Tacho wiring help required

[Pintogogo]

I have a Smiths 4cyl negative earth tacho I want to wire into my car but unsure of what connections go where, I have searched several sites and all say 4 wires to tacho but there is no spade for earthing casing ….. is my diagram correct? any help would be much appreciated

I was going to connect wire from -ve side of coil to No 2 on pic and No1 on pic to -ve terminal on coil then connect No 3 to permanent +ve supply

 

Edited October 24, 2019 by Pintogogo

[IanS]

On some of the smiths instruments the ilumination bulb provided to earth.

2 wires to the bulb with the earth being permenantly connected and the ilumination supply coming from the side light feed.

When the bulb is plugged in the bulb holder provided an earth connection.

[nelmo]

Blimey, that's a bit of an antique - can you not stretch to a new one which will hopefully have a wiring diagram? 

[fry61]

Does this meter need to be connected in series with the coil or does it just take a signal from the coil?  If my memory is running to-day the early tachometers were just measuring the current drawn by the coil so in effect the meter is just an ammeter & so only gives an idea of rpm.

[Longboarder]

Most tachos of the 60's, 70's and 80's are three wire, connecting to ground, 12v ignition live and to the 'negative/switching' side of the coil without disrupting whatever switching wiring is already in place. The switching wire may come from points, ECU, or some other electronic module. Just leave that in place and add the wire from the tacho (by convention often green) to the negative pole of the coil.

[Pintogogo]

In reply Nelmo,

Yes I could stretch to a new one thanks, but having already splashed out on a new set of ETB gauges & senders etc which are basically in my opinion not up to job I thought I would try this 'vintage' piece of kit to see if it was any better having been manufactured in a decent period & not banged together

…… but thanks for your helpful comment

[blue]

Hi I have a new smiths flight rev counter sitting on a shelf £80  posted. 

[Peter Bell]

Mines just like that, there is also an earth connection to the case, has worked most times for the last 23 years in the car. No idea how old it really is.

[nelmo]

On 10/25/2019 at 11:04 PM, Pintogogo said:

In reply Nelmo,

Yes I could stretch to a new one thanks, but having already splashed out on a new set of ETB gauges & senders etc which are basically in my opinion not up to job I thought I would try this 'vintage' piece of kit to see if it was any better having been manufactured in a decent period & not banged together

…… but thanks for your helpful comment

Any comment that provides a realistic, alternative solution to your problem should be taken in that spirit and not abused. I was not aware you had already tried a new one - had I known,  I wouldn't have replied.