How to tell if alternator pulley is bad

A failing alternator pulley clutch may trick technicians because its symptoms are so subtle and varied. After all, a bad pulley clutch could create problems ranging from a barely audible noise to a no-charge condition. These tips will ease diagnosis. This month, I’ll mention some alternator pulley clutch applications and briefly describe two types of clutch designs. Then I’ll focus on practical test procedures.

The alternator pulley clutch, introduced in the 1990s, first appeared on some European and Asian imports. Later, both Chrysler and GM used it. Eventually, Hyundai and Kia adopted it, too. Overall, Toyota probably has the largest number of these clutches in service.

First, as its name suggests, a pulley clutch is inside the alternator drive pulley. This is a one-way or overrunning clutch; your shop manual may describe it as an overrunning alternator pulley (OAP). This clutch locks up in the direction of normal alternator rotation so the drive pulley turns the rotor. Spinning the rotor makes the alternator charge.

However, the clutch freewheels in the direction opposite alternator rotation, allowing the rotor to coast during sudden engine deceleration. The rotor inside a modern, high-output alternator contains lots of iron and copper. This considerable mass and its momentum try to continue “driving” the alternator pulley during engine decel. Freewheeling the pulley clutch—and therefore, the rotor—prevents belt slippage during decel. This not only reduces belt wear, it also prevents brief but annoying belt slapping, chirping or squealing noises.

Second, the isolator decoupler pulley (IDP) is a more advanced version of the alternator pulley. For your information, Toyota basically has popularized the IDP by using it extensively for more than 10 years.

Like the OAP design, the IDP has a oneway/ overrunning clutch inside the drive pulley. But it also contains a horizontal coil spring that “isolates” the drive pulley from the alternator’s rotor. This means that power flows from the drive pulley to the isolator spring, then from the spring to the rotor. The spring reduces noise, vibration and harshness (NVH)—no small feat—by isolating the rotor from the alternator pulley and the entire drive belt system.

Failure Symptoms
The vast majority of pulley clutch failures occur on high-mileage alternators. So, the most cost-effective fix usually is to replace the entire alternator. Often, distinctive sounds you hear when the engine decels are keys to recognizing a failing alternator pulley clutch. For example, a brief but telltale buzzing may occur when the engine shuts down or during a hard-acceleration upshift. (The driver floors the gas pedal and notices the sound during the 1-2 upshift.)

When in doubt about the source of this buzz, carefully position your stethoscope or listening tool on the alternator. Ask a co-worker to hold the vehicle’s throttle at a steady 3000 to 4000 rpm, then turn off the ignition switch. Usually you can hear the buzz as the engine shuts down under these conditions.

According to an experienced auto-electric specialist, this diagnosis may elude some techs for several reasons. First, the noise that a failing one-way clutch makes during engine shutdown may sound much more like a metallic rattle than a buzz. Second, they may doubt or overlook the customer’s input. For example, experience suggests that a female driver may hear the failing clutch during engine shutdown long before a male driver notices it. Third, you may have to repeat the engine shutdown test I just described many times in order to hear the bad pulley clutch acting up.

Last but not least, the specialist noted that the more frequent pulley clutch failures he has seen occurred on vehicles enduring a particular kind of service—extended idling with lots of electrical accessories turned on. “Under these conditions,” he told me, “I have seen pulley clutches wear out long before the brushes or bearings inside an alternator did.”

A pulley clutch that locks up and fails to freewheel is not a common occurrence. But if it does happen, you may hear the noises during decel that I listed earlier—belt slapping, chirping or squealing. Occasionally a locked-up pulley clutch causes a drive belt to jump the pulley during sudden decel.

But a bad clutch that freewheels full time is not uncommon. The big clue is a dead-battery/no-charge condition with no obvious causes. By all means, perform the traditional electrical checks first. When in doubt, carefully shine a light into the alternator while the engine is idling. Hopefully, the alternator housing openings are large enough for you to see the rotor and/or an internal cooling fan mounted on the rotor. (Fortunately, many popular alternators have an internal cooling fan.) If the drive pulley is turning but the rotor/rotor fan is not, then you know the pulley clutch is slipping severely.

Now, suppose you suspect a slipping pulley clutch but cannot see the rotor or rotor fan very clearly. In that case, shut off the engine and remove the drive belt. At this point, some techs claim they can feel a slipping clutch simply by twisting the pulley rapidly in both directions with their fingers.

However, you may need to hold the rotor stationary to do a conclusive test here. Cautiously slide a screwdriver, pry bar or equivalent tool into the drive pulley side of the alternator. Be careful; you want to snag the rotor or rotor fan just well enough to hold it steady for a moment or two. Then twist the drive pulley in both directions with your other hand. Under these conditions, you usually can feel that a failed clutch is freewheeling in both directions. Furthermore, the more often you perform this check, the more familiar you’ll become with the feel of a good vs. a bad one-way clutch.

Although partial pulley clutch slippage is not common, an ace Toyota specialist told me he has encountered it. Each time, he said, the vehicle had undercharge symptoms; the pulley clutch seemed to work okay at idle. He gambled that stressing the clutch might make it fail. To do this, the tech loaded the battery with a conventional (carbon-pile) load tester while watching the alternator.

For one thing, a 20 to 30A load made the pulley clutch slip. For another, the alternator could not produce more than 20 to 30A during each load test. Replacing the alternator (pulley clutch included) fixed each of these vehicles for good.

Broken Spring Symptoms
Typically, you troubleshoot an IDP just like an OAP because it utilizes the same style of one-way clutch inside the pulley. Not surprisingly, the common IDP issues simply are clutch failures at high mileage. And although isolator spring breakage has not been commonplace, it has happened. One clue to a busted spring is a no-charge condition, likely accompanied by a drumming sort of noise. Another clue is abnormal drive belt fluttering or slapping noises.

Finally, don’t try replacing an IDP with a traditional, solid alternator pulley. The vehicle owner will notice the difference and won’t be pleased.

Okay, everyone be careful out there. Stay healthy and I’ll look for you here next month.

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