Elan Sprint Rev Counter Problem

[pereirac]

Still putting my Elan back together....

Can't get the rev counter to work although it used to. I have a Lucas electronic ingition unit fitted - on Peter Days recommendation many years ago, unlike the Lueminition unit it uses a hall effect switch rather than an optical one so does not suffer from dirt.
The Lucas unit has three wires, one to the distributor (b/w), one to +12V (white) and a black earth.
According to the Lotus wiring diagram there is a white wire from the rev counter which goes to the positive side of the coil and a b/w wire from the coil to the distributor (or in my case the electronic ignition). The white wire from the rev counter seems to have 12V on it so I use this for the ignition unit as well

Everything else I read seems to suggest that for a negative earth car the rev counter should be connected to the negative/distributor side of the coil.

The car runs but no rev counter.

Any suggestions welcome.

Carl

[steveww]

On my S4 there is a white wire from the ignition switch to the tacho, then from the tacho to the coil +ve. Additionally there is a green wire to the tacho.

The tacho uses the white circuit to sense the ignition pulses and the green wire to power itself.

I have a labeled picture of the rear of the tacho if this helps.

[Rob_LaMoreaux]

Move the white wire to the hot when ignition on side of the fuse block. This should get the tach working again.

Powering the ignition module from the coil can cause a continuous current draw that can confuse the current sense tach.

The power to the coil goes through the tach and the current through this wire is what the tach senses, so it needs to change in time with the iginition firing. the difference in current needs to be enough to be sensed, which is why many electronic iginitions won't work.