For all 3.5, 4.5, 3.8 and 5.0 engines with air conditioning.
If you have no idea when this pulley was last replaced, just plan to replace it when you do your V-belts. The bearings can get quite noisy long before it actually fails.
1970 to 1985 V8 A/C Belt Idler Pulley
$38.60
SKU:
GEG-EV8IDLERPULLEY 
Product Details
Location of the idler pulley.
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420SEL 560SEL 560SL 560SEC V-Belt and Fan Clutch Replacement Kit
If you are going to replace one of these you probably should replace the other.
Both items you see above with complete video instructions. Click on each product included for more info. There are other parts you may want to replace at the same time such as the water pump, thermostat, short hose, and radiator hoses at the same time. See related products below.
$67.80
Early 4.5 450 V8 V-Belt Replacement Set w/ Detailed Video Instructions
Fits 1972 to 1976 280SE 4.5, 300SEL 4.5, 450SE, and 450SL
Four new Continental belts with detailed video instructions. You should consider replacing the following at the same time, as they are commonly due to age and neglect.