Tachometer not working

[Dermott]

Hi all,
My tacho hasnt been working for a while, ( '96 pd classic ) are their any checks I can do to see if it is dud or broken wires, does it work on an output from the alternator or ignition? have a multimeter. any suggestions appreciated.
D

 

[Beemerboff]

The electronic ones have usually have a black wire from the Z terminal to the live side of the coil, and switched power to the + terminal. The - terminal, and sometimes the case, are earthed.

So that should be easy enough to check for continuity.

 

[Dermott]

Thanks, Beemerboff, I can do continuity ok, but is the z terminal variable voltage or triggered/pulsed, and what would it show on a digital multimeter?

 

[Beemerboff]

No idea, but if it is connected to the live side of the coil it is getting a pulse, and the strength of that pulse will depend on exactly which ignition system you have.

I have a few tachos around the place, and a multimeter, so if you want any resistance readings just specify where you want them and I will try and get them for you.

 

[cookie]

Airhead tachos are generally driven by a wire from the switched (points or electronic ign. unit) side of the coil, not the live side.

 

[Beemerboff]

Sorry I thought the switched side was the live side, but if the power is supplied continuously via the negative terminal as Cookie says then I am wrong.

 

[cookie]

Just to clarify,

The live is supplied to one coil terminal and the other is switched to earth via the points or electronic ignition. It is that terminal that is connected to the tacho.

 

[Dermott]

Thanks guys, I'll do a few checks over the weekend, don't fancy the price of a new tacho.