Tacho not working at low Revs

[Cronullalad]

Hi all,
I have an interesting problem with the Tacho on the Port Engine in my 1998 Challenger 1800. Basically at idle it reads zero but at about 3,ooo rpm it comes to life........I can run around for ages but if I let the revs fall I below 3K it goes back to zero !!! The other engine / Tacho is fine. What could cause this.....
Thanks, Geoff

 

[Dr Honda]

Never heard that one before. BUT, I would start by simply swapping the signal wires, and see if the problem follows the wire. If it does... then there's an issue down in the MAG. If it stays at the gauge... then it's bad.

 

[Cronullalad]

Thanks Tony, will give that a go this weekend.

 

[Cronullalad]

Well, I was about to swap the Tacho electrical blocks over but decided to give the boat a run first just to confirm the problem still existed........strangest thing, both Tachos functioned perfectly !!!!! Can't provide an answer why but am very pleased that things are back to normal. I guess there is a loose connection some where that is now firmly connected.
Thanks for your support Tony.
Geoff