Hit it with some kroil or pb blaster. you can also try to get the bottom lubed...it comes thru the bracket just forward of the adjusting nut near the oil cooler lines. It goes thru the bracket, then a spacer, then the adjustment mechanism then thru the compressor
You may need a breaker bar.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT