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Discussion Starter · #1 ·
Hi guys just wondering if you could help! I recently had my crankshaft pulley replaced along with all the belts as it was on its way out and vibrating, since having that done my deflection pulley located below the alternator keeps snapping at the long bolt that goes though it and into the engine block! It’s happened 3 times now in the space of a week! The funny thing is though it’s only snapping when starting the car up not whilst driving, I was wondering if anyone has come across the before and could offer any advice to rectify this as it’s annoying! The mechanic is clueless as to why this is happing only on startup and I’m at my wits end with it!!

Any advice would be greatly appreciated!

Motor vehicle Automotive design Font Auto part Automotive wheel system

Pulley 5 keeps falling off due to the bolt that goes through it snapping off only on startup though
 

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Very strange fault that is,how’s the alternator clutch pulley and does alternator spin nice a freely by hand? Also wonder if the alternator is massively loading up on initial start?
Clutching at straws really as can’t see what else could cause the bolt to keep snapping
 

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Discussion Starter · #4 ·
Yeah the pulley spins nice and freely by hand, mechanic has checked each and every pulley whilst the bolt was last replaced and there was no issue with everything turning and working like it should, having gone to bmw dealers 3 times in 1 week for the same part I was questioned why I keep buying this I then told the technician who then called a colleague and they also were confused as they never heard of this issue before

the other alternative my mechanic has suggested is not bothering putting this pulley back on and getting a smaller belt fitted to miss this, that to me sounds a bit suspect and don’t know if it is a viable suggestion
 

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As said the alternator.
It is the only thing that can alter it's strain but in my 50 years under the bonnet i have never seen this.
Will it snap every time ? taking the main wire of the alternator will stop the strain but it could cost you a new bolt.
 

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Bizarre this ... Only other things I can think of is pulley/belt alignment, but that will have been checked by the mechanic, surely .. Has he checked whether the alternator pulley is a clutch type and that it's working properly? And, as you replaced the crank pulley, is the new one definitely the correct one for the engine? Is that a clutch type and is it working properly? When he checked all the other pulleys rotations, did he force them to an angle to see if they deflected from the plane they operate in, or just spin them?
Have you watched it at start-up to see if anything glaringly obvious happens?

Clutching at straws and thinking aloud about something stopping the belt moving somewhere along it's run and forcing the bearing out of line, thus snapping the bolt.
 

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Has this by any chance only started occuring since the cold weather?
Was just thinking, since you said it's only happening on start up (presumably cold starts..?) that maybe if the coolant was too diluted it could freeze up, jamming up the water pump?
I'm just taking a wild guess here but surely it wouldn't be hard to rule that out.
Also, I presume the tensioner pulley has been checked that the spring loading is at the correct tension and the arm can move freely? I presume this was also replaced when the belt was done?
If there is nothing impeding the rotation of the belt then it must either be a tensioning issue or an alignment issue I would have thought.
 

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Discussion Starter · #10 ·
Right guys a bit of an update on this saga!!

ive had a fair few mechanics have a look at this and they are all clueless as to why it’s happening, even a bmw technician. Everything has been done correctly with the change of the crankshaft pulley and they are all working fine and moving/turning like they should, the problem persists wether the car engine is warm or cold but as stated before it is only happening on start up. The tensioner pulley was also changed and is working fine as for the alternator again no sticking and working fine.

As stated before my mechanic suggested leaving this pulley off completely which we decided to do! Took having to root through 30 different belts but finally managed to find the right size and had this fitted, as this pulley was located near the alternator the worry was it would not generate enough power but having checked with a voltage meter it was fine and generating the same power as when this pulley was fitted.

the car is driving absolutely fine with no knock on effects (yet) but funnily enough it’s also quieter than before too (even after crankshaft pulley change the car was not this quiet as it is now without this deflection pulley) which makes this all more strange and confusing, I’m planning on leaving it like this for the time being without the deflection pulley to see it anything else goes wrong or I notice anything else. Hopefully i won’t regret this decision and end out having to fork out more than I already have!
 

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You might find the belt wears more quickly due to slight slippage at the alternator. Be alert for belt squeal indicating slippage. #Buy a spare belt of that size to carry in the car!
 

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Make a note of the belt details you fitted just in case lol.
Will have a number denoting length and number of rib's .
 

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The only thing which worries me about this theory is what happens when you create a high electrical load on the alternator,i.e headlights ,heater fan,heated rear window + other misc electrical loads + power steering on lock.
Will the belt still hold good.?
 

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My first thought was a faulty alternator creating a high load on start up.
But to test could cost another bolt after disconnecting the wiring from it.
 

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The other issue to be considered is the purpose of the Deflection pulley.
The answer lies in the name.
Thrasing and deflection of long belt runs is dealt with by means of this pulley which shortens belt runs for this very reason.
If deletion of the pulley seems to work then good luck but I would feel nervous about long term prospects of your solution.
 
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