Today I finally managed to measure the signals on the digital speed sensor. I don’t own any of the Rz-1 digital cluster parts, but luckily, a friend borrowed me his sensor and allowed me to reverse-engineer it.
The speed sensor has a connector with three pins: black (right), yellow (middle), red (left).
Based on what I measured, black is GND, red is +5V supply, and yellow is the speedometer signal. The based on my scope prints, it seems that per revolution of the speedo cable we can see 24 pulses:
When I rotate the pin of the speed sensor at about one revolution per two seconds, the cluster shows me – very roughly – 13 km/h.
When only connecting the +5V and GND, but leaving the signal pin open. the signal looks idential. When rotating the pin of the sensor, the 5V pulses on the signal line are still visible.
Therefore, it seems we can just use a 0-5V PWM, and bring it onto the cluster, and it should display the speed signal.