# Drive Belts and Alternators: Squeal, Charge Warnings and When to Replace **By Sawley's Auto and Marine** ยท 2026-09-27 A drive belt is the cheapest part on the front of a marine engine and the one most likely to put you on the radio. On a sterndrive or an inboard it turns the alternator, the circulation pump and, on many engines, the raw water pump as well, so when it lets go you lose charging and cooling in the same second. This guide covers how to read a belt before it fails, how to tell a belt problem from an alternator problem, and what to carry so a squeal on the water stays a nuisance rather than a tow. ## Two kinds of belt, two kinds of trouble Older engines run one or more V-belts, the narrow wedge-section type that sits down in a pulley groove and is tensioned by swinging the alternator on its bracket. Later engines, including the Volvo Penta D series, run a single wide poly-V or serpentine belt with a sprung automatic tensioner, driving everything in one loop. The V-belt fails by wearing down into the groove until it bottoms out and slips, or by stretching past the adjustment on the bracket. The serpentine belt fails by cracking across the ribs, shedding chunks of rib, or by the tensioner losing its spring so the belt flaps and slips under load. Both go glazed, which is the shiny, hardened flank that comes from slipping, and once a belt is glazed it will slip no matter how tight you pull it. Volvo's own service note is blunt about it: a glazed or cracked belt is renewed, not retensioned. ## Reading the belt With the engine cold and the key out, twist each run of belt a quarter turn and look at the working face. Fine cracks across the ribs of a serpentine belt, more than a few per hand span, mean it is at the end. Missing ribs, frayed edges or a belt that has walked to one side of a pulley point to pulley misalignment or a seized idler, and a new belt will go the same way in a season unless that is fixed. Black dust on the engine below the pulleys is belt material, and it means slipping. On a V-belt, a cross-section that has worn down until the top of the belt sits level with or below the pulley rim is finished. Check tension by pressing the longest run between pulleys with a thumb: about ten millimetres of give on a span of three hundred millimetres is the usual figure for a V-belt, while a serpentine belt with an automatic tensioner should need no adjustment at all, and if the tensioner arm sits at the end of its travel the belt has stretched. ## Squeal, chirp and rumble A squeal on start-up or when a load comes on, such as the fridge compressor cutting in or the battery charger asking for current after a night on the hook, is slip, and the cure is a new belt plus whatever caused the slip: a worn pulley, a tired tensioner, or coolant or oil on the belt from a leak above it. A rhythmic chirp that comes and goes with revs is usually a misaligned pulley or a damaged rib catching. A rumble or growl from the belt area that changes when you release tension is a bearing in the alternator, the idler or the water pump, and you can isolate it by slipping the belt off and spinning each pulley by hand. Rough, gritty or noisy means that bearing is going, and on a raw water pump it will take the seal with it. ## Belt or alternator? The charge warning light, or a voltmeter reading low with the engine running, is the common thread between the two, and five minutes with a multimeter separates them. With the engine running at a fast idle, read the voltage at the battery terminals. Somewhere between 13.8 and 14.4 volts means the belt is driving the alternator and the alternator is working. Battery voltage with nothing on top of it, around 12.5 volts and falling, means no charge, and the first check is whether the alternator pulley is turning with the belt or the belt is slipping over it. If the pulley is turning and there is still no charge, the fault is in the alternator or its regulator, or in the excite wire from the ignition switch that tells the alternator to start working. Above 15 volts is an overcharging regulator, which boils electrolyte out of flooded batteries and cooks sealed ones, and the alternator comes off the engine either way. Alternators also fail quietly. A failed diode in the rectifier leaves the alternator charging but putting alternating ripple onto the twelve volt system, which shows up as flickering instruments, a tachometer that reads high or wanders, electronics that reboot, and batteries that never seem to come up fully. Set the multimeter to AC volts across the battery with the engine running: more than a few tenths of a volt of AC is a diode on its way out. Our [marine battery guide](/blogs/guides/marine-battery-start-deep-cycle-dual-purpose) covers what a healthy battery should read at rest, which is the other half of the picture, and if you run a house battery as well, the [second battery wiring guide](/blogs/guides/wiring-a-second-battery-vsr-dc-dc) explains why a smart alternator may not be charging it at all. ## Marine alternators are not car alternators On a petrol inboard or sterndrive, the alternator and starter live in a closed engine bay with fuel vapour, and both must be ignition protected, meaning built so a spark inside the housing cannot ignite vapour outside it. A car alternator bolted on to save money is the one modification a surveyor or an insurer will not forgive, and the brushes and slip rings are open to the air. On a diesel the risk is different but the environment is the same salt-laden, hot engine room, and a marine-rated unit carries the corrosion protection to live there. Genuine units for Volvo engines are in the [Volvo Penta charging and alternators range](/collections/volvo-penta-charging-alternators), with belts, pulleys and tensioners in the [belts and tensioners range](/collections/volvo-penta-belts-pulleys-tensioners), matched to engine and serial number so the ribbed count and length are right first time. ## What to carry A spare belt of the correct part number, the tools to change it, and the knowledge of which pulley swings to release tension. On a serpentine system that is a socket or a square drive on the tensioner arm, and a photograph of the belt routing on your phone saves twenty minutes of guesswork with the boat drifting. Change the belt at the interval in the engine's service schedule whether it looks tired or not, and change the tensioner or idler when its bearing starts to sound dry. If you want the right belt matched to your engine, ring [0412 532 984](tel:+61412532984) with the engine model and serial number and we will check it against the parts list before it ships. **Tags:** Electrical, Guides, Servicing, Volvo Penta --- > Source: [Sawley's Auto and Marine](https://www.sawleysautoandmarine.com.au/blogs/guides/drive-belts-alternators-squeal-charge-warning)