RVI TACHOMETERS AND ELECTRONIC IGNITION

QYour April 2017 issue contained a useful feature on fitting electronic ignition. This is something I have been intending to do on my Reliant Scimitar GTE. You didn’t address the issue of how to ensure electronic rev counters continue to function. My GTE has a rev counter marked RVI. Can you advise please? John Chetwin, New Zealand

AA tachometer is normally connected to the negative terminal of the coil and senses the rise and fall of the voltage as the points open and close. It’ll work on any electronically triggered system. AnRvi type instrument differ sin that rather than riding piggy-back on the coil, the current must pass through the tachometer on its way to the coil. Effectively, it works by sensing current rather than voltage change.

Electronic ignition can therefore prevent an RVI instrument from working. The electronic amplifier is often wired so that it takes its power supply from the positive terminal of the coil. This means that when the coil’s switched off in its working cycle, there’s still some current flowing through the tachometer to the amplifier. Thus, the instrument never sees a clean break in current.

The solution is simple: wire the ignition amplifier so that it takes its power source from somewhere else – ie, from the ignition switch or from some other circuit that’s live when the ignition is on. The coil and the tachometer wiring can remain unaltered.

One final point: measure the resistance of the original coil (including its ballast resistor, if it has one) and don’t use a replacement with much less resistance than this. A conventional contactbreaker coil (or coil plus ballast) won’t be less than 3Ω. The tachometer will be designed to cope with this. A modern electronic ignition coil may be as low as 0.5Ω on its own, which will consume six times the current and ruin an RVI tachometer. A higher-resistance coil will work perfectly well with electronic ignition.