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Sierra Rev Counter and NoDiz


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I'm looking for some help please as I am struggling to wire up my original Sierra rev counter in my 2B now that I have installed a NoDiz electronic ignition system.  I have built the following circuit...


The negative feeds from the coils go to two 1N4004 diodes which then go to a 1N5355B Zener diode and then onto my Rev counter.  When the engine starts the rev counter initially bursts into life but then quickly stutters and gives up.  Will this circuit work with the original Sierra rev counter and if so how can I go about testing things?

I have read that an alternative is to use a coil from a relay. I have an old NoDiz that has a 5V tacho and the rev counter did work when I had the original coil and distributor.



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I am no electronics expert.... so I leave others to correct what is about to follow. From your diagram I am assuming that you have two coil packs and therefore a "lost spark" system. That is why the circuit is combining the negative sides of both the primary coils. I have spared no expense in making a rough drawing of a suggestion for a slightly different circuit.


A spark is generated by the secondary coil (High Tension Coil) when the voltage at the primary coil collapses from 12v to 0. This collapse causes the magnetic energy stored in the core of the coils to collapse and the resultant "flux" is what gives the high voltage in the many turns of the secondary coil and thus a spark. In old fasioned engines its the points that disconnect to cause the collapse, in electronic systems the collapse is controlled very accurately by the ECU. Essentially the ECU decides when and how long to connect the "negative" side of the primary coil to earth (or zero volts). A crude graph of voltage at the -ve side of a primary coil against time is thus.


When the ECU stops earthing the -ve side of the primary coil it suddenly rushes up to 12 volts again, however, in all this excitement the high flux in the core causes the voltage to "overshoot" and return to 12v in a voltage spike, displayed in red above. I am going through all this to get a clear picture in my head about what the elements of the suggested circuit are doing. Firstly the two normal diodes are essential because they prevent the earthing of one coil by the ECU from earthing the other coil and causing an unwanted spark. The Zener diode is connected to earth so that any voltage higher than the breakdown voltage (in the case of your zener I think it's 13.5v nominal) is discharged to earth rather than blowing up the Tacho. Now there might be other elements needed like resistors and such like (I am NOT an expert) but I hope this suggestion might help. 🙂

Edited by Sparepart
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A bit more seriously though, would I be wrong in thinking that if either diode fails and becomes "short" rather than open circuit, then both coilpacks could discharge at the same time ?  so you might want to consider the long term reliability of this solution.?

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