Chevrolet dealer technical service bulletin 99-06-04-029A stated:
Condition: Some customers may comment on an intermittent accessory drive rumble noise when performing a garage shift and/or during parking lot maneuvers, such as turning the steering wheel to the lock position. This noise may also occur when turning on the air conditioning at idle conditions.
Cause: Engine idle speed may drop below 600 rpm while performing a garage shift, turning the steering wheel to the lock position, or turning on the air conditioning. This noise may be amplified by a large rate of change in engine rpm. This condition can also be aggravated by different low engine rpm driving conditions.
Correction: A new generator with decoupler pulley and reduced damping accessory drive tensioner was implemented for all Corvettes with automatic transmissions at 2001 SOP (start of production). The generator with decoupler pulley was used only on Corvettes equipped with an automatic transmission. The reduced damping accessory drive tensioner is used on all 2001 Corvettes.
That was then, this is now. Fourteen years later, we are seeing more vehicles coming from the factory with alternator overdrive decouplers (AODs). Meanwhile, millions of the decouplers have been in service, wearing out and showing up for replacement.
Car, and many light truck, interiors have become havens from the outside world. Some argue that it has become too much so and reached the point of drivers ignoring the primary job of driving. But that is an issue for another story. Suffice it to say the motorists will not abide unwanted noises, especially in high-end vehicles.
Over-running alternator decouplers
The first versions of over-running clutch pulleys were simple one-way clutches. They turned in one direction, but not the other. They were called alternator over-running clutches (AOCs) or alternator decoupler pulleys (ADPs).
The latest version, the over-running alternator decoupler (OAD) differs from earlier components by having a spring inside that absorbs torsional vibrations (base engine vibrations). Most techs just call all of these devices pulley clutches. However, there are differences and the various styles are not interchangeable.
Simple, one-way clutches allow the alternator pulley to continue spinning when the engine is shut off and during shifting. This helps prevent the belt noise and chirping that often occurred when the belt stopped, but the alternator tried to keep spinning.
After all, there is a lot of mass on a high-output alternator rotor that wants to keep rotating. The higher the alternator output, the more massive the rotor and the more inertia.
Engines suddenly decelerate when switched off. But the mass inside the spinning alternator resists stopping. This happens not only when the engine is switched off, but also when any sudden change in engine speed occurs, such as when the transmission downshifts into passing gear or whenever the driver aggressively shifts gears at wide-open throttle.
Sometimes, the problem occurs during slow speed maneuvers such as parking or when the air conditioning compressor kicks on at idle, particularly when the transmission is in drive.
One of the advantages to carmakers is that the OAD allows them to use narrower drive belts and lower reduced effort belt tensioners. This reduces the bearing load on the other components such as the air conditioner compressor, power steering pump, water pump and idlers. Improved fuel economy from lower idle speed and reduced parasitic loss is another benefit.
“The OAD is the device absorbing the energy in the belt drive and, as such, is a wear item. It is the punching bag for the accessories,” says John Lussier of Tendeco Sales Inc., which distributes OADs in North America.