your compressor was probably cycling off and on before. Im not sure what the low pressure cut out is set to open your compressor cluch circuit.(a good tech manual will tell you) However what happens when your clutch cycles is; it cuts out on low pressure and as soon as the pressure rises enough your clutch engages again...and so on... Therefore: even though your comp. is running you will have no cooling, or poor cooling(unless you "hot wire" your compressor,which is a bad idea).
This being said i still believe that your suction pressure is too low, because it will drop when you drive. Before you add any refrigerant i would clean your evap., because a dirty evap can cause a low suction pressure.
Now with gauges attached and engine running pull your throttle cable and watch your suction pressure drop. see if your comp. clutch disengages. Idealy you want a suction pressure that corresponds to about a 50 degree evap temp,with your car ideling. You also must be sure that you arent bringing liquid back to compressor. You will know if your bringing liquid back because your comp will sound bad and you can feel liquid on the bottom of your suction line. This is bad.
Now if your comp runs constantly and you have hot air, this can indicate many things.
1. Dirty cond coil (high pressures all around)
2. Inoperative condensor fan
3. Restriction in the system (most likely in the orfice). With a restriction you would have a low suction pressure and a reletively high discharge pressure. eventually your low pressure cut out will turn off your comp.
4. Overcharge (high pressures all around)
5. Air in your system ( high/inconsistent pressures)
6. Mixed refrigerant (for those of you wondering r-12,and r-134 dont mix,even though it may work for a while, it will kill your system.)

Here are the pressures you should get when the system is charged and compressor is running.

http://www.mercedesshop.com/shopforu...9&d=1145327904

Thanks to both of you for your help, guys.

A/C is definitely not my area. Normally, I would have just taken this straight to a mechanic I use for the few things that I am not confident in working on, but funds are tight right now, so I decided to try to learn about the system from the FSMs and from the knowledgable members here.

I can post up the charts from the FSMs for the '97 if you think it would benefit the club.

Echo,
I'd like to see the FSM charts, if you dont mind posting.

OK, what next? I'll look for leaks under the hood this AM. If I don't find any, should I put in a can of refrigerant to aid in the soap bubble method of leak detection?

Is there a complete set of o-rings available?

Item # TEM801800 at napa claims to be complete seals kit. I would also change the schraders on your access ports, as these are known to leak. If you have no pressure in the system you must raise the pressure. The best way to do this is by boosting the pressure with dry nitrogen.. You dont need pressures above 7-10 psi to bubble leak check. Never boost pressure with "canned oxygen" pure oxygen and oil can cause an explosion. play with the soap water mixture if its too thick or too thin, it wont bubble. depends on the soap...50/50 or two parts water one part soap.

Umm...can I find "canned nitrogen" somewhere?

Not unless you know somebody with a cylinder. Plus they would need a pressure manifold regualtor because the cylinders have about 10,000 psi in them. You can use refrigerant, it would just be all lost.

Guess I'm stuck, then. I'll try it today.

Thanks for all your help.

OK, reporting in,

As the system was discharged, I added 2 cans of R134A. The compressor kicked on after the first can was added (quickly) and I had no chilled air. Second car fixed that. I now have a 33 degree differential between cabine ambient and and air exiting the ducts. (43 degrees for the exit air). It'* been 10 minutes and I see no decrease in the pressures.

Pressure show 33 psi on the low side and about 125 on the high. I couldn't see any leaking with the soap bubble test, but I'm going to put the car up on the ramps and soapsuds the compressor in a few minutes. No oil stains that I can see anywhere.

Advice, so far? How are my pressures and air temp?

Thanks.