The owner of this 2005 Mercedes A170 diesel, spent most of Sunday replacing the belt. After the job was finished, they were upset to find the problem had not disappeared. The thudding vibration noise given off by the belt was the same as it had been with the old belt fitted.

This noise did diminish when the engine was revved slightly, but on tick-over, the distinct thudding of the belt was disconcerting to the owner.

I could see that the problem was not the belt, it was the tensioner. It was putting insufficient tension on the belt to maintain its position.

The assembly comes complete with the idler pulleys, and is tricky to fit as the engine needs to be dropped slightly to access the fixing bolts. But the end result meant that the thudding vibration that the owner was now getting quite tired of had gone.

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