Rev counter wire question

[thackers]

I've just bought a small digital rev counter with 3 wires from it. Red goes to the ignition, Black goes to earth then there's a Green wire. This is the one I need help with. As the clock came with no instructions, a little research has come up with it (green) going to the negative side of the coil, or in my case a 12v cdi unit. Can one of you clever chaps in easy terms tell me which wire I'm connecting to? White, red, green, I can't see it going to the earth wire from the cdi to the frame it has to pick up a current doesn't it?
Thanking you in advance for any help

[Grumpy225]

It may not work for your ignition but if it were me I'd common the black and green to ground and see what happens.

[Monty]

If you are sure about the feed and earth Most rev counter pickups take the pulse from an induction wire wrapped round the spark plug lead. Use a length of solid cored wire, wrap it round 6- 10 times close to the plug end. Thats not going to put any high voltx down it so should be good to try. I would use a 9v battery to sort the live and neutral out. Are you running this on DC?

[mick1]

Your rev counter will have a + wire, a - wire and a pickup wire. It sounds as though your spare wire is the pickup so as said above wrap it around your HT cable....

[thackers]

Thanks everyone, Grumpy's method was the one that worked.

[thackers]

Cancel that (sorry Grumpy) the way with that wiring saw it blank out very early there were figures showing but as soon as you moved the throttle enough the display was just 4 _ _ _ _ 's
Now the three wires are red, black and green but I've discovered the black is positive (+) and the red is negative (-) I've tried the green wrapped to the H.T at the plug end but no luck just a 0 reading. Can anyone help find the cure? Thanks if you can.

The wiring is to my ignition switch for the current and earth to the frame, non battery. If that's of any help

[Grumpy225]

If black is positive and the others are green/red and neg/ground then switch them. Are you sure the tach is not a DC only set up or maybe it doesn't like the cdi?

There's only three wires so that gives you 9 combinations total.

[thackers]

Thanks for the help Grumpy, I've just about done my nut in with this I've tried loads of combinations with the wiring and even wrapped some thick wire round the H.T lead and tried the combinations again including a wire from the cdi on each terminal at a time. All I got was either no digits, a 0 reading or as before where opening the throttle took it off its scale. I'll have to have another think as to wether or not there's a point to one. Cheers.

[mick1]

Just a thought but a lot of the rev counters on the market are not suitable for a single cylinder 2 stroke engine. Is the rev counter for a 4 cylinder which when wired on a single cylinder would read 4 x too much and go off the scale, like i say just a thought

[thackers]

This is about the same conclusion I was coming round to. Even though I asked about a single cylinder 2 stroke application with a positive response. Too late now as I have left feedback and it was from our Chinese cousins
I was thinking along this one
http://www.ebay.co.uk/itm/Tachometer-Re ... 232e0bef84
What's the verdict on it and what rev counters are you boys using? I'm not about a expensive all singing and tea making one, it's just to fill a hole really