I was advised to replaced the fan clutch on my 89 420 to help the AC work better. The temp gauge stays at around 85-90 never gets to 100 degrees. The freon is OK he says.

Sometimes when I turn off the engine the fan stops in 1/2 turn. Other times when I turn it off the fan will make 10 or more turns before stopping.

Is that normal?

Yes, the clutch resistance is determined by temperature. If it stops in 1/2 turn when hot, I'd say it's working fine.

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2012 E350
2006 Callaway SC560