Alternator decouplers: Servicing these vibration-reducing pulleys

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Alternator decouplers: Servicing these vibration-reducing pulleys

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.


The first 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 flange 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.

What makes the OAD different is a spring inside that absorbs torsional vibrations. Think of it as a “suspension system” for the alternator. It smoothes out the pulses, the speed-ups and slow-downs in belt velocity.

As you know, the crankshaft speeds up and slows down between power strokes causing torsional vibration. This conditions smoothes out at higher rpms, but is very noticeable at low rpms, especially on smaller engines or engines that feature selective cylinder-kill.

The internal torsion spring design is patented by Litens Automotive Group of Toronto, Canada. Replacement OAD pulleys are available from Gates, Veyance (Goodyear), Schaeffler (INA) and Advance Auto Parts.

As you know they also are available from the automobile dealerships’ parts departments, but you may want to compare prices. The aftermarket companies mentioned above sell OEM quality components.

Installer beware

Beware of counterfeit parts. According to Tendeco, “... a number of global distributors and installers contacted us regarding our 2.0L VW/Audi timing belt tensioner. The complaints have focused around difficulties during installation. All the samples submitted were counterfeit parts produced using recognizable brand names as well as the OE logos and part numbers.”

Also, install only quality remanufactured alternators. The selected OAD must be tuned to the application. Using the wrong replacement will almost certainly result in a comeback. Be suspicious of any rebuilt alternator with a substantially low price.

Beware that many times alternators are unnecessarily replaced, when simply replacing the clutch pulley will solve the problem at less expense. In the aftermarket, OAD prices range from about $50 to $120, depending on application. Labor, however, can run from less than a half-hour to several hours. Be sure to consult your labor guide before giving a quote to the customer.

Corvettes and many Toyotas are among the easiest to replace. Chrysler minivans fall somewhere in the middle. However, the Dodge Caliber and its kin (Avenger, Sebring, Compass and Patriot) are among the most difficult. The air conditioning compressor is in the way and access is from below. Experts recommend replacing the OAD at the first indication of any noise or vibration on these vehicles.

Diagnosis is fairly simple: look, listen and feel

Look at the belt while the engine is running. It should not be fluttering, but running smoothly throughout its circuit. Look at the belt tensioner. It should not be fluttering, dithering or vibrating excessively. Look for signs of grease around the pulley indicating a bad seal. If you can get a view inside the alternator, the rotor should continue to spin down when you shut off the engine. If the charging system warning indicator is on, the OAD may be the culprit, not necessarily the alternator.

Listen for belt rumble while the engine is running which usually indicates a problem with the OAD. With the transmission in park (neutral for manuals) and the parking brake applied, have an assistant rev the engine up to 2,500 to 3,000 rpm then switch off the ignition. The alternator’s rotor should coast down for up to 10 seconds and if you hear a buzzing sound, the OAD bearing is probably shot.

Using the appropriate special tools for the application, hold the pulley stationary and rotate the alternator shaft both ways while noting how it feels. The OAD should rotate smoothly in one direction with a bit of spring-back when you relax. If it will not rotate in either direction, rotates in both directions, or if there is roughness while turning, replace the OAD.


Replacing the AOD

There are three different ways to replace a clutch pulley and apply to both the alternator overrunning clutch (AOD) and overrunning alternator decoupler (OAD).

The first, and easiest, requires the special tools. This method is the quickest and can be done with the alternator in the vehicle in many cases.

Pop the protective cap from the end of the pulley. Some can be pried off. With others, you need to pierce the cap with a pick or small screwdriver and then pry it off.

Holding the pulley shaft with the proper tool, rotate the alternator’ rotor shaft with a box-end wrench. Once it is free, simply spin off the pulley by hand. Spin the new pulley onto the alternator shaft then install the attachment bolt using the same special tools. Caution: you MUST tighten it to the specified value (usually about 65 ft.-lbs.) using a torque wrench.

The second method requires you to remove the alternator from the vehicle and remove the rear housing from the alternator. You then clamp the rotor in a vice. This eliminates the need for special tools, but complicates the job unless you are also rebuilding the alternator at the same time. Be careful not to damage the rotor or fan.

The third method requires an impact wrench and torque stick. Place the alternator in a vice and hold the pulley by hand or with a band wrench. With the impact on its highest setting, pull the trigger two or three times. The pulley’s shaft will turn, but with a little luck the impact will turn faster.

This method is the least desirable. And, it is hard to get the proper torque when installing a new pulley. You also risk damaging the rotor or new clutch pulley.

However, this is the only method that works with many Mitsubishi Electric alternators, since most do not have a typical installation feature on the end.

This method also requires you to reset the OAD clutch after installation. To do so, rotate the shaft in the overrun direction at least two times by hand until you can feel the spring force.

Finally, install the new cap that comes with every OAD pulley. Many cavities are now grease filled and without the cap, it could get quite messy under the hood.

Multi-rib accessory drive belts were commonly made of neoprene and generally lasted about 50,000 miles, but since the mid-1990s, drive belts have been made of EDPM and can last up to 100,000 miles. The key words here are “up to.” Check the belt, tensioner and clutch pulley every 50,000 to 60,000 miles.

Since the OAD, belt and tensioner are a system, it is a good idea to replace them together.

They likely have the same time and mileage on them, so do your customer a favor and prevent a future breakdown due to a weak charging system or broken belt. At the very least, inspect these components whenever a belt is replaced.   ‚óŹ

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