300 TDI Rev Counter problem

Hi all first post for a newbie.My brother inlaw has a problem with his 94 TDI Rev counter,it is showing about half the revs it should.It has been fitted with a new alternator and this is when the problem started.The alternator drive pully is smaller than the previous one fitted, would this effect things,or would it be somthing else.Your thoughts please.(It is not a factory alternator)

HAPPY NEW YEAR

Well if the pully is smaller then its going to be going faster, so would show more revs.

So it could be that the belt has not been A properly routed (which many mechanics seem to struggle to do right) and its there for slipping or its not been tensioned properly and allowing slipping.

When you mean factory alternator do you mean LR genuine? Even at that it should be more or less the same unit, if no better.

Who needs a rev-counter on a tdi anyway? Sell it on fleabay and fit something more useful like a boost gauge

That has given my brother inlaw food for thought,any other possible causes?

Check the connections on the back of the alternator. One of the wires on the back of the alternator connects directly to the rev counter. It is easily accessible, although you may have to remove a cover plate (3 nuts) to get to it. There should be a plastic cover that hides the spade connectors. Take of the female connector, compress the female connector (to get a tighter fit) and then clean up the male connector. Add a bit of electrolube too.

Odd though, as normally a bad connection will mean no rev reading at all, but it might help!

What type of alternator is it? Marelli? Britpart? Are there any 'squealing' noises coming from the engine?

No squealing he could well have a go at the terminals Friday.The alternator box went with the knackered one so he does not know the type.Interestingly the alternator seized and stopped the engine dead,hence the need for a replacement.Many Thanks