The 'E' Type Forum

Fellow owners: A couple of years ago I had my working electronic tachometer, along with all the other instruments and gauges, sent to a well known USA instrument company for calibration and cleaning. It did sit a while as the car was being restored. When the shop got to installing the dash, the tach did not work consistently. The vendor was not very helpful other than suggesting a new wire from the coil, which they did. After taking the completed car home, the tach worked intermittently, appearing correct sometimes and reading 0 other times. Then it began staying pegged at 6000rpm. Eventually it did not work at all. I just sent it to another vendor, received it back and it still doesn't work at all. This vendor sent a video of the tach working on his bench top. In the car, the green wire to the tach has 12 volts, and I reversed the coil wires to see if that helped. Still doesn't work. Any thoughts folks?

If your car has been converted to electronic ignition, it will produce the symptoms you describe . In which case, the internal circuitry of the tacho has to be altered for it to work reliably. There are quite a few threads on this if you search the board . RVI and/or RVC are probably the best search terms

This is always an odd one.
My series 1.5 has the edis system from Ray L . ( I transferred this from my series 1 before I sold it)

I fully expected the symptoms you describe but it works faultlessly with any wiring changes. Cant say for other electronic ignition systems but as Christopher says many threads on this.

I'm running a 3.8 positive earth car and changed to a 123 dizzy some years back due to arthritis making gapping points difficult. I've had not a problem at all.

Thanks for the replies gents. I have the standard ignition system from the factory, so some other fault yet to be discovered.

Fred : right, since it is points ignition , and the tacho works on the bench , then it must be a fault in the ignition to tacho circuitry , and since the moving coil appears to be getting 12v from the green wire we can eliminate that for all practical purposes . That leaves the white wire as the likely culprit, but the strange thing is that if there were any discontinuity on that, one would expect that the car would not fire up at all . That suggests that it is the run of the white wire which is wrong ( it has to be coiled round the inductive pickup, usually twice, and in the correct direction ), or that there is a fault in the plug to the tacho . There are two types of these tachos : the early ones have the pickup external to the case, and the later ones just have the white wire disappearing into the case with the pickup being internal. Which is yours ? and if you can post photos of the run of the wiring at the back of the tacho, it might help

PS I should have thought of this earlier - make absolutely sure that the tacho case has good earthed ( grounded ) contact with the instrument panel . If need be, run a wire to earth from one of the holding down nuts

A thought. could it be the magnet on the end of the camshaft moving?

Christopher,
Thanks for your suggestions. My tach is the earlier kind with the external white wire. The plastic bit that holds the wire loop is only big enough for one loop. The metal strap that surrounds the plastic loop holder is essential to the operation of the tach I am told, and it is indeed in place. Your suggestion of direction of current flow is likely the answer. When I get the car back from rear crankshaft seal replacement, I will give that a try. I am fairly certain about the ground since the lights work, but that is always a good thought.
Fred

Sarthe : this is not the camshaft driven type - it is electronically driven by counting the number of pulses in the low tension ignition circuit

Teach me to learn to read better! Apologies all.