RVI rev counter bounce

[stprobert]

Sorry to drag this up again. My Elan S3 has Lumenition and a Smiths RVI tacho, used to work OK now it bounces over 4000 rpm. I know I could have the tacho internals replaced for a cost but I am intrigued to fix mine, I have experience of electronic design and have a scope and other test gear.
My tacho works fine all the way to 8000 rpm if you remove the optical sensor from the distributor and test trigger the lumenition with a toothed wheel on the end of a small electric motor. I am obviously doing this without running the TC engine. Is this an indication that the problem must be interference from the generator/regulator through the tacho supply line? Seems likely to me. If so it should be easy to fix with a decent earth wire and filtering on the tacho supply line? I would try it right now... but its raining. Any one out there tried this?

[mark030358]

Hi there,
I have just had my tacho converted to voltage pick up as I could not get away from the failins after fitting an Aldon ignitor. However, even after the conversion the tacho still misbehaves. Reads slightly high, slightly low or complete drop off occasionaly. When I inject with a signal generator the tacho is 100% accurate and never misses a beat, even when I vary the duty cycle (on to off pulse size ratio). So either I have a bad intermittent earth (not likley) or noise induced in either the supply or impulse cables. I would be interested what RC circuit combination you fit to tyhe supply and signal cables. Remembering that the RC is a first order network and can reall slug the tacho signal.

cheers
Mark

ps I would be interested to see what your pulse train looks like on the dizzy side of the coil. I have some pictures of mine if you wishto cross reference.

[stprobert]

Your experience backs up the idea that it is noise on the supply lines, I'll keep you posted, thanks

[elansprint]

If you suspect it is noise why not try a 3W 18V zener diode from the coil to earth instead of an RC network this should clip the back EMF spikes from the coil. Some cars seem to have the indicator feed wired from the coil positive & rev counter goes mad when the indicator is turned on

[stprobert]

I think its not so much spikes from the coil as noise on the tacho supply line from the generator and regulator, plus a few bad earths, very common on an old Lotus. Inside the tacho case the circuit is earthed by a simple contact pressing against the case, I may try a soldered link, can't do any harm. Just had a little look at it all this PM. but no definite result yet. Thanks for your comments.