Modern ECU to Smith's Tachometer interface


I am posting in here because I am guessing the “Lump group” has more experience with making non-Jag tech work with Jag tech. I have done EFI conversion to my '74 XJ6, I am using a LINK ECU which has lots of options to configure various inputs and outputs. I replaced the distributor with a AEM Enigine Posision Module (hall effect device). So the computer knows the RPM, but I don’t unless I have the laptop plugged in. Does anyone have any knowledge what kind of signal the Smith’s gauges work off of? Its the XK engine, 6 cyl so some of the issues I’ve read here going to a V8 shouldn’t be an issue. Thanks!


Eeek! Almost a month and no one has any advice…

Obviously the wrong place to post then!

For the small gauges, they work with a 10v supply and a sender which runs from 240ohms to ~40ohms.

You can mimic this with an opamp circuit that has a power transistor in its feedback loop. I posted such a scheme back in 2012. See

There is obviously some text in the archives from around this time, so search for that. Essentially all the opamp does is take a voltage as its input and adjust its output so as to match the inputted signal. In the process, it drives a gauge via a transistor.

For the tacho, it will be happy with a 12v square wave signal and you’ll see plenty of examples on the Megasquirt website of how their ECU drives a standard gauge - yours is almost certainly no different. The tacho can typically be driven through a diode (or diodes) taken from the coil or directly from the ECU, whose “tach out” signal is specifically designed for this.

kind regards

My lump relies on two things. to get an accurate tach reading.

  1. An adjustment within the tach to alter the phase from one developed for a 5 6 cylinder and the 8 that are now present The maker of the kit I used did that. But, check Jaguar-Specialties. Andrew’s web site covers that.

  2. The kit vendor suggested that I skip the PCM for the signal and go to the coil. . I did that.

I suspect either will work. Including the PCM might effect some management issues. If so, I have not noticed any in just regular running and driving…

In the donor car, it did have something to do with the the Power steering. And possibly fast idle for AC operation…


Thanks MarekH! I’ll check into that link you sent!

I am happy to report that I was able to connect into the ECU, and configure the Tach… and it worked!