When an alternator decoupler pulley fails, the alternator may still be producing the correct volts and amps. The decoupler pulley is a “one way clutch”. When an engine is at a cruising RPM of “3400 RPM” and the engine suddenly drops to an idle around 800 RPM the inside moving parts of the alternator do not have to slow down at the same time as the engine. Mass in motion stays in motion. The one way clutch or “decoupler” allows the rotating mass “heavy parts of the alternator” to rotate freely, independently of the engines drive belt. This decoupling pulley reduces the flywheel effect placed on the engine and the engines drive belt. When the decoupler goes bad. Fuel economy will be reduced.
I have clients that think they need a tune up because they are not getting the same fuel mileage as they use to and it turns out the one way clutch on the alternator could be the cause. Many things on todays car can be broken or very worn and you will never know they are bad. Some broken parts will never turn on a check engine light. During a major service interval, I may have the drive belt off during the service and that is the best time to inspect the alternator decoupler.
I have had some clients, that watch fuel mileage for work related jobs. Tell me that they noticed up to a 1.5 MPG change after replacing the faulty decoupler. That may not seen like that big of a deal, but over a years worth of driving it adds up.
