Tachometers and the Megajolt

Electronic tachometers seem to be difficult to get to work with the tachometer output from the Megajolt unit. Also, in the case of the TR7 the tachometer is designed for a four cylinder motor. I have changed the particular resistor described in “How to Improve your TR7” but to no avail.

In the case of the Rover P6 (see left) the tachometer detects coil current not voltage (that is the 12 volt supply to the coil passes through the tachometer and there doesn’t seem to be a way to make it work except with some sort of additional interface circuit.

If you are using a third party rev counter such as the ETB unit, this should work straight off the tachometer output on the Megajolt.

The simplest solution is to retain the original coil and distributor which will continue to supply the rev counter with the signal it needs. However, you will need to connect the high tension lead on the coil to a large resistor with the other end connected to the chassis. This will provide the coil with a load which will prevent some nasty effects such as the coil or the electronics driving it (if present) burning out if the coil is left open circuit.

I used ten 10k, 1 watt resistors soldered in series in a long chain contained in some silicon tubing. A fair amount of power is dissipated in these resistors and also a high potential difference (perhaps 30,000 volts) is present which one 100k, 10 watt resistor might not be able to resist without a flash over!

I secure the tubing with P clips to the bodywork in an inconspicuous place.

The sketch below shows how this can be done.

Resistors in silicon tubing.