Alternator (overrun) clutch

[PierreC]

This is a strange one ... I had noticed a rythmic growling noice when my car is cold and idling ... sounds like a bearing grumble but much louder and not contant i.e. what I can describe as "rythmic". Took it to my dealer for a diagnosis and was told that it was either the serperntine belt tensioner but more likely (because of the rythmic sound) the alternator's overrun clutch. The fix is a new altornator ... (a new alternator is about 30% more expensive than the overrun clutch alone, and on a car with 125.000 miles, replacement of the complete alternator is "recommended". Labour is about three times the time to fit the new overrun clutch compared to just fitting a new alternator, so the cost difference is not that much: about $70 - $80).

Has anybody heard of this before? Is this for real? Is it serious?

Oh, and the steering fluid resorvoir is leaking ... I need to replace it. I will do a complete steering system flush at the same time.

[MacNoob]

This is a strange one ... I had noticed a rythmic growling noice when my car is cold and idling ... sounds like a bearing grumble but much louder and not contant i.e. what I can describe as "rythmic". Took it to my dealer for a diagnosis and was told that it was either the serperntine belt tensioner but more likely (because of the rythmic sound) the alternator's overrun clutch. The fix is a new altornator ... (a new alternator is about 30% more expensive than the overrun clutch alone, and on a car with 125.000 miles, replacement of the complete alternator is "recommended". Labour is about three times the time to fit the new overrun clutch compared to just fitting a new alternator, so the cost difference is not that much: about $70 - $80).

Has anybody heard of this before? Is this for real? Is it serious?

Oh, and the steering fluid resorvoir is leaking ... I need to replace it. I will do a complete steering system flush at the same time.

I had never heard of an overrunning clutch used in an alternator before. A quick google told me that they do indeed exist - whether the XC alternator has this clutch is unknown to me.

I can tell you for sure that a bad stator in the alternator can make a rhythmic growling noise.

[cattlecar]

Overrunning clutches are in a high percentage of alternators on new vehicles. Volvo has been using them for 14 years, maybe more.
They are used because the rotor in the alternator has a lot of mass and the engine rotational speed changes that occur several times per crank revolution make the serpentine belt get more abuse and the belt tension is in constant motion. With the overrunning clutch the rotor can overspeed the engine for the einstant when cranks slows for compression stroke after having been accelerated by the crank speed up from cylinder combustion.

[PierreC]

Thanks MacNoob & cattlecar

The diagnosis therefore seems fair and I will order a new alternator.

I wonder ... is there a simple test to confirm it is in fact the alternator that is going?

[MacNoob]

I wonder ... is there a simple test to confirm it is in fact the alternator that is going?

It will be obvious once the alternator is removed and put on an alternator testing machine. If you have time and the local resources you might consider removing the alt and taking it to an alternator rebuilder.

I know the last bad one I had (not Volvo) was a rebuilt unit right out of the box, identified as bad stator at the local shop.

[cattlecar]

I would start with belt removal and inspection of the tensioner and fixed idlers to make sure they are not the cause.
The overrunning clutch should be smooth when it is reverse rotated. To do this turn the alternator clockwise sharply and then hold the pulley. The rotor of alternator should turn freely with no coarse feeling when it is overrunning.
The alternator should continue to spin CW when you stop pulley due to the overrunnng clutch.