Rev counter pre FL MK2

[Jake]

I saw an interesting article in issue 226 of Six Appeal this month about changing the dash to include a rev counter. I also saw the tacho tested before fitment, but what I want to know is about connections for the earlier type of Mk2 tacho as this has 3 connections. (Discounting the light and the black earthing cable that I'm guessing is only for the light?)

I'm changing from a later type rev counter with 2 connections (That works fine) to the earlier type to match the other instruments. (The third wire is not there )

Looking at the Haynes manual;
The green wire is the same coming from the voltage stabiliser. (On the input side so just 12v?)
The white and slate wire comes from the neutral side of the coil on the later type but across the ballast resistor on the earlier type (Mine has no ballast resistor by the way)
Can I view this as being the same???
The additional white wire used on the earlier type of tacho comes from the battery condition indicator. (So surely also 12v?)

Anyway with the Black, Green and White/Stale wires connected it doesn't work!
Do I need the other connection?
Is the Slate/White wire ok to connect is it appears it want's to be (to the male terminal)

Advice of those that know better gratefully received

[Mike Stevens]

Hi Jake,

I'm away from home at present so this is entirely from memory!

The early tacho is current sensed, so has the wire to the coil going from the +12V, into the tacho and then out again to the coil - the tacho is in series with the +12V feed to the coil.

The later tacho is voltage sensed, so it just has a wire to the coil/points connection. Obviously, either type will work in pre and post facelift, with suitable wiring changes.

So, if you want to replace a later one with an earlier one, you'll probably have to change the coil feed so that it goes through the tacho (hence the 2 connections, in and out!)

BTW, if you have an early tacho and try to start the engine with no tacho connected, it won't run as the coil feed isn't there. Also, the later cars had the ballast resistor built into the wiring loom so, although you may not be able to see it, you still might have one in the system. I think the ballast wire colour is pink - but see below...

I'm afraid I can't comment on the wire colours as I don't have the manual to hand here in France!

Cheers,
Mike.

[Firewatcher]

Hi Jake,
Bit more info on colours from my BL workshop manual it lists 4 connections for the Smiths RV1 thus: Green - Instrument (ignition controlled) positive supply (voltage stabilizer connection). Black - Negative Earth (body of instrument). White - Pulse lead, ignition controlled positive supply (battery condition indicator). The White/Slate - to ignition coil via ballast, that is, to the coil positive terminal. States that tacho' is suitable for use with or without ballast resistor.
Pete

[Tinsmith_Skippy]

Found this quite informative piece http://www.triumphclub.co.nz/wp-content ... Smiths.pdf

The kamacarze auto electrician in me would try using a jumper lead from a white ignition circuit to the other terminal.

But I think reading the above might be more successful:

[Jake]

Thanks all for the input, rev counter duly connected and working correctly!

I used the Haynes manual but the wiring diagram appears to be a copy of the factory manual.
A few changes to how is was with the later rev counter but I just copied the info from the manual.
Easy enough once Mike said it was in series