I have a doblo 2007 1.9 diesel MJet, and I have noticed a strange engine noise.

It's almost like a deep turbo whine or transmission noise and it sounds like a slow air-raid siren when the engine is rev'd.

The note of the noise follows the revs, and it happens irrespective of whether it is in gear or not.

I have recorded the sound - see here:

Other than the noise, it behaves as normal.

Any ideas?

Thanks

Yooj

Not a lot of help I know but that is definitely worthy of further investigation as it sounds far from right. Great idea to record it and post it up.

I would start with anything belt driven, alternator, power steering, aircon, water pump and idler pulleys. I saw car once that did this that had picked up a piece of plastic debris that was rubbing against the belt. Also check the belt hasn't frayed. Fortunately it doesn't sound Turbo related.

If you are very careful, you can use a long screwdriver as a stethoscope to identify the problem component. Place the tip of the screwdriver against each of the main belt driven components and put your ear to the handle of the screwdriver. You will know which component is failed as the noise will travel up the driver to your ear. You must be careful doing this though as its easy to get the screwdriver caught in the belt with serious consequences.

Thanks Yobbo,


I took the aux belt off and the noise went. Giving the tensioner pulley a spin, and it appeared to have a bit of noise, so I thought this was the culprit, and so I got a new tensioner and tensioner pulley and belt and installed these.

However, the noise is still present with the new parts, so it must be one of the other parts.

There are four pulleys in total - two large and two small. I have already replaced a small one (tensioner pulley), and the other small one is the alternator pulley. One of the large ones is the crank pulley, (with the rubber damper), but I do not know what the other large one is.... can anyone shed any light what this pulley is for?

Thanks,

Yooj

not unknown for alternator to seize, take off belt again and spin alternator by hand to see if it turns freely.

Thanks T14086 ,

Good point! I forgot to mention I did spin it by hand, and it did not seem rough or noisy.

Does the alternator pulley have a clutch mechanism? If so, could this be seized?

Thanks,

Yooj

Hi Yooj, other pulley is Power steering pump, check clutch pulley hasn't seized on alternator as they are common for failing,obviously check steering fluid level ok as this will make the pump whine if low,hope this helps,John.

Thanks John,

I checked the PAS fluid level today and it was fine.

I am starting to think more and more that it is either the alternator or it's clutch pulley.

I will whip the belt of again tomorrow (weather dependent) and see if the clutch pulley is seized or not, and pay a little more attention to the alternator bearings.

Assuming that it is the alternator clutch pulley, can this be removed with the alternator still in situ, assuming I have the correct tool for removing the pulley?

Thanks,

Yooj

Alternator clutch pulley? would someone please explain how and what this does?

The clutch pulley is a freewheel pulley similar to a rear sprocket on a push-bike, in that it locks in one direction and spins freely in the other direction. I allows the pulley and alternator rotor to freewheel when engine speed reduces through gear changes etc. This reduces stresses on the alternator and tensioners in these situations.

Yooj

Thanks for the explanation, it makes great sense but I never heard of this before! Amazing what you can learn on this forum!

I've had a chance to have another look... well, i'm not much the wiser...

The clutch pulley appears to physically operate correctly, in that it locks in one direction, and turns independently (i.e. free-wheels) of the alternator rotor in the other direction.

However, when it spins/free-wheels, it is not loose, in that it does turn but does not spin and continue to spin without me turning it. I am not sure how loose the pulley should be to turn... should it be almost frictionless when it is free-wheeling?

Furthermore, when it free-wheels, there is a noise, not a grinding noise or anything, more of a 'friction interference' noise, again, should the pulley be noiseless or is some noise normal.

Secondly, I checked out the alternator again, and there is definitely no roughness nor looseness in the rotor shaft.

Finally, the alternator appears to be a real pig to remove... therefore, is it possible to remove the two (i think) bolts on the front of the alternator, and effectively rotate the alternator down enough for me to remove the pulley so I can fit a new pulley first prior to having to remove the alternator?

Thanks

Yooj