How can i diagnose a jumpy speedo on my 1991 K75RT ?

[xcaret]

I have just noticed my speedo needle is jumping ,and or hesitating by about 5 to 10 kilometers. where do I start looking for problems ?
Thanks in advance, Neil

[frankenduck]

That is sometimes caused by a warped faceplate so that the needle rubs on the faceplate.

Look up from the bottom to see if the top edge is close to the leans. Here's an example of a warped tach faceplate.

[xcaret]

Thanks for your suggestion, sadly it's not a warped face plate . Having searched "speedometers " on here,  I find all kinds of terrifying things from circuit boards to faulty electrical wiring , someone  even suggested checking the differential ( taking it apart) ..
 
As I mentioned this bike has been parked for over 3 years. The other day I noticed the rpm's kept creeping up to 3000 at an idle. I stopped at a bike shop where there was a BMW mechanic, he told me they would first set all the valves ,then check the fuel pump and on and on he went with more things to investigate. As it was I had left the choke on and he was good enough to start the bike before wanting to book it in..  I mentioned this because at $140 per hour ,I would soon be spending more than the bike would be worth. I now shudder thinking what remedy that shop would have if I left it to them to correct my jumpy speedo.   
 There are at least 2 more shops in town that still work on older BMW's ,tomorrow I'll stop by to see if they too need to tear my whole bike apart and relace everything in order to correct this problem.. If it comes to it I can still judge my speed from the RPM gauge !   

[Gabriel70]

Hi

I experienced a similar issue many years ago and managed to fix the problem.
Inspect the inside of your instrument panel, in particular the Speedometer amplifier board.
In my situation, it was a faulty capacitor. Marked C3.

this is the board.

good luck.

[Laitch]

Gabriel is recounting the experience of more than a few around here. That could be one of the sources of your problem.

The simplest thing you could do right now is take the Brick for a ride then replace the fluid in the rear drive with a carefully measured amount of the correct grade. I use 80w90. Remove the screw from the holder of the speedometer sensor from the top of the rear drive and carefully extract the sensor from the drive. Clean the sensor's body with brake cleaner or a different fast-drying solvent then reinstall it. Follow the sensor's wire up to its connector, open the connector, clean the pins and sockets with electronic contact cleaner, reassemble it, and go for a drive.

If that didn't help then go to this page, brought to us by a world-renown shade tree Brick mechanic who apparently has put himself through hell so we don't need to go there unless that's one of our goals, and follow the instructions carefully. Maybe that will do the trick.

[Gabriel70]

Hi

If only it was that simple.
The link you posted takes us to a solution for a loose connection.
The issue is often more complex.
The sensor rarely breaks down.
The speedometer amplifier board should be checked. I replaced my c3 capacitor, a simple solution and my speedo is now working and the indicators self cancel.
The circuit board tracks often corrode due to condensation inside the instrument case.
A known issue.

[Laitch]

If only it was that simple

Sometimes things are simple, Gabriel, but vast experience can sometimes cloud that. That's why experienced climbers have died on Mt. Washington.

Where I come from, simple solutions are tried first.