Slipping belt in wet weather.


Feb 28, 2011
Volvo c70 T5 / BMW 316Sport
I've an interesting little thing going on with my 2007 B180 CDi which has a slipping belt somewhere in the engine bay. I've only heard it on wet days and whilst the engine is relatively cold.

When slipping there is a slight effect on engine rpm, perhaps by 50/100 rpm drop which recovers once the slipping has stopped. It normally only happens once whilst driving away from home in the morning so replicating it would be really difficult.

It's very weather dependant too, only happening after very heavy / persistent rain.

Any assistance would be greatly appreciated. The car has 33500 miles on it and had a clear 'star' check in March with a dealer.


MB Master
Jun 22, 2003
Perth, Scotland
W204 C200CDI Estate
The belt will have an automatic tensioner which may be worn allowing the belt to slip. The other possibility is that overrun clutch on the alternator pulley is faulty. Both items are relatively cheap to replace. When the engine is started from cold there is a considerable drain on the battery which is rapidly made up by the alternator but this means its under load just after start up so any slight defects in pulley belt drive system will become apparent. Unfortunately due to the somewhat cramped layout on the B class I don't know how accessible these components are to replace. perhaps some B class owners can comment?

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