When faced with an illuminated charge light diagnosis on most late-model vehicles, do not automatically assume the problem is inside the alternator and potentially unserviceable. The problem could be the pulley and belt drive system. Neglecting to test and diagnose the pulley can lead to unnecessary replacement.

Almost every late model car or truck is equipped with an overrunning alternator pulley (OAP) or an overrunning alternator decoupler (OAD). An OAP is a one-way clutch like a socket wrench that turns in one direction and locks when turned the other direction. An OAD operates in the same manner, but has a special clutch and spring that absorbs vibration to smooth out vibrations in the drive belt system. Regardless of the type, the pulley should be checked before condemning and removing the alternator.

These new pulleys allow the alternator to “free-wheel” or “overrun” when the belt suddenly slows down. This prevents the belt from slipping and reduces vibration. Best of all, the system need less tension and even a narrower belt can be used. This can result in a 1.5% to 2% fuel economy improvement. However, these pulleys have a limited lifespan due to how they operate internally.

 Inspection Procedure

1. Raise engine speed to 2,000-2,500 rpm in Park (auto trans) or Neutral (manual trans) and then shut off the engine. Listen for any noises from the OAD after the engine is shut off. A worn-out bearing will generate a “buzz” noise during this test. If the OAP is noisy during this test, replace it.

2. Remove the cap, and with the proper tool inserted into the front of the OAP, rotate the alternator’s shaft in both directions. In the overrun direction it should feel smooth and in the drive direction it should have a spring feel.

• If the pulley is locked up, replace it.

• If the OAP has no spring feel in the drive direction, replace it.

• If the OAP requires more than 9-13 in./lbs. (1-1.5 Nm) of torque to turn in the overrun direction, replace the OAP. • If the OAP is not smooth in the overrun direction, replace it.

Like most other systems on late-model vehicles, charging systems have become smarter and more complex.  Today’s computer-controlled charging systems tailor the charging rate not only to the electrical demands on the battery and alternator, but also to changing driving conditions. That makes diagnosis much more difficult when something goes wrong. Alternators have one of the highest return rates of any repair part – often because of misdiagnosis. Bench testing an alternator on a test stand should verify whether or not its output is within specifications.  If the unit tests bad, your customer needs a replacement alternator. But if the unit tests good, the problem is something else such as a bad voltage regulator, PCM or wiring harness. Loose, corroded or damaged wiring terminals at the back of the alternator are common causes of charging problems. Wiring connectors and terminals may appear to be okay on the outside, but have loose, corroded or broken wires inside.  Other sources of trouble include loose, corroded or damaged battery cables and ground straps, blown fuses in the power center, or a blown fusible link in the wiring. Another source of trouble can be miscommunication or lack of communication between the PCM and alternator or regulator.  You can have a good alternator that is capable of producing the required charging voltage and current, but it may not work properly if it doesn’t communicate properly with the PCM. Another problem that sometimes occurs is that some “economy” reman alternators that are listed to fit a particular application are not totally compatible with the charging system controls. The alternator may bolt right in, but it fails to communicate with the PCM preventing it from charging normally.  Another item that can affect an alternator’s output is the pulley. Many late-model vehicles do not use a solid alternator pulley. Instead, they have an Overrunning Alternator Pulley (OAP) or an Overrunning Alternator Decoupler Pulley (OAD). An Overrunning Alternator Pulley (OAP) has a one-way clutch mechanism inside the hub that allows the belt to turn the alternator in one direction, but allows the alternator to free-wheel and spin at its own speed when the engine suddenly decelerates.  The pulley should lock up when it is turned one way, but freewheel when it is turned in the opposite direction. If the internal clutch mechanism is bad, the pulley may not drive the alternator, or it may remain locked all the time increasing noise and vibration. An Overrunning Alternator Decoupler (OAD) pulley also has a one-way overrunning clutch inside as well as an internal torsion spring to further dampen vibrations in the belt drive system. The spring acts like a shock absorber to cushion the hub.  This reduces noise at idle and low engine speeds, and helps dampen harmonic vibrations at higher speeds. If the clutch or spring inside the pulley has failed, the pulley may fail to drive the alternator, or it may create vibrations and noise. OAP and OAD pulleys usually thread onto the alternator shaft whereas solid pulleys are typically a straight slip or press fit with a large bolt on the end of the alternator shaft to hold them in place. Some replacement alternators come with pulleys and some do not.  If an alternator with an OAP or OAD pulley is being replaced, and the replacement unit does not come with a pulley already installed, the original pulley can be removed from the old alternator and installed on the new unit – provided it is in good condition. However, on high-mileage vehicles, replacing the original OAP or OAD pulley with a new one is recommended to assure trouble-free operation.

Dear all , GP PRECISION will attend the Automechanika Shanghai 2-5th Dec 2020.

Our Booth No : HALL 4.1 .  F47 

Welcome to visit us !