 AM by Cussboy

ggenovez on Mon July 19, 2010 11:48 PM User is offline

I'm thinking of finding something that will fit over the schrader and allow it to depress. My concern w/ solder is, with my luck it will met the rubber O ring...

ggenovez on Fri July 23, 2010 3:32 PM User is offline

Thank God for beer!

I used a bottle cap, cut a little sliver, and bent over the stem.

TADA! got it to depress.

So today 95 outside, on the R12 pressure I'm seeing 50 on the low and 165 on high. Of course on the PSI side it's much higher... Is that much of a swing normal?

Am I looking at the wrong side on the gauge? on the PSI side of the gauge it's 55 low and 275 on high.

ACProf on Fri July 23, 2010 3:41 PM User is offline

PSI is PSI whether its water pressure, tire pressure, or refrigerant pressure, so you SHOULD be reading only the PSI scale when you are measuring pressure. THe other scales on the gauge are TEMPERATURE of the various refrigerants at the measured pressure. Ignore those additional scales for now.

ggenovez on Fri July 23, 2010 4:38 PM User is offline

I see. So do the readings indicate over charged?

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