Alternator Pulley (Overrunning Pulley)
The alternator pulley transfers drive from the accessory belt to the alternator; many modern ones are overrunning pulleys that freewheel in one direction.
What is Alternator Pulley (Overrunning Pulley)?
An overrunning alternator pulley (OAP) or decoupler (OAD) contains a one-way clutch, and in decouplers also a spring, so the heavy alternator rotor can keep spinning when the engine slows suddenly. This reduces belt flutter, tensioner wear and noise, especially on diesel engines with strong crankshaft pulses. Simple solid pulleys are still used on many petrol engines, and the pulley usually comes off with a special splined tool.
How it works
The one-way clutch locks when the belt drives the alternator and slips when the alternator tries to run faster than the belt.
Why it matters for your car
A worn overrunning pulley can cause belt chatter, a vibrating tensioner, squeal on start-up and charging irregularities. Manufacturers often recommend checking or replacing it together with the accessory belt and tensioner.
Other terms
- ABS Warning Light
- Atkinson Cycle
- Catalytic Converter Theft
- Ceramic Coating
- Control Unit Reprogramming (Flashing)
- Coolant Temperature Warning Light
- Dielectric Grease
- Immobilizer
- Megawatt Charging System (MCS)
- Open Circuit
- Pedestrian And Cyclist Detection
- Plug & Charge (ISO 15118)
- Rescue Sheet (EV Rescue Card)
- Small Overlap Crash Test
- Trailer Sway (Snaking)
- Transmission Fluid (ATF)
- Trolley Jack (Floor Jack)
- Vehicle Speed Sensor
- Windscreen De-Icer
- WLTP (Worldwide Harmonised Light Vehicles Test Procedure)