1990 mercedes 500slJuly 26, 2014when at idle a/c blows cold air ,when driving no cold air. Keywords:500sl 1 thought on “1990 mercedes 500sl” Freeautomechanic July 31, 2014 at 9:44 am Without your gauge readings, I am going to go as far as I can with this. If you can post the pressure readings for your high and low sides (with A/C on, and the engine at roughly 1500 rpm) as well as the temperature around the car when you take them, I will be able to help more. You have two pressure safety switches inside your system. One for the high pressure and one for the low pressure. The low pressure cuts off the compressor when the pressure drops below a certain point, typically between 16 and 26 psig) At idle your compressor is not turning as fast therefore your low side pressure switch stays engaged. The faster the engine revs, the faster the compressor turns therefore it is going to pull a lower pressure on your low side, and may be disengaging your switch. It is very possible that your refrigerant charge may be low due to a small leak. As the engine develops higher rpms, it pulls the low side pressure below the opening point and shuts down your compressor. With your pressure readings I can expand more, but from the sound of it this is the most likey cause. A low refrigerant charge. The other switch, the high pressure switch, protects your system from overly high pressures. The main problem of high pressure is a dirty condensor. The condensor is located in front of your radiator. Check it to see if there are any leaves blocking airflow through it, dirt or sand in between the fins, or if more than 20% of the fins are bent/damaged. If it is dirty, a simple water hose will do wonders for it. If the fins are damaged/bent, a tool called a fin comb is available at most auto part stores and used to straighten them back out. I hope this information helps you out. Try cleaning your condenser and if that does not help send me your pressure readings. The A/C world is controlled by pressures and you would be amazed at the different things they can tell a technician. Comments are closed.