incorrect Tacho reading

[95disco]

I recently replaced the alternator (magneto merelli) in my 95 V8 disco and now my tacho is doing wierd things. Initially its sits at idle revs until the motor reaches running temperature then springs into life, however, the reading appears incorrect as when you accelerate the reading seems to go progressively out @100klm/hr reads at around 3100rpm the engine doesnt sound like its reving that hard and i'm certainly not setting the world on fire speed wise.

Any ideas on how to fix this or what needs to be done.

[p38arover]

The tacho is driven by a feed from one (AC) phase of the alternator.

Start checking the tach lead to the alt. It could be a faulty alternator.

Don't forget that if the pulley sizes have changed from the old to the new tach, that will throw the readings out.

[DiscoveringAuz]

Pulley size will be the answer, my guess is you upgraded to the larger amperage alternator, as such it has a smaller puller so it produces enough ergatrons at idle, this throws it out across the range,
fix is to take the pulley off your old alternator and swap them (you will need a rattle gun), problem this will cause is less power at idle,

do you know the amperage of the alternator you fitted?

[95disco]

Thanks gents will give the pulley thying a go.

[Rad97]

Mate its a simple one there is one wire with a spade on the back of the alternator that is not making a good connection or has a broken wire in it,

I had the same problem and what I did was put a new spade electrical connection on it and all sweet.

Ralph.

[CheekyD1]

Hi
I am new to the wonderful world of Discos and was interested because my tacho does not work at all.
I am off now to look ath the alternator.

The pully size problem would be a uniform ratio of "out-ness" there is a kit you can get at Jaycar, to correct the ratio of Out-ness you get with your electronic spedo, when you put on larger wheels than standard.

Maybe this could help with this problem, just a thought:cool: