Puzzling Speedometer behaviour

[plc]

G'day All,

I am having some strange speedometer problems that don't seem to fit with the standard problems I've read about on the forum.

For a couple of months now my speedometer has wavered terribly often dropping to zero then jumping back up. The speedometer was serviced about 3 years ago, and I know it isn't the damper in the speedo itself. So thinking it was the cable I disconnected the cable from the gearbox, connected a drill and found the speedo ran perfectly and steady at 65 km/h, if I slowed the drill it faithfully slowed without any wild fluctuations. It operated as I'd expect.

So I then removed and checked the speedometer drive gear and while having signs of use (shiny spots where it engages output shaft), there are no obvious wear problems.  And the cable seems to engage with the drive unit faithfully.

So this leaves me baffled, the cable and speedo work fine on a drill, and the drive unit looks ok. Now the one other part is that it is a bigger problem in cold weather. As the afternoons warm up the speedometer flickers less and runs truer as the temperature rises. So on a really hot day (our summer is coming) I figure the problem may be gone.

Could it be gearbox oil causing it? I use Penrite 85W/90 oil in the box. I thought about the cable's lubrication but it works fine off the drill.

So to the collective wisdom of the group, what could be causing this erratic speedometer behaviour?

Cheers to all,
Paul

[Bob_Roller]

I'd lubricate the drive cable with a light oil and see if it helps any .

[Adrian]

The speedometer was serviced about 3 years ago, and I know it isn't the damper in the speedo itself.

Hi - I had the same problem some years ago and I gave the bike to a shop that specialises in gauges of all kinds. The problem turned out to be the damping so that was fixed easily. However, 3 - 4 years later the same problem came back and I had to get it done again. So maybe this is happening to you too. Might be worth having an instrument shop look it over for you just in case. Hope that helps .....

[nhmaf]

When you tested it with the drill, did you have the cable still routed on the bike  with all its bends?

I agree with Bob - these speedometers (all BMW airheads, not just R65) have a tendency to be prone to dampening problems.  With the drill bit, things are nice and smooth and not bumpy, whereas on the road, it is usually another story.

[bruce_launceston]

I had the same problem for many years until I cleaned and regreased the nylon gears inside the speedo at the top end of the cable. They are just accessible once the cable is removed.

[plc]

Bruce, did you use a silicon grease to lube the nylon gears?

Cheers,
Paul

[tvrla]

It doesn't really matter what grease you use - just as long as it's light weight. Don't use very much as it will just fling off anyway.

But I don't believe that's your problem - though it could be. When turning the speedo input by hand, does it feel stiff? Lots of resistance? It should feel fairly free, not sticky, though you can tell you're driving something.

My first thought is the cable. Perhaps clean it out (however one would do that?) and lube it properly. Ensure no tight bends - nice sweeping curves.

And no, it's not your transmission or the speedo drive gear. Definitely not!

[plc]

Hi Guys,

Well I lubed the cable by running a light oil through it. At first black oil dripped out but eventually clean oil came out. Tested the speedo again with a drill and the needle was flickering with the drill this time. On the road it was even worse dropping to zero for a minute or two.

It registers a speed of I hit a bump or if I manipulate the cable just at the bottom of the fairing.  I'm now thinking my cable is damaged in some way so now will be getting a new cable.

Just for the record I bought a Venhill cable from Motobins years ago but it doesn't fit because the sheath which goes over the brass drive at the speedo is too tight and causes that brass part to bind when the cable turns it. So a genuine article this time.

Cheers,
Paul

[tvrla]

There must be some way to expand the cable end...