Tacho Behaving Strangely !

[ElvisWedgely]

The tacho on my 400SE is working fine when the engine is running, but when I switch the engine off it settles at around 4000 revs. When I put the ignition switch back on, it goes back to zero before starting the engine, and then starts working when the engine starts, until the engine is switched off again and settling back at 4000 revs and so on.

I've never come across this problem before. Obviously it's electrical and seems to be receiving power or signal when the ignition is switched off. Has anyone had this problem before? From my experience a tacho either works or it doesn't. This one is behaving very strangely. Any ideas, tests or information most welcome.

Tony. TCB.

[GV]

My old one did that too. Nothing to worry about.

[Jack Valiant]

Usually symptomatic of the tacho about to fail but could be a faulty coil discharging back through the instrument at switch off. This can be caused by a faulty earth elsewhere thus finding its way to ground via the tacho. Check the ground path for the instruments is good, some cars used the bonnet release catch !! If in doubt ground the instrument temperally to check symptoms.

Cheers

Chris

[ElvisWedgely]

Jack Valiant said:

Usually symptomatic of the tacho about to fail but could be a faulty coil discharging back through the instrument at switch off. This can be caused by a faulty earth elsewhere thus finding its way to ground via the tacho. Check the ground path for the instruments is good, some cars used the bonnet release catch !! If in doubt ground the instrument temperally to check symptoms.

Cheers

Chris

Thanks. When I get a chance I will run an earth from the tacho direct to the negative on the battery to see if it has an effect. I don't understand the bit about the coil discharging on switch off. Are all coils meant to do this or is it because it could be faulty and if so why are there no other symptoms? Everything else in the electrics seems fine. Do you think the tacho may be drawing current continually when switched off and draining the battery? though the battery hasn't lost power as far as I can tell.

Tony. TCB.