Gauge experts - wiring an old unknown electronic rev counter?

[Colin Lindsay]

Here's one I'll throw out to the forum gang - how to connect up an electronic tachometer / rev counter especially an unknown brand. The Triumph versions ie from Spitfire are straightforward but I have a few old models, especially two inch versions, that I want to use in the Herald if suitable. All ideas on wiring will be very welcome.

First one is a 2", no identifiable make but a white kite / aeroplane / arrow mark at the bottom, marked 12v and a rather enigmatic mark reading 43.5.16. The two screw terminals are marked + and - , both insulated from the clamp, with a second blue and green wire on the rear. There is no internal light. The 46 cylinder presumably means it suits either 4 or 6 and not a 46 cylinder engine... but not sure if it's positive or negative earth. 

  

Second one is 3.5 inch, an awkward size for mounting anywhere. Again no makers mark but the same little aeroplane / white triangle symbol on the face as the first one has. Three wires here, blue, yellow and black and again a printed figure on the rear of 43.9.4. This one is switchable between 4 and 6 cylinder and has separate illumination.

  

Last one may be the easiest, a 2" gauge by Checkman. It's switchable between 4, 6, 8 and 12 cylinders, has internal integrated lighting, and has green, red, white and black cables. White I think is for internal lighting, black earth, red for power and green for trigger from the coil - correct?

    

I've checked a few online sites recently but some are way off, or else much too modern for these. Any thoughts / pointers / comments? 

 

[Jon J 1250]

Hi Colin,

Similar thing here, I have this 3" Smiths electronic pulse tacho and pod to fit also. (Wish I could have found a 6k RPM one, but hey ho)
The fuel gauge means nothing, its going to have an ammeter, a dual temp/oil press some indicator lights and switches, and the tacho when finished. A nice period accessory.

I've included the instructions if it helps.

Jon

[Peter Truman]

Both our Spit & Vitesse have/had the std mech tacho's, as a spare I brought a cheap Chinese 2in electronic tacho and fitted these internals into the Jaeger 4in mech tacho case. The electronic tacho is 4/6/8 cylinder switchable, and besides the red and black backlight wires there are 3 other wires coming out of the electronics, Red to 12V positive ignition supply, Black to Battery earth, & Green to the coil negative supply, if using an electronic ignition (eg HEI, MSD, or Crane) connect the green wire to the tacho pulse supply.

[NonMember]

1 hour ago, Jon J 1250 said:

I've included the instructions if it helps.

That wiring diagram is for a "series" connected tacho, which were common a long time ago, needing the existing coil -ve wire to be removed in favour of one that passes through the tacho. Most tachos from the 1970s onward are "parallel" (or voltage sense) type, with a single thin wire connected to the coil -ve alongside the existing wiring.

[Jon J 1250]

14 hours ago, NonMember said:

That wiring diagram is for a "series" connected tacho, which were common a long time ago, needing the existing coil -ve wire to be removed in favour of one that passes through the tacho. Most tachos from the 1970s onward are "parallel" (or voltage sense) type, with a single thin wire connected to the coil -ve alongside the existing wiring.

Ah, I did not know that, it is indeed an old NOS tacho as modern versions of it cannot handle positive earth.

Presumably it won't affect the performance of the engine to connect in series, although I can understand parallel connection would be more resilient.

JJ

[mark powell]

Some more wiring systems.

[Colin Lindsay]

I have that one, it's fairly common and was one of the first guides I downloaded - the Smith's gauge appears to be positive earth and looks to be a two-white cable so it's an RVI model with one wire to the coil and one to distributor.

Assuming in the top one above that the two clamp terminals are power in / earth, then the yellow and blue are the trigger cables, so one to coil and one to distributor? (In paralell, or in place of, the black and white wire already linking the two?)

If the second one has the same yellow and blue cables, plus a black (presumably earth) then where does the power go in? I need to identify that brand then research that.

[Josef]

I think that diagram is not particularly helpful, the Smiths one it’s showing is an RVI type and I’d be surprised if any of those aftermarket ones use that system. My best recommendation would be to take the guts out of the gauges. It should then be possible to figure out which wire is which. 
 

My guess is that they’re all RVC equivalents that require switched power, earth and a coil -ve connection. For number one I’d think that the blue wire is the coil connection for 4 cyl operation and the green for 6 cyl, or vice versa. Number two, one wire I’d suppose is power and the other the coil connection, and ground goes via the case. 

[Colin Lindsay]

I think you're right in that they're all RVC, but I've no way of dating them at all to 60s, 70 etc even with regards to positive or negative earth. I may be lucky in that if incorrectly wired they just don't read at all or else fly across the dial, rather than blowing up or melting. There's actually very little online about connecting old tachos, that diagram appears on quite a few websites but there's very little in the way of 'proper' guides. JohnJ's diagram further up the screen looks to be the same RVI two-white wire as in the coloured picture.

Worst case scenario is to transplant the internals from a modern gauge into the smaller case, using the original period face. I have a modern 2" rev counter but it won't suit the car, especially as it lights up in lurid blue...

[Pete Lewis]

way back i had a pos earth tacho  ( loop trigger)   mounted it in my Imp in a  insulated    ply panel and just reverse wired it 

that  found that  7k  in top gave around 106 mph  well off the speedo  clock 

Pete

 

 

[Josef]

I built the tacho for my Spitfire from a Honda bike tacho, custom circuit board and a fair bit of fiddling to make everything sit nicely in the case. I did buy something similar to the modern one you have as a donor to start with, but it was way too flimsy and I ended up destroying it… I do think with a look at the circuit boards inside the ones you have it’ll be possible to take an educated guess at which wire is which. If that doesn’t work out, I’d give trial and error a go. It’s unlikely to completely destroy the electronics, but if it does then a replacement board would be a better option than trying to transplant the modern one into an old case (happy to help with that if it turns out to be needed). 

[Colin Lindsay]

4 hours ago, Josef said:

I built the tacho for my Spitfire from a Honda bike tacho, custom circuit board and a fair bit of fiddling to make everything sit nicely in the case. I did buy something similar to the modern one you have as a donor to start with, but it was way too flimsy and I ended up destroying it… I do think with a look at the circuit boards inside the ones you have it’ll be possible to take an educated guess at which wire is which. If that doesn’t work out, I’d give trial and error a go. It’s unlikely to completely destroy the electronics, but if it does then a replacement board would be a better option than trying to transplant the modern one into an old case (happy to help with that if it turns out to be needed). 

Your help will be very welcome! Take for example that top tacho, the little 2" version. How to bench test? If I connect power from a spare battery to the +, an earth back to the same battery to -, what should I try the other wires with?

This little guy is the modern version I bought; red / black / green / yellow / white cables. Sadly the face is white - it never even occurred to me when I bought it - but it must be transferable between cases, if the other won't work, but that's a last resort - I prefer the period version working properly.

[Josef]

This method should work for you (providing you have an old school battery charger, which I guess is likely. It doesn’t work with my ‘smart’ Halfords one so I ended up buying a small signal generator.)

Using the setup in the linked page, for tacho 1 I’d connect +\- power and then one of the other wires as the sense (coil) connection. My thinking is as it has no switch for 4/6 cylinders then it must be one wire for 4 or the other for 6 (one will connect through an extra resistor). So I think if you connect one wire you’ll see a different reading to if you connect the other. 

[Colin Lindsay]

Well I've identified the gauges as Yazaki, so that's a start. The wires on the left-hand of that diagram almost match, but what does it mean when it shows a wire to the distributor and then to the coil - do I splice to the black and white wire running there already?

I also found an interesting upgrade for rev counters:

https://www.ebay.co.uk/itm/185589851884

[NonMember]

41 minutes ago, Colin Lindsay said:

what does it mean when it shows a wire to the distributor and then to the coil - do I splice to the black and white wire running there already?

It means you connect the blue wire to the points, alongside the existing wire to the coil. If you have a distributor with flying leads, so that it's not easy to do so, then you can perfectly happily connect the blue wire to the coil "CB" (or '-') terminal, where the wire from the distributor is attached. Leave all the existing wiring as it is - you're only adding to it, the tacho observes but doesn't take part in the operation of the engine.

[Colin Lindsay]

Yes. Much obliged! For some reason I was looking at the diagram and seeing one cable running straight through, touching off the dizzy then going to coil. The colour change should have told me. One blank moment sorted!