Speed sensor fusing: power / current requirements

[FazerNoob]

I did a good amount of searching before posting. Apologies if this is old ground.

So...as we all know the speed sensor has 3 cables: live/power, signal/drive, and earth.

I'm replacing the speedometer and the OEM speed sensor live is actually out of the speedo unit itself rather than from the switched live like everything else.

I've had the speedo apart and done some testing on the circuit board. As far as I can tell it's some kind of voltage regulation that's going on, seems like this could protect the sensor and/or make it more accurate. Does anyone know the reason?


In any case I'm replacing the speedo, so the sensor will just have to connect to the switched live now. So to make sure I don't fry my speed sensor I'm planning to put a fuse in. Does anyone know what power/current it would draw? I have the electrics in bits at the minute so it's tricky for me to measure with a multimeter!

Cheers

[FazerNoob]

UPDATE: In case anyone finds this post in the future with the same issue.

The circuitboard of the speedo connects the meter light input live (blue wire) back to the speed sensor output live (blue with yellow trace wire) via some kind of voltage regulator: In parallel there is a capacitor and a zener diode.

I’ll try and upload a pic of my half arsed circuit diagram!

[kebab19]

Good work, always great to increase the knowledge of the Fazer hivemind 

[FazerNoob]

Quick update for anyone in future looking for this issue on an fzs600 fazer speedo replace:

Summary: If you're replacing the speedo and keeping the original speed sensor it will probably work without this step, but I did it anyway.


- The speedo has this capacitor/zener diode combo between (a) the meter light input to the speedo (which is a secondary switched live) and (b) the speed sensor power output from the speedo.

- My previous diagram probably had the zener the wrong way round...

- I'm not clear what on it does. I'm pretty sure it doesn't reduce voltage. I guess it's either to (1) reduce "noise"/needle wobble on the speedo; and/or (2) to protect the sensitive speed sensor from voltage fluctuations.

- I expect that 25 years ago a talented yamaha engineer designed this feature into the circuit for a good reason anyway (let's hope he's now retired on company pension enjoying golf and warm saki...). I think these little details are behind the famous fazer reliability anyway, so how could I just bin it?!

- I couldn't confirm the specs for either the capacitor or the zener. So I bought an old wrecked speedo on eBay and cut them out and soldered (I'll add pics)


Side note on speedometers: The 1999 Fazer speedo is very different from the 2000 speedo. The circuit board is actually a completely different design with components in very different locations. I expect that they're probably 99% the same components wise. Future electronics tinkerers beware!