Squealing or Shrieking Noise From Under the Bonnet

What Does The Noise Mean, And is it Serious?

If you hear a high pitched shrieking noise from under your bonnet, especially when you rev your engine, the fault will almost certainly be with your alternator belt.

The alternator is a round looking object that is bolted onto the side of your engine, and which looks a bit like a large electric motor. But what it will have on the front of it is a pulley wheel. This pulley wheel is driven by a rubberised belt, and this will be wrapped around another pulley that is driven by the engine.

The purpose of the alternator is to provide electrical power to charge the battery, power the lights and other electrical equipment, such as your CD player.

The shrieking noise is the belt slipping on one of the pulleys and it will do this most vigorously when a lot of electrical current is required from the alternator.

Typically this will happen just after you have started the engine for the first time that day, or if you are running your heated rear window. The more current that is required the greater the resistance within the alternator, so the harder the pully becomes to turn.

What Happens If I leave It?

Whilst the belt is not shrieking it doesn’t mean it isn’t still slipping, and if it is slipping then the alternator isn’t out–putting the right amount of current. When that happens your battery will gradually discharge, so that one day, when you go to start the car, nothing will happen. It can die altogether when driving along, whereupon your engine will just stop.

Another thing that can happen is the belt will snap, simply because of all the slipping that is going on, so wearing the material so thin it cannot cope.

On some vehicles the alternator belt not only drives the alternator, but also the water pump. So, when the belt snaps, no coolant is being pumped around the engine. Continue driving and the engine will overheat and that can cost you a complete replacement.

Why Does The Belt Start to Slip?

There are two common reasons for this, and the most common is the tension of the belt has not been adjusted correctly the last time it was worked upon. Alternator belts should be tight as a drum, and if there is any play in it, where it runs between pulleys, it isn’t tight enough.

The other common reason is the belt it very old. It has stretched over time and no longer has the capability to retain the tension.

So, How Can You Tell if Your Alternator Belt Has Snapped?

After all, when you are driving along you can’t see under the bonnet.

The usual tell–tale sign is the red ignition or battery light will come on, showing your instrument panel. This signifies there is no electrical charge being sent from the alternator to the battery. If you see this, pull over as soon as you can and investigate.

Two Vital Things to Remember Here.

NEVER put your hands anywhere near the engine bay when the engine is running. Before doing anything under the bonnet, switch the engine off and take the keys out of the steering lock.

Once you have switched everything off, BE CAREFUL of hot components, as these will hurt you as bad as putting your hands onto the hotplate of your domestic cooker with the power switched on.

Assuming you have made all precautions, try to identify the alternator. It would be useful to identify certain engine parts before anything goes wrong and in the comfort of your own home area. It isn’t so easy when you are in the dark, stopped on the hard shoulder of the M40, and when it is throwing it down with rain.

If You Can See The Pulley, The Belt Has Snapped

If you can see the pulley on the front of it then the belt has snapped and will probably have fallen out of the bottom of your engine bay. To be safe you are going to need to call for help, because to drive any further can mean overheating the engine and⁄or draining your electrical power to nothing.

The answer here to not having to cope with all of this is to make sure you keep your car well maintained and properly serviced by a reputable workshop. If you begin to hear a shrieking noise from under your bonnet, or any strange noises, get it checked out as soon as possible before something happens that will cost you dearly.

Similar Symptoms, Different Problem

If your shrieking noise only happens when you press the clutch pedal dowm, it is not your alternator belt that is the problem. This is most likey to be the thrust bearing on your clutch. If that is making a shrieking noise, and it is not sorted out, it has the potentail to do a lot of damage if you keep using it.

Speaking again of red ignition warning lights showing up on your dash panel, if the battery light is glowing softly this will be a fault with the alternator. The part that has failed when this happens costs less than a pound, but if you are not an auto–electrician, the problem will cost you a new alternator (about £95.00). To continue driving like this is unlikely to harm the car, but your battery may well die on you through insufficient electrical charge being maintained.

One of the most common causes of the faint battery light glow problem is using a battery charger to re–charge a battery, but with the alternator still connected.

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This page was last updated
Wednesday, 02-Feb-2011

Car Maintenance – Electrical Faults