Alternator Pulley

[rovertone]

Does anyone know whether the pulley on an 03 CDTi incorporates a one way clutch.
I ask this as I have a mysterious problem with an auxillary belt that bounces at tickover causing the belt tensioner to bounce which eminates a knocking sound, as soon as the revs rise a fraction all goes smooth and quiet.
I inherited this problem with the car and thought that i had found the cause when I rectified a wrongly fitted crank pulley (it was running 'buckled' due to not being correctly seated), at the same time I changed the tensioner. The knocking and belt flapping improved slightly but the 'flapping' moved further round the belt to just before the alternator. I have just come across a comment on a Mercedes forum where someone has said that if you experience this it will certainly be the pulley clutch that has failed but I cannot find any reference to any such clutch on R75.
Any advice or thoughts would be most welcome.
Thanks Tony

[klarzy]

From what you describe it sounds like the belt is bouncing for 1 of 2 reasons and I would err towards number 1...

1. the belt is too loose
2 the belt has a defect which is putting it out of balance...

the bounce starts on one pulley and dependent on the speed of the belt, the distance between the 2 pulleys, the tension of the belt and the harmonic of the belt material the bounce and slap occurs..

change any of the variables and the slap stops. (goes away when you rev the engine..)

as you want it to stop at idle you cant change the speed
you cant change the distance between the pulleys
you cant change the material of the belt (unless you replace it)

so that leaves the tension...

[James.uk]

There isn't a clutch on the crank pulley of a diesel engine.

As stated, it's too long, the tensioner should take the play out though..
...

[klarzy]

Originally Posted by James.uk

There isn't a clutch on the crank pulley of a diesel engine.

As stated, it's too long, the tensioner should take the play out though..
...

the tensioner may be weak or have play...

[okenora]

crankshaft pulley on it's last legs possibly......

[HarryM1BYT]

It was the alternator, he was asking if there was a clutch on and no there is no clutch on that. There is a sort of clutch on the a/c compressor.

I wouldd be inclined to check the bearings of each pulley, check the run out of each and make sure nothing has gone stuck to the drive face of them, like a sticky label. Also check the crank pulley for wear.

[rovertone]

Thanks for your responses.
When I rectified the crank pulley problem I renewed the tensioner which was very weak compared to the new one and also replaced the belt. It only needs a few revs above tickover to smooth the running of the belt and after a run the knocking occasionally disappears so I'm taking a view that it is not too big an issue during normal driving conditions.
I bought a new crank pulley but the existing one was solid so I returned it. I think it had been changed anyway, someone had fitted it and put a 'home made' key into the exposed slot but had left it a few thou too proud thus the pulley wasnt seated, they were obviously not aware that the keyway is for the key further in!
I will keep monitoring and will certainly check out all the other pulleys more closely as suggested.
Thanks again Tony

[HarryM1BYT]

My Aux belt flicks up and down a bit (few mm), but I wonder if yours might be moving more, due to an unstable tick -over? Maybe a mismatched injector, or a partially choked one.

Have you used any injector cleaner recently?

[rovertone]

Hi Harry,
No I haven't but I was thinking of doing so anyway as I have 97k on the clock, working on a diesel engine is breaking new ground for me.
Tickover and performance seems fine, car goes well but I like things right and this problem is irritating me!
Thanks for the tip, I am away at the moment and will certainly try that when I get back.
Thanks again Tony

[klarzy]

Originally Posted by rovertone

Hi Harry,
No I haven't but I was thinking of doing so anyway as I have 97k on the clock, working on a diesel engine is breaking new ground for me.
Tickover and performance seems fine, car goes well but I like things right and this problem is irritating me!
Thanks for the tip, I am away at the moment and will certainly try that when I get back.
Thanks again Tony

do you have a timing strobe? one with variable setting so you can mark the belt and see if the oscillation is in the same place on the belt or the same place in the engine cycle?