Thread: incorrect Tacho reading

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.

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?

Thanks gents will give the pulley thying a go.

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.