Speedo started acting up a while back and cleaning some plugs seemed have helped for a while. So this time I replaced the plug between the sensor and the harness, that didn't do it. Cleaned the contacts on the instrument pod, that didn't do it. Removed and clean the sensor.
Anyway
If I get out the old welder soldering iron and hold it near the sensor the speedo reads a nice 72km/h and will do so all day it seems.
I am left wonder if anyone has seen theses sensors act up only when warm? It seems to me that is behave a bit better when the bike is cold and especially on cooler days.

What would you do next?

The sensors can go bad and may need replacing. One way to isolate the failing sensor from any other problem in the gauge cluster (and avoid random parts replacement) is to use a PC running Karamba to feed an audio signal into the harness to exercise the speedo.  The tutorial write up is here:  https://www.k100-forum.com/t2038-karamba-speedometer-calibration-program-tutorial

The upgraded Karamba program is now called Kdiag. You'll find a link to the Kdiag program on the K100 Forum Tech Page.

Thanks for the info. Is this not similar to running the 60hz transformer near the sensor? In that test every thing seems to be in good nik. If the speedo its self or the wire were faulty that test should fail as well right?

Yes, the solder iron test is similar to Kdiag except that Kdiag tests the whole range of frequencies but the solder iron tests only one frequency. Also, Kdiag eliminates the sensor allowing you to isolate the problem to one component. 

Can you describe the symptoms in more detail; what do you mean by "acting up"?

At the start of a ride the speedo usually is working but register a little slower then it should. There is usually some fluctuation in speed shown as well.
This gets progressively worse until the speedo is registering very slow or not at all. The odometer  works when the speedo is.
Sometimes it will come back and work for a bit.

Yet with the transformer near the sensor all is fine.

Hope that gives you a better idea.

Because the speedo works at first and sometimes comes back again, I'm thinking the problem might be inside the gauge cluster.  The signal from the VR sensor on the rear drive is very small and any corrosion can interfere. You didn't say whether you opened the gauge cluster and looked at cleaning the connections inside the cluster. If you haven't done that, I would recommend you attempt to address any corrosion within the cluster.

It may well be in the case but it was looked and and clean a couple of months back. At that time I went through the whole system and did get some improvement. The innard workings are clean, dry and corrosion free
Although I don't have a lot of proof I think it is heat related, again this could be in the instrument itself or in the pulse generator.

The speedo works so well using the transformer near the sensor I can't help think the pulse being generated is to weak or intermittent (I will check the voltage from the speed sensor).
As usual everything work fine in the shop.

Sorry just kind of typing my thoughts out.
I appreciate you taking the time.

It is looking like a bad bit of wire between the speed sensor plug and the plug for the instruments. I get high resistance and even an open circuit a couple of times.
While work on a bypass wire for further testing I think I have located the issue near the instrument cluster plug. There may some damage done by a tie-wrap and a GPS mount that used to be on the bike. I don't need the GPS as people are kind enough to tell me where to go.

I am hoping that is it as it is a fairly easy fix and doesn't involve buying parts

Hi

I experienced a similar issue many years ago.
Flaky speedometer eventually ceased working.
I suggest you check the connections inside the gauge.
Very, very carefully remove the rubber gasket covering the main connector pins and check for corrosion and clean with deoxit.
Remove the long thin circuit board at the bottom right corner inside the  gauges and check the circuit board tracks for corrosion.
I fixed my speedometer when I replaced the capacitor.


 if the indicators auto cancel, 
the sensor works and the cause of the fault must be found downstream of the amplifier;
 if the sensor works, the indicators do not auto cancel,and the wiring is correct, the problem may be in the amplifier.

Thanks for the info Gabriel.
I think my issues were for now at least are sorted out.Who knows what repair are in my future though.