Part explained
Auxiliary (serpentine) belt: interval, price, symptoms
The auxiliary belt — also called poly-V belt, accessory belt or serpentine belt — drives everything that isn't connected directly to the crankshaft: the alternator, the AC compressor, often the water pump and the power-steering pump. It is **not** the same as the timing belt (a common confusion). This page explains the difference, the interval, and what replacement costs.
By the onderdelen.autos editorial team · Updated · About the editorial team
What does the auxiliary belt do?
The auxiliary belt runs around the crank pulley on the outside of the engine and transmits mechanical energy to a row of accessory units. It has lengthwise V-shaped grooves (hence "poly-V") that grip against the pulleys. Tension is held constant by an automatic tensioner — a spring-loaded wheel that maintains fixed pressure. Wear is in the rubber: after years of UV and engine heat, hairline cracks appear across the grooves. If the belt snaps or jumps, the alternator, AC and (where applicable) water pump quit instantly — the engine still runs, but the battery drains and the engine may overheat.
Symptoms — what to look and listen for
- Squealing or chirping at cold start or under load (AC on).
- Visible hairline cracks across the grooves, or shiny wear spots on the back (slight mirror sheen).
- Bits of rubber around the pulleys or in the engine bay.
- Battery warning light (alternator no longer driven).
- AC cools poorly at idle.
- Persistent rattling from the tensioner or idler — not the belt itself, but directly coupled.
When to replace
Indicative 80,000 – 120,000 km or whenever cracks become visible. Some manufacturers specify a fixed interval (often around 90,000 km), others only call for visual inspection. Rule of thumb: have the belt checked briefly at every service after 80,000 km. The tensioner often lasts a bit longer, but replacing both at the same time is sensible — a worn tensioner can't apply correct pressure to a fresh belt.
Typical interval: 80,000–120,000 km
What does the belt (and tensioner) cost?
The auxiliary belt itself runs € 15 – € 50 depending on length and brand. The automatic tensioner is a separate part: € 30 – € 100 for the tensioner. Workshop labour: 0.5 to 1.5 hours for the belt, slightly longer with the tensioner. A used tensioner from a scrapyard is possible, but new parts are cheap enough that it rarely makes sense.
| Condition | From | To |
|---|---|---|
| New | € 15 | € 50 |
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DIY?
Difficulty: Medium
On an accessible engine bay, an auxiliary belt swap is a 30- to 60-minute job. Biggest pitfall: the belt routing. Before you remove the old belt, photograph or sketch the path — otherwise you'll spend an hour puzzling out the correct route around all the pulleys. Release the tensioner by turning the hex with a ratchet or a dedicated tool; tension comes off and the belt slips free. In cold weather a new belt may squeal briefly the first few minutes — that disappears once it's bedded in.
- A poly-V tensioner tool or a half-inch ratchet that fits the tensioner
- Pen and paper to sketch the belt routing before you start
- Possibly a new tensioner
- Good lighting — inspect routing with a torch
Frequently asked questions
Is the auxiliary belt the same as the timing belt?
No. The timing belt hides behind a cover and drives the camshaft from the crankshaft — if it breaks, the engine grenades. The auxiliary belt is on the outside and drives accessories; breaking is annoying (flat battery, no AC) but not engine damage. Different intervals, different price, different job.
Does the tensioner always need replacing too?
Not required, but sensible after 150,000 km or if the tensioner rattles. A new belt on a worn tensioner won't get enough tension and will squeal sooner.
My auxiliary belt squeals — do I need to replace immediately?
Squealing can come from cold rubber, a wet engine bay, or wear. Inspect the belt for hairline cracks and shiny wear spots. No cracks? Try a rubber conditioner. Cracks? Replace.
What does a workshop charge?
Expect € 50 – € 150 including the belt for an accessible engine. With tensioner and idler: € 150 – € 300.
Can I buy a used auxiliary belt?
Not recommended. Rubber ages with time and UV exposure, even off the car. The difference between new and used is € 20 — not worth the risk.
Related parts
Auxiliary belts from Dutch inventory
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