The A/C on my 300CE24 91 (R12) is not working since I bought it, and now it's time to take care of it...

So, conditions:

- Outside temp = 72 F = Vent temp,
- low side pressure = 70 PSI,
- high side pressure 0 !
- through the sight glass = heavy stream of bubbles,
- compressor not switching (turning ON and OFF when activated by the inside command, but not cycling),
- when A/C is off: low side = high side = 50 PSI.

Low refrigerant? Expansion valve?

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From 300CE to SLK230K

You sure you have the gauges connected correctly? There is no way (that I'm aware of) that the high side can be lower than the low side with the compressor running and refrigerant in the system.

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1989 300 SEL that mostly works, but needs TLC

The high side service valve might be broke. If you're seeing bubbles streaming, that indicates that the compressor is indeed pumping something which means you should be getting at least some pressure, say maybe 30, or 50 psi (certainly not 0). I would double-check your high-side fitting and/or look for obstruction or poor connection from the hose to the gauge. If it all checks out, your valve might be bad...

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

My high side hose/connection/gauge seems fine: when I turn off A/C (without disconnecting anything), the high pressure side then indicates, the same pressure as the low side (which I think is expected), showing 50 PSI (so measuring something).

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From 300CE to SLK230K

Surely, the compressor hoses are not reversed causing refrigerant to go backwards. Did not think they could be connected backwards.

When both read 50 with system off, does the one on the high side drop to zero when the system is turned on?

Never have heard of anything like this.

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Lawrence Coppari

2002 SLK32 AMG
2005 Acura TL
1987 328GTS
1986 944T

My mistake: high and low side were connected backwards (I missunderstood the text and picture on the service manual, but the first page diagram helped me understanding...).

So now the problem is low side = 0 and high side = 70 PSI; just a low charge?

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From 300CE to SLK230K

Your LOW gage should be connected to the large diameter hose coming back from the evaporator in the passenger compartment.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

This is a 91, so R12 = easy to mistake the fittings since they're the same size (good thing you weren't charging from a can).

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Start with getting the charge correct, and look for leaks.

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1989 300 SEL that mostly works, but needs TLC