Magazine for the auto service professional
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56 | September/October 2014 the primary job of driving. But that is an issue for another story. Suffce it to say the motorists will not abide unwanted noises, especially in high-end vehicles. Over-running alternator decouplers The frst 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 pul- leys (ADPs). The latest version, the over-running alter- nator 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 alter- nator 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 alter- nator 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 con- ditioner compressor, power steering pump, water pump and idlers. Improved fuel economy from lower idle speed and reduced parasitic loss is another beneft. "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. The frst over-running alternator pulley was installed on a 2000 Opel in Europe. That was followed in North America by the trio of Chrysler minivans: Voyager, Caravan and the Town & Country. Today, there are over 52 million vehicles equipped with OADs. Industry experts predict that that number will climb to 160 million by 2018. Currently, 95% of all Toyotas have OADs; 100% of Chrysler's 4-cylinder and V-6 engines have them. All of GM's engines currently under development will come equipped with them. Ditto for Hyundai and Kia. Why don't all engines have over-running alternator decouplers? Probably due to cost. A carmaker that looks to save a few pennies on an unnecessary bolt must be convinced that the component is worth it. Traditional pulleys, whether driven by a V-belt or multi-rib belt, are attached to the alternator with a nut. OAD pulleys have a fange on the front that sticks out past the belt grooves. OADs are attached to the alternator shaft with a bolt, usually an "Allen" or Torx type. You will need special tools to diagnose or replace these pulleys. Connections Look closely at this photograph. The cap pro- truding from the nose of the Jeep Renegade alternator means that it has an over-running alternator decoupler (OAD). Photo by the author