Testing an S4 rev counter?

[Yellelan]

Hi guys, I've already looked at tacho stuff on here and not found what I'm looking for. I'm renovating an S4 from bits, Engine runs well but tacho doesn't work. I've definitely got 12V onto the tacho 12v spade terminal, and also 12v from the ignition switch to one bullet connector, and a direct wire connection from the other bullet to the +ve coil connection (by-passing the spagetti), and an battery earth to the body, so I believe the thing is wired correctly. The only doubt I have is whether one of those screw connections on the back of the instrument should be earthed directly as well as to the casing, and whether there's a specific way round the bullet connectors must be wired. One of the bullets is loose but still connected because I know for sure that current is passing into and out of the bullet connectors as this is the only thing powering the coil and the engine runs ok.
So unless someone spots a connection problem, it's likely the tacho isn't working. Is there any way of testing these independent of the car? Are they easy to strip to fault find?
Thanks for any help given.

[tonyabacus]

Is your car standard setup or has it got anything like Lumenitio or Aldon Ignitor fitted, some of thgese are wired differently
Tony

[Andy8421]

Hi guys, I've already looked at tacho stuff on here and not found what I'm looking for. I'm renovating an S4 from bits, Engine runs well but tacho doesn't work. I've definitely got 12V onto the tacho 12v spade terminal, and also 12v from the ignition switch to one bullet connector, and a direct wire connection from the other bullet to the +ve coil connection (by-passing the spagetti), and an battery earth to the body, so I believe the thing is wired correctly. The only doubt I have is whether one of those screw connections on the back of the instrument should be earthed directly as well as to the casing, and whether there's a specific way round the bullet connectors must be wired. One of the bullets is loose but still connected because I know for sure that current is passing into and out of the bullet connectors as this is the only thing powering the coil and the engine runs ok.
So unless someone spots a connection problem, it's likely the tacho isn't working. Is there any way of testing these independent of the car? Are they easy to strip to fault find?
Thanks for any help given.

It does matter which way round the current flows through the bullets to the coil, originally one was male, the other female so that it could only be installed one way around. If your wiring has been messed with, it might be worth swapping the connections around to see if it makes a difference.
If it is still a dud, then there are many self-help pages and videos on youtube of how to repair these. Google is your friend.
The rev counter was widely used on British sportscars of the era and the MG and Triumph owners clubs also have lots of info. The circuitry is laughably basic, but is also quite subtle and both the coil and the points are required to make it work properly. Electronic ignitions and points substitutes can cause issues.

If all fails you can replace the electronics with a modern substitute, or if that fails there are firms that will rebuild the rev counter for you.

Edit: Should have mentioned that there is more than one type of rev counter depending on its vintage. Check the web for details.

Good luck.

[Yellelan]

Thanks both guys who responded. There's no electronic ignition. I'll try swapping the wires to the sensor.