Rev Counter Wiring

I can give you the BS standard wiring colours.

Red - sidelights. Black - ground White - base colour for ignition circuits. Green - feed to auxiliary devices.

The ground is self explanatory as is the aux feed. Sidelights means in this case the illumination. The ignition connection is more problematic, as there was more than one way they used to acheive it. Most common was the feed to the coil being looped through an inductive sensor - so a white in and out as it were. Note this is on the switch *not* cb side of the coil.

You're sure it is a positive earth device, finally?

uk.rec.cars.classic if you haven't already

Well if you isolate it and connect power / neg backwards it still wont work cos its trying to count negative (positive in this case) coil pulses!

Unless it was designed for pos earth...

But it MIGHT count pulses even the wrong way round.

So you could try it. If its modern.

It really only needs three wires.

Black = earth or rather modern negative or common... Red = +12V or rather the battery or power.

You need to reverse these. And make sure its isolated (insulated by chassis)

Now comes the problem... The two wires. One will be an instrument light. The other should go to the negative coil wire. The switched wire by points in your case!

How you work out which is which is easy. Add the non chassis power wire to each in turn. One will light it up.

The other is the ignition signal (switched side of coil) wire. In a pos earth system it may not be able to count them. Or it might!

Think that's your first mistake. ;-)

Smiths units usually used the 12 volt *feed* to the coil - there's enough of a pulse on that to trigger without running the cb circuit half way round the car - which is also bad practice, by the way, as it might interfere with correct operation of the ignition, as well as radiate more RFI.

Most rev counters of this era are marked positive or negative earth on the face.