Air conditioning '98 ML320
 

Is it possible to tell how much charge is in the air conditioning on a '98 ML320 by reading the low side pressure alone? I read 35 psi at idle, at 85 degrees ambient with a hot engine. Seems high but it blows cool air, not cold. Should the compressor run constantly at idle or cycle on and off?

I understand vehicles with a variable displacement compressor you can't tell the charge by reading pressure, you have to evacuate the system and refill with exact quantity of refrigerant. Is this one of them?