Tacho - reading when ignition turned off

[Matt Elan]

Oh Joy

It's time to break out my electrical test tools. My tame mechanic could get my Plus 2's horn working but he did do all the niggly little jobs that I didn't want to do like changing the roll bar bushes, changing gearbox and diff oil etc so that job no 1 - and I suspect its earth hunting, but I've ordered a new two tone air horn anyway. The second is that the Tacho flicks around when I turn the ignition off - now I'd assume this another earth issue but is there anything else that causes it?

[tdskip]

It needs some power source to register, presumably from the ignition circuit. Does it work normally otherwise?

Sneak current?

[Matt Elan]

Thanks Tkskip - yes it does seem to work normally andits only just started to act up! I've been messing with earths behind the dash, and have a couple of other issues - such as the horn not working, so its back behind the dash with a probe and meters this weekend, and I'm also replacing the ign switch and solenoid to hopefully sort out an intermittant starting issues - i.e turn the key to start and nothing, waggle it maybe get a click, waggle again and everything works normally - well normally for a Lotus!

[mbell]

The tacho works by measure current pulse to the coil which it can "convert" to RPM (two pulses a cycle). So it could be detecting pulses or back noise from the coil (guess). So I check the behavior with the coil disconnected. I'd start by unplugging the coil and testing. If it still does that I remove the coil supply and wire from rear of the tacho and re-test.

If it still does it you know the problem is in ground or 12v supply. I'd probably create a temp gnd connect to tacho as next step.