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Old 07-02-2003, 09:16 PM
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Join Date: Jul 2002
Location: Stony Brook, NY
Posts: 160
Question What should high & low side pressure be for A/C?

Before I bought my 1988 300SE last year, the prior owner had the car converted to 134a. All the requisite parts were changed and the work was done by a reputable Mercedes shop. I saw receipts for thew work. I want to make sure the system is charged properly. I bought a nice set of gauges to check both the high and low side. Now my only problem is that I can't find any information as to what the pressure of my converted system should be with the car off and with the car running and A/C on. So, anyone know what the pressure should be so I can use my nifty new gauges?


Ron Brooks
Ron Brooks
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Old 07-02-2003, 09:43 PM
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Join Date: May 2000
Posts: 376

Sorry no answer for you, but I have the same question for 190e with R12?

I have the factory air conditioning manual for a W124 not a W201?

Does anyone have the specs? Or know of where we can find them on the web? Should I just aim for the W124 specs????


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Old 07-02-2003, 11:06 PM
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Join Date: Jun 2003
Location: Here an' there.
Posts: 2,464
Speaking of 134a ...several cars and truck I've done with this freon have had reasonable sucssess....however, any older car (Especially 911's ) stay with R12 for as long as you can.
From temp' measurements I have made the following is somewhere normal ish' !!
1800 RPM
Lo side = 28 to 35 psi.
Hi side = 240 to 315 psi.
Humidity 75 to 90%
This is with the system stabalised, MAX COOLING . BLOWER HI, doors closed, windows rolled up, and ambient temperatures around 80 to 90 degrees F. (I have set my gauges to zero at 1200 feet above sea level.)
The variables are amount of oil in the system. Your vacuum pump quality. Expansion valve 134 compatible and drier the same.
Visual check on evaporator core, where possible, for any obstruction and a blend door system that works as designed.
On systems with a sight glass in the drier DO NOT LOOK FOR A CLEAR SIGHT WITH 134a. it doesn't happen, as the pressure rises the glass will go milky....134a will be fully charged with just a few bubbles runing past the sight glass......I fill the system by monotoring the vent temperature with the vehicle in the shade, ie: no sun load. When the vent temp rises 1 degree from the lowest reading system is as cool as it is going to get......
This is often 44 to 48 degrees F, a few drop to 38 or 36 F but this is very rare ....134a will not chill as cool as R12 system for system...difference is between 6 to 8 degrees warmer..Remember too, you also need different oil for 134a as the R12 oil will not dissolve in the 134a .......
I know there will be some tech's who have their own way and figures, but many of the change over 134a jobs are slightly different and the text book case is quite rare in my experience.
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Old 07-04-2003, 11:25 AM
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'94 S500: only 793 sold!
Join Date: Mar 2001
Location: San Diego, CA
Posts: 1,924
Check my page for . . .

a typical Mercedes air temp/pressure/humidity chart contained in MENU #16. About in the middle of the page.

Although it for the 140s, it wil be close to what you should expect. Click on the "www" icon below to get to my page.
Regards . . . . JimF
'94 S500 Cpe

Visit my Mercedes Web Page
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Old 07-05-2003, 02:39 PM
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Join Date: Oct 2001
Location: California
Posts: 278

Please post the URL for your web page. Thanks.
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