A/C Pressure: Low side = 70 PSI / High side = 0?
 

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?

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.

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...

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).

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.

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?

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

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).

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