Ammeter and Rev Counter

[Simon Rines]

I have Smiths analogue ammeter and rev counter from 1960s/70s. I'm not sure how best to wire them in. They are going into a Triumph Herald 13/60, which runs off a dynamo and has had no conversions - any tips? Thanks

Rev counter

Ammeter (the wires that are in pic are obviously for the night time illumination - it is more a case of how to wire so I don't have to run the full current across the instrument). 

[glang]

Hi not so straight forward this one. The ammeter first as its the easiest and is normally connected into the wire from the starter solenoid to the voltage regulator box. I think on your car this will be a small brown wire but theres two connected to the terminal with the big supply wire from the battery. The other brown wire supplies one of the cars fuses and you dont want this as the reading you need to see is the charging current from the regulator to the battery. Once connected the right way round the ammeter will then show a + current flow after starting which should reduce to almost zero after running for a while. If it indicates - current at any time then the battery is discharging and something is wrong with the system.

A couple of warnings: this brown wire isnt (and cant be) fused ie its connected directly to the battery so when installing the ammeter into it the connections must be well insulated so that nothing can accidently short them to earth. Also by installing an ammeter you are increasing the resistance of the charging circuit slightly which might affect its performance.

Now the tacho which being electric is more complicated as Triumph originally only used mechanical ones driven from a drive on the distributor. I think to operate yours you will need to generate a voltage proportional to engine speed and hope someone else on here might have an idea of how to do that....

[RobPearce]

The ammeter is designed to have all the current flowing through it, wired as glang said. You can't really avoid that.

The tacho probably connects to the coil. It won't need "a voltage proportional to engine speed". It will need a pulse train at a frequency proportional to engine speed - i.e. the coil signal. However, there are two completely different approaches to sensing that.

Option 1, which is a bit old-hat but was common in the 50s/60s, senses the coil current. The gauge will have two wires that connect between the distributor and coil, i.e. one wire to each, replacing the existing LT wire.

Option 2, used on 70s cars including Dolomites and very late Spitfires, senses the voltage. It has one wire to connect to coil -ve alongside the LT wire.

The other wires on the gauge will be a supply and ground for the built-in electronics, and either one or two wires for the illumination. So that makes four, five or six wires. I see five in your photo. So now you just need to work out which option you have and which wire is which!

[Simon Rines]

Thanks for your help Gland and Rob. I'm assuming that the two white wires on the Rev counter go to the coil/distributor. The green is presumably earth, the red the power source for the unit and the red/white for the internal bulb. My concern is whether I could damage it if I connect the two white wires incorrectly. So one goes to the -ve on the coil, not 100% sure what the other attaches to... are you saying the small lead that goes to the distributor? and will it matter which way round they go? - as you can see, they both look similar.

On the Ammeter, that does seem straightforward - so really appreciate that - thanks again :-)

[RobPearce]

I'd instinctively agree with your guess at the wire colours but it's worth googling the exact model if you can find an identifying mark. Assuming it is a current-sense with the wiring as you say, then I don't think you'll damage it by connecting the white wires backward. It may well not work, though.

If you have a multimeter to hand, check the resistance between the wires. If you're right, there should be almost no resistance (well less than an ohm) from white to white and very high resistance (at least some megohms) from the whites to any other wire.

[RobPearce]

Here's a couple of useful links:

http://www.theminiforum.co.uk/forums/topic/284095-2-types-of-smiths-rev-counter/

https://www.classictiger.com/techtips/motach.html

As one of the posters in the first points out, if you have an "impulse" (current sensing) tacho, it doesn't actually need to be between the distributor and coil - it will work just as well in the coil's other wire, which is probably easier on the wiring. The white wires then connect to the ignition switch and the coil +ve respectively.

[Simon Rines]

Thanks Rob - much appreciated! Best wishes - Simon