This will be the chain that drives the oil pump.
VW modified their old PD engine which originally used a chain to drive the oil pump.
VW added ballancer shafts & ran this off the oil pump drive chain, then they bolted on the oil pump on the end of one of the ballancer shafts via a spline conexion.
The ballancer shafts set up an oscillation in the chain at certain revs which destroys the chain drive & the spline conexion to the oil pump. Whichever one fails first stops the oil supply. A great design.
VW then came up with a replacement gear drive which cures the problem but costs about £1k. But of course you generally have a wrecked engine by then.
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