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  #1  
Old 06-29-2017, 01:17 PM
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Arrow AC Charge Question - High Side and Low side identical? SOLVED

Hi AC folks -

W123 / 82 300D

- 30 minutes vacuum pump time, vac held overnight properly
- charged with 2.2 lbs of R12 this morning
- compressor engages when turning cooling system on
- 40 degrees now at the vents in idle, in park
- measure with the gauge and the high and low side pressures are identical 30PSI with the car idling
- ambient temp is 80F, 50% RH
- compressor does not seem to be cycling, it's always on

its working...but the high side is totally confusing me . the whole system is new, flushed, vac'ed, pressure tested etc.

thank you in advance :-)


SOLUTION:

OK - so problems identified and solved.

1. My gauge set hoses don't depress the valve core. The set I have, the brass tab isn't long enough to push in the core, so it can't take a reading. Grr. Ordered another of a different brand, but it looks like it has the same issue.

2. I had no valve core in the low side - it didn't come with one. So the reading I was getting were indeed just from the low side. There was also some pilot error on my part operating the gauge set.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Last edited by dieseldan44; 07-08-2017 at 09:41 AM.
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  #2  
Old 06-29-2017, 01:26 PM
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If the unit is cooling then likely your gauge is bad. It's pretty typical for ac to run non stop at idle. If you raise the idle to about 2k rpm it should start to cycle
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Old 06-29-2017, 01:39 PM
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Were booth valves off before you hooked up gauges? If yes the that would lead me to believe the expansion valve is stuck open which is slightly irregular or the compressor is shot and is bypassing pressure internally.
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Old 06-29-2017, 02:53 PM
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Do you have the typical 3-hose "gage set" and have you used it before? In my HF one, the valves at the hose ends open (screwed in) to apply pressure to the low (blue) and high (red) gages, resp. The valves on the body each open to connect that side to the center (yel) hose (for vacuum or fill). If you open both those valves, you will connect your cars high & low side together, which will bypass the compressor. If not, then since compressor is engaging, either the flapper valves and/or pistons are leaking, or the expansion valve is stuck open.
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Old 06-29-2017, 03:15 PM
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High and low sides equal when running it would not cool. You are cooling very well it sounds like.

Last edited by barry12345; 06-29-2017 at 03:25 PM.
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  #6  
Old 06-29-2017, 03:51 PM
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Quote:
Originally Posted by barry12345 View Post
High and low sides equal when running it would not cool. You are cooling very well it sounds like.
Exactly, it's a gauge problem, or some issue with the manifold (the gauge valve system).

If you weighed in 2.2 lbs of r12 and have cold ac disconnect those gauges and go have a beer. Your done, success, don't over think it
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  #7  
Old 06-29-2017, 07:37 PM
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Thanks everyone

I do have the ubiquitous Harbor Freight Gauges. Thanks Bill for clarifying how those ones work.

I think the high side valve core must be stuck.

New problem. I went to go take the gauge set off, and the low side service port just spit refrigerant at me. I quickly put the gauge set back on.

This must mean I have a bad valve core on the low side? Please tell me there's some way to fix this without removing all the refrigerant. That hose is brand new, do they not come with valve cores? I remember checking for one and seeing it.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car
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Old 06-29-2017, 11:04 PM
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Are you saying a bad valve core AT the base of the pressure line where it connects to the car ?
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Old 06-29-2017, 11:06 PM
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There IS a special tool which can remove an old schraeder valve core and put a new one in while the system is charged...
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Old 06-30-2017, 01:47 AM
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Be careful with those gage sets. At the ports, screw in the valve just enough to open the Schraeder valve, maybe 1 or 2 turns after you see pressure on the gage. If you over-do it (like most people), the Shraeder valve often gets bent. You can usually use needle-nose pliers to re-bend it so it doesn't stick open, or at least pull the stem out so it stops leaking. I think you can also get caps that will seal the port even if it leaks.

If determined to replace it, know that even if you totally remove it, it might take 2 min for all the refrigerant to escape since it must boil off to do so. I have replaced one while venting only ~2 sec (negligible loss). I used a nice tool from auto parts w/ a big brass cylinder for your fingers, but you even see removal tools in dust caps for bicycles. Use an AC Shraeder valve, not one for car or bike tires (rubber will degrade). BTW, compression gages use a very special Shraeder valve (white plastic) w/ very weak spring so they don't affect the reading (acts as a check valve). I bet many people install a regular one and wonder why compression reads low. EPA rules make it illegal to vent any refrigerant to the atmosphere, so hope their drones aren't watching. True, even for the HC refrigerant I use, which is the same environmental impact as a cow fart, and they vent all day long.
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Old 06-30-2017, 08:04 AM
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Quote:
Originally Posted by dieseldan44 View Post
Thanks everyone

I do have the ubiquitous Harbor Freight Gauges. Thanks Bill for clarifying how those ones work.

I think the high side valve core must be stuck.

New problem. I went to go take the gauge set off, and the low side service port just spit refrigerant at me. I quickly put the gauge set back on.

This must mean I have a bad valve core on the low side? Please tell me there's some way to fix this without removing all the refrigerant. That hose is brand new, do they not come with valve cores? I remember checking for one and seeing it.

I found it really helped me in understanding the plumbing of the manifild gauge set by pressure testing. I put the red and blue hose ends in a bowl of water and blew compressed air in the yellow hose and observe the gauges and when you see / not see bubbles. Once you've done a pressure test with a good manifold, you should know that to take a pressure reading of the system, both knobs should be turned off (CW).

As for hi/ lo side pressure being the same. It would be if taken with the system off and stabilized at static pressure, otherwise your gauge is off or you read the scale wrong.

You said that's a new part with the suction port that's spewing refrigerant. Did you check the tightness of the Schrader core before use? Before replacing the core, maybe worthwhile to try tighten it a little and see if that'd stop the leak.
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Last edited by funola; 06-30-2017 at 08:18 AM.
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  #12  
Old 06-30-2017, 09:02 AM
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thanks again for the help -

i see the schrader valves listed on this site. are they the same as generic ones at an autoparts store? im hoping theres nothing special about them...

what does this magical tool look like to service the port while full and where do i get one?

i am an inexperienced AC tech at best - i probably over tightened the hoses on the car and maybe on the gauge set. good thing i have a second gauge set.

i also just scored a 30lb tank of R12 on CL for $100 :-) ... so i am ready to get this all fixed.
__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Last edited by dieseldan44; 06-30-2017 at 09:12 AM.
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  #13  
Old 06-30-2017, 11:03 AM
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The tool is so magical that I am not sure I have ever seen it ... but I read about it...and posted the information.. AND VStech confirmed that it existed....
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  #14  
Old 06-30-2017, 11:34 AM
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Quote:
Originally Posted by dieseldan44 View Post
thanks again for the help -

i see the schrader valves listed on this site. are they the same as generic ones at an autoparts store? im hoping theres nothing special about them...

what does this magical tool look like to service the port while full and where do i get one?

i am an inexperienced AC tech at best - i probably over tightened the hoses on the car and maybe on the gauge set. good thing i have a second gauge set.

i also just scored a 30lb tank of R12 on CL for $100 :-) ... so i am ready to get this all fixed.
This Yellow Jacket 18975 1/4" 4-in-1 Ball Valve Tool - Core Removal w/ Side Port | eBay

https://www.youtube.com/watch?v=xhJGHkaPOPg&spfreload=10

should be the tool if you have reverted back to R12 fittings.

Did you see my earlier suggestion to tighten the core first?
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  #15  
Old 06-30-2017, 01:31 PM
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There is a cheaper tool version on ebay direct from China. For now you might just be able to screw on a cap with an incorporated rubber gasket. Some caps come with them. Then even forget about the tool and system for now.


For those that have never used this tool. I unscrew the valve with it installed. Pull the vacuum for smaller heat pump installations. Let a little pressure escape until it shows two to three pounds positive in the system. Then screw the valve back in. Then release all the pre contained system pressure.


This was not the intended use but it does avoid any intake of moisture back in and seem to speed up the vacuum draw down time at least to me.


I hit 100 microns quite fast and assume it is doing a better job when still running with less restriction. Might be overkill as well but it is what I do. If you are intent on purchasing one of these tools.


I have a very strong high volume vacuum pump and may be wasting my time with my approach. When you could buy a really good brand of commercial big name lightly used vacuum pump for a song on ebay .


Why buy a toy in comparison? It is somewhat heavier by about a factor of two in comparison to the usual less durable vaccum pumps.


There is one valve changing brand out there that claims it is really tight. The others are known or suspected to have a small leakage factor. When used. Although I see no evidence my cheapie leaks.

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