A quick overview of the three different tacho types

Current driven tachos are wired in line with the primary coil wiring, such tachos can be easily identified by the cable loop at the back of the tacho. Smith tachos of this type are labeled "RVI". The tacho is not actually wired to any of the ignition components, the wire connected to the coil (+) terminal just induces a magnetic field for each ignition impulse via a small wire loop on the back of the tacho, which is then picked up by a small coil within the tacho. This type of tacho was installed until the late sixties to early seventies, when the automotive industry moved to ignition kickback driven tachos.
Kickback controlled tachos have a single wire connected to the negative primary terminal on the coil, and use the inductive kickback of each ignition, which creates a short impulse with several hundred volts at the primary terminals to display the engines RPM. Smith tachos of this type are labeled "RVC".
In the early eighties, voltage controlled tachos, also called "electronic tachos", were installed when the industry switched to electronic controlled ignition systems. A voltage controlled tacho receives a 12 volt square wave signal to display the engines RPM. Nowadays, most if not all aftermarket and oem tachos units are voltage controlled tachos.

The tacho types listed above are not interchangeable. Connecting a modern voltage controlled tacho directly to the coil in an older kickback setup might damage the tacho. Also, connecting a kickback controlled tacho to a modern capacitive ignition system can also damage the tacho due to the higher primary voltage.
For many setups tacho adapters are available from a range of companies. As example connecting an electronic tacho to a kickback setup can be easily handled by a standard tacho adapter. There is also a another tacho adapter type available to connect older kickback tachos to newer electronic ignitions by means of a tiny transformer to create the kickback impulse. As current driven tachos are difficult to integrate with an external adapter, the scope of this document is to offer an easy way to do a conversion instead.


How does the conversion work

The Datsun 240Z current driven tacho is based on a two transistor circuit, which amplifies the induction impulse from the coil wire loop. The conversion circuit listens to a 12 volt square wave input from a modern electronic ignition and then creates a 500mV 1ms induction impulse signal similar to the original one, which is then directly fed to the current driven tachos input stage.

This is realized by a relatively simple circuit with just one transistor and a voltage regulator, the signal processing is handled by an AVR ATtiny13A microcontroller. You need an AVR compatible programmer such as the low cost AVRISP mkII, or someone with a programmer to flash the microcontroller with the firmware. The firmware is provided for downloading below. Cost for all parts is 6-8€, The installation is simple as the circuit is soldered directly to one input pin of the tachos pickup coil like a daughterboard. The tacho itself is not modified, so the conversion is easily reversed if required.




Building and installing the conversion circuit

Here is a shopping list of the required parts:

2. Remove the 240Z tacho from the dashboard by unscrewing the two winged nuts on the back and pulling the tacho forward. Make sure its the correct type with the white wire in a loop on the back. Remove the screws holding the black plastic bezel, then remove the electric terminals, the white coil loop and the two screws on the back to get to the PCB.

3. Install a third terminal using a screw and nut in the unused hole for the 6V terminal (see picture below on the left). Solder GND (black wire) to the ground post in the middle, +12V (red wire) to the +12V post on the left and TACH_IN (orange wire) to the new contact on the right side.

4. Identify the small coil below the current loop terminal (the coil is on the right picture below in the lower left corner). The black wire of the coil connects to a terminal on the tachos PCB, on the right picture below marked with a green circle.

 

5. Solder the conversion boards TACH_OUT solder pad (blue marking in the PCB picture above) to the terminal with the black wire coming from the coil (green circle on the picture above right). Take care that the board does not touch the right terminal/red wire.