Ask Sail: Soot or Something Else?

There is a soot-like buildup on the face of the alternator on my diesel engine. I have not noticed any fumes or exhaust leakage in the engine compartment.
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SOOT, OR SOMETHING ELSE?

There is a soot-like buildup on the face of the alternator on my diesel engine. I have not noticed any fumes or exhaust leakage in the engine compartment. What are your thoughts or comments on what it might be?
Mike Asbury, Dandridge, TN

Nigel Calder is an author and expert on boat systems and diesel engines

Nigel Calder is an author and expert on boat systems and diesel engines

Nigel Calder Replies

This is most likely very fine belt dust from accelerated belt wear. There are three likely causes: the belt is a little loose and is slipping under load (mostly right after the engine starts when the alternator goes to full power); the pulleys are worn so the belt is bottoming out in the pulley grooves (belt loads should be transmitted through the sides of a belt, not through its base); or the pulleys are misaligned. If your pulleys are rusty, this will also accelerate wear. The most likely cause is that the belt is a little loose and is slipping when the alternator is at high levels of output. This will cause the belt to get hot and become hard and “glazed” over time. If you look closely you will see hairline cracks running from the center of the belt toward its perimeter. Quite often, a slipping belt will also squeal, sometimes just momentarily after the engine is cranked. When checking belt tension you should not be able to depress the longest stretch of belt more than 3/8 to 1/2in with quite a bit of finger pressure.

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