Well, I talked to my mechanic and was told that on the V8 W140's checking the voltage regulator requires removal from the engine bay because the back of the alternator is not accessable without removal- a one hour (read $185 labour for remove and reinstall) process. The rebuilt alternator is $150 as opposed to the $38 for the voltage regulator. $335 for the whole job as opposed to $223 for the regulator only.
I decided to go ahead and replace the alternator. With my luck he'd replace the voltage regulator and the alternator would go bad the next day.
