sdbenz:

Low charge (hopefully) or bad compressor. A good compressor will vibrate some, but definite knocking noises are internal damage or a bad clutch (note that a leaking compressor shaft seal will cause the rubber in the clutch to fail -- operation is then erratic and noisy as the friction plate is loose).

On the W126 monovalve, pull it and check. It's up by the firewall in the heater hose, four small screws holding down a plastic cover. Pull the valve out (solenoid winding comes off). If the rubber boot is swollen or cracked, replace. If there is any sign of corrosion, you probably have had coolant leaking into the solenoid, and you may need to replace the coil as well.

A leaking monovalve will result in poor cooling as there is heat in the heater core all the time, reguardless of flap position. I would also test for vac leaks -- not closing the recirc flaps will make the car slow to cool off.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!