I'm attempting to sort out a friends 300 and need some guidance.

The a/c compressor run only after the car is started and then the clutch disengages anywhere between 15-40 seconds. Low-side is 32 and high-side is 150 more or less. The car must be turned off and then restarted for the compressor to engage again.

Even if I bypass the pressure switches on the drier the compressor does the exact same thing. The a/c control panel is brand new and my friend said that the klima "relay" thing is only a few months old.

My guess is that the compressor is bad. But, based on normal compressors I would be able to diagnose it better. With 3 wires going to it, the blue gives 11.8v before it suddenly disappears and the clutch disengages.

Ideas?

If there is belt sippage, then the compressor will disengage. Check the belt tensioner to see if it is good and the tension in the belt.

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1986 300E (3/2002) Rear ended and totaled (10/2009)
2000 Xterra (5/2000)
1992 400E (11/2009) -Sold
1986 300E (12/2009) - Sold
2004 E500 Wagon (6/2013) - Sold

Since the car has to be restarted, sounds like textbook speed sensor on the compressor. They eventually go bad. Its not worth replacing just the sensor...its more cost effective to just replace it. In short, its giving you a false belt slippage reading and killing the power. It saves that until the car is turned off, then it resets, hence why it works on restart.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

check the compressor clutch air gap, if its too much the clutch may slip which kills compressor power till the engine is restarted. This is my car's clutch hub - it toasted the magnet coil

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Since he has not responded, I will assume the problem is still present. The belt tensioner behind the power steering pump has a tension gauge on the cast bracket. The arrow on the gauge must be to the left facing the engine to confirm the belt is properly tight. A 17 mm socket and short extension is used to loosen the tensioner pivot, then a 13 mm wrench is used on the tensioner bolt to move the tensioner pulley. Turn the 13 mm bolt clockwise.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.