Many owners put ignition work and fluid changes first and forget the accessory drive belt and its rollers. The belt may offer the best return for the money anywhere under the hood. This guide covers what the system includes, how long it lasts and what to look for,.
What the accessory belt system includes
Accessories, sometimes called ancillaries, usually include the alternator, A/C compressor, power steering pump and water pump. They sit on or around the engine's front end and are turned by pulleys that the crankshaft drives through the accessory drive belt. The accessory belt system proper is made up of the belt, the tensioner and the idler pulleys. The accessories are not part of it.
How a drive belt is built
A drive belt has tensile cords at its core, covered in a rubber that resists oil, for example neoprene or another synthetic. The cords give the belt its strength and tension, so it can carry load without sagging between pulleys. Sag loads the accessories unevenly and can bring on early belt failure.
One face of the belt carries molded ribs that seat in the accessory pulley grooves; the opposite face is comparatively smooth, sometimes lightly textured, and runs over the idler pulleys and tensioner. Between them they maintain enough tension that the belt can grip and turn the pulleys, staying on and not slipping. These are simple rotating parts, yet they are frequently what brings down a neglected system.
Accessory belt versus timing belt
A timing belt turns the camshafts, so the valves can open and close clear of the pistons. Its materials are similar, and it also has a tensioner plus idler rollers, but it is normally toothed instead of ribbed, to suit the camshaft pulleys.
Neglect is far costlier here. The most common reason a timing belt breaks is missed maintenance, and the consequence is much worse than from a frayed accessory belt or a seized accessory pulley. In an interference engine, a snapped belt brings pistons and valves into contact, and the engine must be disassembled.
One example is the Audi 4.2 liter 40-valve V8: its timing belt interval is about 75,000 miles, and skipping it almost guarantees damage. How often belts are due and how often they fail varies with the make and the engine, so if you are not sure when it was last done, research the timing belt history of your engine.
Service intervals
Each part has its own service interval range, and wear depends on how the car is operated, so no single mileage applies. On the European makes covered here, a change of the accessory belt is suggested somewhere between 60,000 and 100,000 miles, with a check whenever other maintenance is carried out.
Belt and pulley replacement
It is harder to judge the health of a pulley, because the belt has to be removed first. Pulleys should last roughly as long as the belt, so replacing the two together is advised, treating belt and pulleys as one complete system.
Signs of a worn belt
A deteriorating belt shows itself in ways you can see: cracking, glazing, stretching, tears, splits and oil soaking, plus squealing at start-up. If the belt is oil-soaked, the oil leak needs to be found and fixed while the belt is being replaced.
If a belt fails, the best outcome is losing the charging system and the air conditioning. A car cannot run for long without electricity, so the extra minutes you might get from electric power steering and the water pump do not change much. With a mechanical water pump, losing the belt creates an immediate overheating risk. Power steering loss makes turning much harder.
Checking pulleys and the tensioner
Idler pulleys and tensioner pulleys have just the bearing to worry about. Spin it and listen: a healthy bearing is silent, while a worn bearing is loud and makes a crunchy, gravel-like noise caused by pitted, loose balls.
Besides the bearing, a tensioner has a roller, which is almost always replaceable on its own, and an arm in which a hydraulic piston or a wound spring keeps the belt taut over its service life. Watch for fluid leaking straight from it, a lack of belt tension, and a knocking sound while the engine runs. Each points to a failing tensioner. If it leaks, or if the belt is slack, that tensioner is broken. A new tensioner, if anything, applies excessive pressure to the belt, so a belt that is less than taut signals trouble. A knock is much less common and occurs when the belt slaps back and forth against the tensioner.
Doing the job yourself
Swapping an accessory belt ranks among the better DIY jobs. How hard it is depends on where the engine sits, but the belt is usually easy to reach, and the job calls for a basic tool set and an afternoon. Get the right parts before you start.
With a belt kit that includes the rollers and tensioner, nothing more needs buying. Febi Bilstein, INA, Continental and Bando are among the OE/OEM brands that appear in belt kits.
The INA range is another to browse.
You can also look at Febi Bilstein parts for this job.
Key takeaways
- The system is the belt, the tensioner and the idler pulleys; the accessories are separate.
- Expect a belt change somewhere between 60,000 and 100,000 miles on the European makes covered, with inspection during other checks.
- Replace the belt and the pulleys together, as a complete system.
- Cracking, glazing, stretching, squealing, tears, splits and oil soaking are the warning signs.
- A tensioner that leaks, or a belt that is loose, points to a broken tensioner.
- The timing belt is typically toothed, is a separate part and costs far more if neglected.
Own a BMW F20, F22, F30 or F32? Check out our Genuine BMW basic accessory belt service kit.
General information only. Always check the specification for your own vehicle.