PaulR
regular
Reged: 09/09/2001
Posts: 385
Loc: Home West Sussex, Boat on Moor...
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been cleaning around our yanmar diesel today and noticed black dust around the alternator settling on the cooling vanes and body of the alternator and below it - all working fine - is this dust -looks like carbon normal (I know I have seen it and wiped it off before) or if not what is likely cause and remedy ?
thanks
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Forethought_of_Gosport
regular
Reged: 13/02/2005
Posts: 349
Loc: Hamble-le-Rice, Hants
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Its the black alternator belt being ground down and slipping.
Sometimes I get a complete dusting of the front of the engine..
A couple of causes I have seen on my 25 year old 1GM:
1. Rust on the pulley surfaces grinds away the belt. A bit of work with emery cloth on the pulleys seemed to help.
2. The pulleys arent lined up, so the belt is constantly sawing on one side of the v-groove - I found about 1mm misalignment between the engine and the alternator, using a straight edge and a lot of fiddling. Adding a washer between the alternator and the engine block on each mounting bolt seems to help.
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fry
regular
Reged: 11/04/2008
Posts: 73
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It may have come from the belt,possibly slipping a bit.
Or as happened to me, the groove on one of the pulleys may be rough and might need smoothing off.
I Won't tell you how I did mine cos you might sue me when you take your fingers off! 
Edited by fry (11/05/2008 21:58)
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csail
regular
Reged: 24/08/2005
Posts: 3306
Loc: Rhoose/Cardiff Bay,UK
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Ours does it too apparently quite normal if the battery bank is low.
--------------------
Marine grade vinyl boat names & graphics made to order just pm me.
Reduced price for you guys seeing as it is only a hobby. http://www.h2o-marine.co.uk/
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wotayottie
regular
Reged: 01/07/2007
Posts: 1892
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two common causes on Yanmars. The first is the use or automotive spare belts of the wrong size - they look the same cross section but they arent. Cant now remember the precise difference but its something like the auto belt is half inch and the Yanmar is 13mm - either way using auto spares allows the belt to sit at the bottom of the pulley groove rather than grip from the sides and it slips.
The second is the pressed steel Yanmar pulley coming apart / becoming loose.
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steve6367
regular
Reged: 28/04/2003
Posts: 242
Loc: Poole, Dorset, UK
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Had this problem on my 1GM in the last boat - changed both pulleys, checked alignment and problem went away.
Steve
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http://www.photobox.co.uk/album/1303358
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tamara
regular
Reged: 11/12/2005
Posts: 62
Loc: Blyth Northumberland at the mo...
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the black dust is coming from excessive wear on the belt. It may be someone has reduced the diameter of the alternator pulley. to increase it's output at low reves. Check your belt size. Both length and width, these dimentions are stated on the outter surface of the belt. Does the belt slip when running?(makes a squeeling noise until the batteries have increased their charge levels) ceck alignment of the pullies. this should get rid of your dust.
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Barry & Debbie
Tamara
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PaulR
regular
Reged: 09/09/2001
Posts: 385
Loc: Home West Sussex, Boat on Moor...
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thanks everyone - as I took alternator off last weekend will be relatively straightforward to sand pulleys and check alignment this weekend - appreciate the advice - ps it doesn't squeal
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srp
regular
Reged: 10/05/2006
Posts: 1243
Loc: IoW
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Quote:
I Won't tell you how I did mine cos you might sue me when you take your fingers off!
I did mine the same way. 
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www.corribee.org
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