Hi Guys & Gals

There are 6 reasons why the Smiths electronic tacho will not work. If you follow through the list below in order, you will solve the problem.

1. The tacho is the only electronic part in your Sprite/Midget, and it contains a diode, through which the current can only pass in one direction. Sprites up to the Mk III HAN9-72040 and Midgets to Mk II GAN4-60459 were positive earth, and after that, negative earth. In small print on the front face of the tacho, you will find it says "positive earth" or "negative earth". A positive earth tacho will not work in a negative earth car. So if the battery your positive earth Spridget ( I hate that word!) has been swapped to negative (often so that a radio/CD can be fitted) then the tacho will not work. So step one is: check which terminal of your battery is connected to the chassis (the earth) and check what it says on the face of the tacho. If the tacho is positive earth, and the negative battery terminal is connected to the chassis, then you will need to have the polarity changed inside the tacho. You can try this yourself, or send it to an instrument repair specialist.

A couple of things to note here. If your tacho face says nothing about the polarity, look closer. It may have been changed in the past, and instrument repairers often paint out the "positive earth" on the face with black paint. If you reflect some light off the face, you might be able to pick up the writing underneath the paint.
Also, when your tacho comes back having had the polarity changed, and you re-install it, it still will not work. You need to see point 5 below!

2. The tacho needs a power source. This is usually supplied by means of a green wire out of the wiring loom, ending in a female spade terminal which connect to a male terminal protruding out of the back of the tacho. Check this wire is connected. And with a test light, check that it has power when the ignition is turned on. No power = no tacho.

3. The tacho needs to be earthed to the chassis of the car. This was usually done by means of two or three thin black wires, all joined to a steel loop terminal, that should be screwed under one of the knurled retaining nuts that hold the tacho in place. These black wires don't just earth the tacho, they earth other things as well. Often when the tacho is removed, the owner doesn't notice that there was this terminal under the knurled nut, so it doesn't go back. In theory, the tacho will still earth to the back of the dashboard through the retaining bracket, but this isn't foolproof and doesn't provide a strong and reliable earth.

4. You have an "impulse" tacho. What drives the needle is the impulses through the ignition wire when the ignition coil delivers power to the distributor, which fires the spark plugs. In all Spridgets, there is a white wire that delivers 12 volts from the ignition switch to the coil. In cars with an electronic tacho, this wire exits the wiring loom behind the tacho, passes in a double loop through a plastic block, and disappears back into the loom. This plastic block should be attached to the back of the tacho by means of a small knurled nut, and between the nut and the plastic block is a small squared U-shaped strip of thin metal, connecting to two small metal terminals that just protrude from the tacho. (Cars after 1970 or so have this loop internal to the tacho, and the white wire is cut with two terminals, one male and one female, that simply plug into the back of the tacho.) Is this plastic plug in place, and is the metal piece there. If no to either, then no tacho. The metal bridge piece is much less than thumb-nail size, and can often be overlooked when it dropped to the floor when the tacho was last removed!

5. Everything OK so far? Good, you are almost finished. Remember we talked about changing the polarity of the tacho from positive to negative earth, and it still will not work. Well, that's because you also need to change the direction of the current flow through the plastic loop. It matters whether the current is flowing clockwise, or anti-clockwise. So if the battery and the tacho have both been changed from positive to negative earth, then you need to reverse the current flow through the loop. The easiest way to do this is to cut the loop completely from the wiring loom (being careful to cut it approximately in the middle of the length of white wire available) fit two female terminals to the two remaining wires coming from the loom, and two male terminals to the end of the wire passing through the loop. (If you like, you could at this stage get yourself a longer piece of wire to replace the short bit running through the plastic block.) Then refit the plastic block (with the metal bridge, remember) and reconnect the loop wires to the wiring loom. Tacho still doesn't work, then unplug the two wires and reverse them.

6. Done everything, and the tacho still doesn't work. Do you know why? It's because your tacho doesn't work. Send it away and get it fixed. Perhaps someone in the past has reversed the battery, and left the tacho connected. Those little diodes don't like current being forced through them in the wrong direction, and they get fried. Unless you are good at electronics, send the whole thing to an instrument repairer. Or to me! I can supply reco exchange tachos.