Could a Tacho cause coil issues ?

[Becky's Dad]

Hi Guys

2.0 CU Aircooled

Back in February I fitted a Tacho, a week or so later the indication was intermittently erratic indicating anywhere between 3000 - 4500.
Two days after this the Hall sensor in the dizzy failed. Couldnt get a rebuild kit at the time so I went for an aftermarket dizzy (Powerspark) which I'm happy with.

After about 500 miles the van played up coughing and banging with the tacho again bouncing around everywhere, at first I was going for a fuel issue as I just paid someone to setup the standard dual carbs but after getting recovered home turns out the coil was failing.
After fitting the new coil all was well for about 150 miles when it totally lost power - coil was open circuit. New coil fitted and failed again 100 miles later with a secondary resistance of 120,000 ohms.

New coil (3rd one!) fitted and tacho disconnected and probably clocked up another few hundred miles running just fine.
So, I'm rethinking things and maybe if the Tacho has an internal fault and shorting out, since its pulling power/signal from the coil I guess its possible that the Tachos been earthing out the coil causing the misfires and destroying the coil ?? I know I could just reconnect it and see if the fault returns to confirm it but I'm reluctant to fry another coil.

Any thoughts? I admit the tacho was an ebay cheapo

[itchyfeet]

yes possible a tacho could affect the ignition but at worst I'd say it shorts the coil to ground when it should not be cuasing misfiring but I can't really see how that would kill the coil or the hall sender.

The hall sender is independant and the coil is designed to be shorted to ground thats what the ignition module does , shorting the coil 12V to earth builds up a magnetic field which when you open the circuit collapes and geneates the spark.

[bigherb]

Tachos can go short to ground which will cause misfires or no spark at all, but unlikely to damage the coil unless the ignition was left switched on for an extended period with the coil shorted out.
More likely another problem.
Which distributor did you get the standard electronic ignition or the high energy version? The coil has to be compatible with the ignition system. What make of coil did you get? If it was the Flamethrower that might be the fault anyway they have a high failure rate.
How did you run the cable for the tacho through the body? Check the cable has not chaffed on any sharp point.

[Becky's Dad]

its a standard Powerspark svda, it came with a matched coil allegedly Bosch but black, coils two and three were 'Viper Sport' and were Powersparks recommended fit but got them to swap the totally failed one for a Bosch blue one which so far is behaving fine.
The tacho wire runs under the body and up behind the dash in a flexible conduit along with oil temp indicator wire and a spare so I guess I could swap it out.