Electronic Ignition and the Tachometer

Matthew Kimmins has been producing electronic ignition modules for a few years now. With later voltage driven tachometers there is usually no problem. The internal electronics is basic but they tend to work. The problems tend to be with the earlier current induced units which use a transformer to pickup the voltage spike created by the opening points. See the picture below.

This is a gauge as opened up for the first time. You can see the white wire that enters the unit and loops round the transformer.

There is no way you can trigger an RVI tachometer from a normal electronic ignition system. The whole principle of its design is that it uses the back EMF of the coil which can be one hell of a voltage spike. Typically 100 volts. The small (by comparison) 12V square wave or less generated by a modern electronic unit will not even touch it. The germanium transistors do also tend to go a bit deaf over the years and require a bigger input voltage to drive them. That might be why it read low? One trick was to wind another turn round the transformer but this will not help in the case of fitting an electronic ignition.

The Solution

The original board is removed from its plinth and the existing wires removed. This includes cutting the wires to the transformer and re-terminating them to form a direct link through the meter. This links the ignition circuit to distributor providing 12V to the points should you wish to go back to a points system. The photo below show the original board. Note the white wire looping through the transformer. This is the pickup.

The new board measuring just 20mm X 40mm is mounted on the base of the chassis with a double sided foam pad.

Connections are made to earth / 12V and tacho out. Also the meter is re-terminated at the movement. No new holes or additional parts are added to the unit apart from the PCB and replacement wiring.

Once assembled the unit is calibrated at 500 1000 1500 etc using a known good signal source. (Thurlby Thandar signal generator) An on board 10 turn potentiometer allows microfine adjustment if needed in the future. The input will trigger from either a points system or electronic ignition tacho feed and its high sensitivity and input impedance mean it wont be damaged by short circuits etc.

The original board features one transistor. The new circuit incorporates 44. It has a typical linearity of +/- 0.3% across the rev range. 500 to 7000RPM

Supply Voltage Range = 8 to 28V Supply Current = 3.5mA @13.8V

Operating Temperature Range = -40 °C to 85 °C Negative earth by default, or for use in a positive earth vehicle please specify Reverse polarity protected.

A wire terminated board roughly calibrated is available for self fitting. It will need calibrating against a known RPM. Cost £17.99

Matthew Kimmins can install and fit then calibrate the board into your unit for a very reasonable fee. You send your tachometer and he will aim to have it back in first class registered post within the week.

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