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During the last few months my car developed a not too loud, but annoying noise (squeak) coming from the auxiliary belt area. Sometimes it starts as soon as the car is started, sometimes it seems to do it only at least a few seconds later. I am not sure, but I think it does it more often when it's wet and cold outside (every day is like this recently, so I can't confirm), I remember not having this during dry days. The noise silences when the RPMs rise.

I am not sure if this is related, but I've had some coolant spill near the belt area. I know coolant makes belts squeak, but this happened months ago. I've also heard that coolant evaporates really slowly, but it's still hard to believe that this could be the cause.

The belt is tensioned automatically, as far as I know. So, what could be causing this and similar problems? What can a home mechanic do to fix them?

  • Does this answer your question? – Zaid Jan 28 '16 at 8:04
  • Not really, it squeaks constantly once it starts - until I park. Also, the belt should be tensioned automatically. I've gone through the posted answers, none help really. Except one that mentions pulley misalignment. Decided that it would be great to know all possible causes. – I have no idea what I'm doing Jan 28 '16 at 8:09
  • What is the year/make/model? Cannot tell you anything otherwise. – Pᴀᴜʟsᴛᴇʀ2 Jan 28 '16 at 11:11
  • It's a 1999 Volvo V40 T4. Aren't the reasons for the auxiliary belt squeak pretty much the same for any car? – I have no idea what I'm doing Jan 28 '16 at 11:23
  • Try spraying some "Belt Grip" on your belt to see if the noise goes away. If it doesn't, then at least you have eliminated the belt as the cause. – HandyHowie Jan 28 '16 at 11:57
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The culprit in this specific case was the steering pump pulley - it developed a slight play along the axis and the auxiliary belt started rubbing the sides of the pulley ridges.

Since no one has a more comprehensive answer for similar issues I can try answering it myself now that I am more knowledgeable in this area.


Causes of the noise

The auxiliary belt can produce an annoying squeaky noise for two main reasons:

  • Slipping on the pulleys;
  • Rubbing a misaligned pulley.

Which one it is can be determined from the noise it makes. If it is a constant "squeal" that occurs or increases in volume when the RPM is increased (especially when the engine is cold), the belt is likely to be slipping on the pulleys, making a god-awful noise. If the noise is a more subtle periodic "chirp" - the culprit is probably the belt ridges/sides excessively rubbing the groove sides of the pulley.

To be sure there is one easy test that can be done. Take a spray bottle of water and lightly spray some on the belt and on the pulleys while the noise is occurring. If it suddenly goes silent - it indeed is a misaligned pulley. You just lubricated the part that was rubbing and the noise went away, unfortunately it will come back soon. If the noise suddenly got louder - your belt is slipping and it temporarily started slipping even more.

You can also inspect the belt itself if you're still not sure, but it usually requires the belt to be removed. If you see fraying on the edge of a belt this usually indicates rubbing a misaligned pulley, if it was happening for long enough. Glazed grooves or edges of the belt indicate that the belt was overheating from excessive slipping.


Causes of slipping

The most basic cause for slipping would be a simply worn belt. It usually shouldn't cause the belt to slip, but it might it if wasn't replaced after previous repairs. The only way here is to replace it.

Another simple cause is fluid contamination. Coolant, oil, power steering fluid and brake fluid can seep into the ridges, attracting dirt and causing slippage. Cleaning the belt and pulleys off with a rubber friendly degreaser should help.

If the belt is in pristine condition and dry, yet still slips - the reason is most likely insufficient tension provided by the belt tensioner. Usually on newer cars the tensioner is spring loaded and retains tension automatically. If the tensioner does not do its job - it needs to be replaced. I believe manual tensioners exist as well, which means they might need to be adjusted as the belt ages and stretches.

A temporary fix for a slipping belt could be belt dressing, as suggested in the comments. It's a sticky substance that basically increases the friction between the belt and the pulleys, reducing slippage. However it also attracts dirt, which will need cleaning off the pulleys when the belt will finally be replaced.


Causes of rubbing

If the belt is rubbing a misaligned pulley, then one of the pulleys has a different offset (parallel misalignment) or a different angle (angular misalignment) than the rest. Parallel misalignment occurs when the pulley is not pressed onto the shaft far enough or pressed too far. This can be checked by placing a straight ruler across adjacent pulleys - they should be perfectly parallel. There might also be lateral looseness in the pulley because of a failing bearing, which would not be shown by this test. If the misalignment is angular, it is likely caused by a worn bearing as well. In either case a bearing problem can be found by taking off the belt and grabbing each of the pulleys, giving them a few tugs and shakes - there should be practically no wobble.

Pulley misalignment types

(borrowed from flowcontrolnetwork.com)


If there are any inaccuracies in this text - please feel free to edit, my knowledge might be flawed here.

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It sounds from the comments that you have eliminated belt slip as the culprit due to the sound of the squeak (and lack of squeal).

If you can pinpoint the squeaking to one specific pulley, it is likely the bearings for that pulley are on their last legs. Bearings perform better under heavier loads, which could explain why the squeak goes away at higher RPMs.

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