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Relay to trigger a high drive tacho

EFI Wiring Fundamentals

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My dash tacho needs a high drive tacho output. This used to come off the negative of the coil but I am running 6 individual coils now.

I tested the Aux output of a Link G4 and it would not drive the tacho. I want to use the voltage spike caused by the coil in a relay collapsing to drive the tacho. I have modified the relay and removed the mechanical latching part. I have wired it up as per my drawing and tested that it works fine.

But I am worried the voltage spike caused by the relay collapsing could damage the ECU ? Is that possible?


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best to ask this to the link tech guys tech@linkecu.com

The tacho signal from the ECU simply needs a pull-up resistor; this is quite common. Some ECUs have it built-in, or selectable in software, or only on certain hardware output pins, so check for that first. If not, then add a 1k or 2.2k 1/4 watt resistor that taps into the ECU tach output signal with one leg, and the other leg will go to ignition or ECU +12v. This can also be done at the back of the tach if it's easier.

Sorry, a bit late to the party. Beau, the original post relates to a high level tacho, the problem is not pull-up related - high level tachos were originally designed to be connected directly to the negative side of the ignition coil so they expect to see 40-50V spike with every spark event rather than a 12V square wave signal. As the original poster suggested the common practice to generate this high voltage spike is using an inductor of some sort - a relay coil as he mentions is common.

Andrew - the ecu is fine with the high voltage spike, what you are doing is common practice and wont hurt anything.