The serpentine belt drives many engine accessories – alternator, water pump (on some engines), power steering pump, AC compressor, and more. The belt itself is a wear item that should be inspected at every service. But the components that tension and guide the belt – the automatic tensioner and idler pulleys – are equally important and often overlooked. Failure of either causes belt squeal, belt loss, and potentially catastrophic engine damage on engines where the timing belt drives the water pump.
Inspecting the Belt
With the engine off, examine the belt for cracks (small cracks across the ribs are normal on older belts; deep cracks or chunks missing mean replacement), glazing (a shiny smooth ridge surface from slipping), fraying along the edges, or oil contamination. Modern EPDM belts no longer crack visibly when worn – instead, the rib material wears away. Use a belt wear gauge that fits between the ribs to check for excessive wear. Most manufacturers recommend serpentine belt replacement every 80,000 to 150,000 km.
Inspecting the Tensioner
The automatic belt tensioner has a spring-loaded arm that maintains constant tension on the belt as the belt stretches and accessories move. Test it by putting a wrench on the tensioner pulley bolt and rotating it against spring tension – it should move smoothly with steady resistance and return to its rest position when released. A tensioner that feels stiff, jerky, gritty, or that does not return is failing and should be replaced. A worn tensioner often makes a knocking or squealing noise.
Inspecting Idler Pulleys
Idler pulleys guide the belt around the engine bay and have sealed bearings inside. Spin each idler pulley by hand (with the belt off if necessary) – it should spin freely and quietly. A pulley that feels gritty, that has any wobble, or that makes noise has a failing bearing and will eventually seize. A seized idler pulley breaks the belt instantly and stops the engine.
Replacement Procedure
Take a photo of the belt routing before removal – belt routing diagrams are often complex and the workshop manual or a sticker under the bonnet shows the correct path. Use a wrench on the tensioner pulley bolt to rotate the tensioner against its spring, releasing tension on the belt. Slip the belt off one of the simpler pulleys (often the alternator or AC compressor). Then route the new belt following the diagram, releasing tension on the tensioner one last time and slipping the belt onto the final pulley.
Best practice is to replace the tensioner and idler pulleys at the same time as the belt – they all have similar lifespans and labour to replace them separately is more than the cost of the parts. See our serpentine belt replacement guide for additional details.
Belt routing, tensioner location, and torque specifications are vehicle-specific. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
— MechanicMate . Questions or a second opinion? [email protected].