Low side of AC goes into vacuum? - Mercedes-Benz Forum
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post #1 of 11 (permalink) Old 05-21-2019, 04:23 PM Thread Starter
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Low side of AC goes into vacuum?

My AC stopped blowing cold air. I assume it's simply low on refrigerant but when connecting gauges it's showing the following:

AC off: low side is around 40, high side is around 40 too.
AC on: low side goes into vacuum (-12), high side is around 75.

Anyone can diagnose what's going on?

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post #2 of 11 (permalink) Old 05-21-2019, 04:43 PM
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You have stopped up expansion valve or not enough refrigerant to get things going. What refrigerant? R12, R134A?

1972 280SEL 4.5
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post #3 of 11 (permalink) Old 05-21-2019, 04:50 PM Thread Starter
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Originally Posted by Witek_M View Post
You have stopped up expansion valve or not enough refrigerant to get things going. What refrigerant? R12, R134A?
It's R134a. Should i add some before trying to identify the issue further?
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post #4 of 11 (permalink) Old 05-21-2019, 07:11 PM
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Originally Posted by patrykrebisz View Post
It's R134a. Should i add some before trying to identify the issue further?
Try to add some, super slow. Do not exceed 60 psi on low side, gas only. When high side gets to ~200 psi stop. Wait for system to stabilize and take another reading.

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post #5 of 11 (permalink) Old 05-22-2019, 09:42 AM Thread Starter
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Originally Posted by Witek_M View Post
Try to add some, super slow. Do not exceed 60 psi on low side, gas only. When high side gets to ~200 psi stop. Wait for system to stabilize and take another reading.
After adding one can of a134r (340gr) my low side was at 5 and high side at 145. Ambient temperature was around 73F (23C) and the vents blew 50F (10C). Should i keep on adding more?
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post #6 of 11 (permalink) Old 05-22-2019, 10:09 AM
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First things first. Do NOT put any more of the "stop leak" crap into the system. If you have a leak, find it and fix it!

Here's a link to help you with diagnosing problems.

R-134a System Pressure Chart - AC Pro<?php/* bloginfo('name'); */?>
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post #7 of 11 (permalink) Old 05-22-2019, 10:16 AM
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With ambient temperature of 73:

low side should be ~35-45 psi
high side should be ~145-165 psi

Reading should be done at 1,500 to 2,000 rpm

Last edited by 260e crusier; 05-22-2019 at 10:21 AM.
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post #8 of 11 (permalink) Old 05-22-2019, 10:31 AM
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You've probably covered this in a previous post but I'm too lazy to look it up, what all was done when the car was converted to R134a?

New compressor, "barrier" hoses fitted, new dryer, system flushed, parallel condensor installed, etc.?

As 260E Cruiser mentioned, readings should be taken at apx. 1500 RPM's.
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post #9 of 11 (permalink) Old 05-22-2019, 10:51 AM Thread Starter
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You've probably covered this in a previous post but I'm too lazy to look it up, what all was done when the car was converted to R134a?

New compressor, "barrier" hoses fitted, new dryer, system flushed, parallel condensor installed, etc.?

As 260E Cruiser mentioned, readings should be taken at apx. 1500 RPM's.
new compressor - york (for r134a), new dryer, system flushed, old hoses but they did hold charge for the 3 summer months last year.
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post #10 of 11 (permalink) Old 05-22-2019, 11:18 AM
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Sounds as though you'll be chasing a leak. What fun!

I'd suggest if you do have to open the system you replace the expansion valve and all O-rings for ones compatible with R134a. This is not what is causing your current low pressure problem but it will help in the future.

It is not uncommon for R12 systems to leak when converted to R134a due to its smaller molecules. That is why barrier hoses are recommended for R134a.

You might also consider retrofitting a parallel flow condensor.

Not a big fan of the York compressors but if it works for you...
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