Correct X type alternator

When looking at the factory manual, it states the alternator for manual transmission has the one way clutch. But the aftermarket catalogs, all say the automatic gets the one way clutch.

Which information is correct?

Can’t help but I’ve never heard of a one way clutch on an alternator.

The X type does. I had never heard of it either till I bought my 05 wagon.

Every VW TDI from 1998 onward uses one. (sometimes referred to as Overrunning Alternator Pulley or Freewheeling Pulley)

What’s the rational for it?

If you ever had a pulley failed (locked solid) on a TDI VW 4 cylinder engine, the belt ‘chirping’ at idle will make you uncomfortable.

The Diesel engine ‘pulses’ (speeds up and slows down with every cylinder ignition) at idle more than a gasoline engine and the inertia of the alternator rotor will cause the belt to wear prematurely and make noise.

It is not as noticeable with a gasoline engine but apparently the engineers thought it necessary with the X400?

Thanks for that, didn’t realise the pulsing on the TDI engines created that problem.