tachometer/rev counter wiring

[julie4morris]

I recently picked up a Smiths Tachometer and am trying to find out how to wire it in to my moggy. It was originally positive earth but has been converted to negative, that much I know.
I understand the spade I have marked (A) goes to earth but what about (B)? I have seen on the net pictures of a piece of wire looped round which is connected to the coil but it did not come with that, so could anyone help? I know (C/D) are for the ignition and the back light. Also feel free to reply with picture of where and how you have fitted yous as I'm after a few ideas [frame][/frame][frame][/frame]

[bmcecosse]

Pretty sure the loop of wire (from coil to dizzy) goes through 'B' without making electrical contact. Interested to know how (and by whom) it was converted to -ve earth..... It needs a power feed - so where is that going?? Why do you think A should go to earth ???

[mike.perry]

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I have the same Tacho as yours and had it changed from pos to neg earth

[julie4morris]

Sorry correction 'A' goes to the ignition side of the coil and yes i was running on the understanding there was a piece of wire to loop round. All i need to do now is find one. well there is a stick on that back and I'm presuming they carried out the work/conversion.[frame][/frame][frame][/frame]

[RobThomas]

RVI (Inductance) reads the pulse from the cable that runs from the coil CB terminal to the distributor. Follow the single wire that connects to the distributor and it should run to the coil. You need to splice a new long wire in there so that instead of running directly between the 2 components it runs via the loop drawn in the picture above that Mike posted. Basically, just re-route the wiggly amps through that loop so that the revcounter can 'view' the magnetic field that the coil pumps down to the distributor. It's a bit like using a stethoscope to listen to the beat of someones pulse.
RVC revcounters us a separate single wire direct from the distributor and it doesn't return. A bit like the oil pressure capilliary gauge, it just looks down a wire to see what is happening at the other end.
Hope this makes sense.

[IslipMinor]

It appears to have the small white plastic block missing from the 'B' connection? It is shown in both the Smiths instructions sheets below.

The original Smiths fitting instructions for the 'Universal' rev counter showed the 'Pulse Lead' connected between the non-fused ignition fed supply and the coil. The connections are reversed for negative earth, but still between the ignition switch and the coil.
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The later Smiths instructions show the pulse lead fitting between the coil and distributor, as do the instructions earlier in the post from Speedy Cables, who took over supporting the Smiths instruments, after Smiths pulled out.
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The 'pulse wire' coils around the 'B' terminal to provide an inductive pulse, and the direction that the pulse flows around the terminal is critical to the rev counter operation. No harm is done if wrong, it just will not work.

Both versions feed the unfused supply from the ignition switch to the coil up to the rev counter, and back, and presumably through the bulkhead? Great care is needed to make sure there is no chance of any damage to the wire insulation that could cause a non-fused short circuit.

For negative earth, the 'A' terminal is for a +ve ignition fused supply (A4 fuse), not earth.

[Mr Spigot]

Apologies for hijacking an old thread, but despite looking at lots of instructions and wiring diagrams, I am still uncertain about how to connect my tachometer. It is a negative earth RVC 1410/00AF - see photos. The car is negative earth.
As far as I can make out, the spade connector attached to the case is attached to earth but I am uncertain about the other two - another spade and a male bullet. Do both go to the coil? Does one go elsewhere? Does it need a live feed off the ignition?

[StillGotMy1stCar]

Spade on case is Earth.
The other spade is switched +12V supply.
The bullet goes to the points side on the coil.
A useful link https://www.triumphclub.co.nz/wp-conten ... ters-1.pdf
Edit; I forgot to mention the spade on the round disc is the night illumination lamp, I expect you realised that and also the gauge is over 40 years old so it may not work. Good luck.
Regards John