Speedo needle bounces when...

[stopintime]

Stable some of the time and unstable the rest.

It typically goes from stable, let's say 80 km/h, and then for a second up to 100/110 or 120. Again and again.


Done some testing today.
- battery terminals voltage: bike off 12.8v / cranking ~9v / idle just after start 13v / idle after a minute 14v / 4.000 revs 14v / 5.000 revs 14v
- no wires or connectors look burnt
- regulator seems to be as warm as usual (yes, it's 13 years and 205.000 km old)

- maybe the most important (?) finding is that when this happens, I can pull the clutch and let it roll at idle and the needle will be stable again.
- it's stable when I'm running at 4.000 revs or less. Starts it's funny business over 4.500 revs
- I tried revving at 5.000 standing still. No needle movement.

Hmmm.... 

[rule62]

Just a guess... The resting and cranking voltage seem a little low. Perhaps, when running, there's some odd voltage fluctuation happening which causes the speedo to get wonky. When is the last time you replaced the battery?

IME with another Italian bike (Aprilia Dorsoduro) batteries being near the end of their life caused all sorts of mystery gremlins. Perhaps give a new battery a go and see if it solves the problem.

[Duck-Stew]

The speedo gets its signal from the Hall effect switch (duh).  Perhaps that has an issue with shielding or grounding?

Very odd issue…

[stopintime]

The battery is 5-6-7 years old. First time that has happened. I usually kill them by leaving the bike with alarm on during late season months. This one has been brought back to life a couple of times and seems strong (or maybe not....)

The poor 'Hall effect switch' is old and not ideally positioned. I'm not sure it's ever been taken off, cleaned or anything. I could start there...  I can't stop thinking though, that if it fails it would be bouncing down...

[ungeheuer]

Reverting to Italian form.   This was normal behaviour for the Veglia speedo on my Morini ...  all you knew was that you were moving at some velocity +/- 30km/h.

Sorry zero help.

[Howie]

You do need a battery if it is fully charged and drops to 9 volts while cranking.  Not your speedo problem though.  Do pursue the sensor and wiring first.

[Charlie98]

The battery is 5-6-7 years old. First time that has happened.

Definitely new battery time.  You might be surprised... a new battery might fix some problems you don't realize you had... 

[stopintime]

So, I bought a swingarm years ago. It included all the stuff the owner wouldn't be bothered taking off.

Fresh sensor  

The rubber boot/sleeve was torn.

If it stays dry I'll test it tonight. Good methodical work   I'll get an answer about the sensor. The battery will be next - if that's what fixes it, we'll know less than if it's the more obvious sensor fail, but more than nothing

[Howie]

Charlie98 made a good point.  As long as you have 12 volts the sensor will be happy, but your dash, maybe not.  Weak batteries play havoc on electronics.

[stopintime]

Rock steady  
Thanks all!    Except Ungeheuer  

[ungeheuer]

Glad we could help 

[stopintime]

Glad we could help 

[ungeheuer]

Sensor or battery?

[ducpainter]

Sensor or battery?

Yup.

[stopintime]

Sensor or battery?

Probably one of those...