'95 S420 A/C system pressure question
 

What is the system pressure at rest? My gauges both read 65 PSI. Today was the first time I tried the A/C and the air just blows warm. I saw a sticker that dye was used last August so it must have had a leak before. Also the refrigerant pressure switch has 3 wires to it. Which should I jump to see if the compressor kicks in?

You can't jump these switches. This isn't the same as an old 300E...The newer cars have pressure sensors, not switches. The rest pressure of the refrigerant can vary depending on the outside temperature. I would guess it should be at least 80PSI or so.

So it would be safe to add half pound or a pound to see if the system works then, since at 65 psi it doesn't?

65 PSI sounds low to me too. I'm not sure what static pressure should be on this R-134a system, but the rule of thumb for R-12 is that static pressure equals temperature (F), and R-134a has a generally higher pressure/temperature curve. So unless it was really cool out, 65PSI sounds low.

Is the compressor engaging or not? If not, it strikes me as unlikely that adding refrigerant will affect that problem. Almost every car - older MBs & just about everything else I've ever worked on - cutout the compressor when static pressure falls below ~30PSI. This function both protects the compressor in a low charge situation and also serves to shut down the compressor when outside temperatures fall below freezing.

If the system is running but not cooling, don't just go adding refrigerant if you don't have a set of guages. If you can't monitor both the high & low sides, you won't know what's going on nor whether you are likely to damage the system.

I checked the system this morning at 45 degrees and the pressure was only 32 PSI. After adding a half pound of R134 the compressor started working. It took another full pound before the bubbles in the sight glass stopped. NOW the air blows cold!

If the system has already has had dye entered in it , ck the a/c drains with the ultraviolet light and see if it is the evaporator which is common. Most of the time you can smell it from the center vents. You can also read how many bar the pressure switch has via the pushbutton control unit .