on my 91' 500SL when i turn the a/c on for the first time in the day the compressor always start,but after the engine reach the normal operating temp. and I turn off the car the a/c compressor will not engage after restarting the car like 15 min later but if I turn it off again and restart the car the compressor will engage. can anybody please give me some ideas. thanks.

Start by checking the engine belt. If it's loose replace it (I think yours has the automatic tensioner), if it's manual try tightening the belt.
Your car has a speed sensor on the compressor. If the engine RPM doesn't match the compressor speed signal it cuts off via KLIMA relay.

If that doesn't work you should check resistance of evaporator temperature sensor. This sensor sends a signal to the compressor according to temperature to avoid freezing.
The sensor is located above the driver's side AC drain.
Resistance below 2.5k ohms is a faulty sensor.