Speedometer fault.

[dougal9887]

Hi,
Hopefully someone can shed some light on this problem, or at least suggest a possible cause. The car is a 1973 Plus2S 130/5 speed.
Basically the speedometer started erratic behavior in about 1979! The car was laid up shortly thereafter.
When rebuilding the car I remembered the issue and tested the speedo out with a drill on the end of the cable. It worked perfectly on acceleration, de-acceration and steady speed. I removed the drive mechanism and couldn't see any problem there so just cleaned and lubricated the cable and replaced everything.
However once back on the road the problem was still there. I removed and opened the speedo up. It was very clean and all parts worked freely. I lightly lubricated according to the instructions and didn't expect any improvement - there wasn't!
Studying the movement of the needle more closely, the issue appears to be load related. On acceleration the needle stalls then on lift off jumps to the correct speed. The needle remains steady on over-run or coasting downhill but when any load is applied the needle drops but jumps back as soon as the foot is lifted.
Could it be that some sort of gearbox wear is causing drive to be lost from the mechanism under load conditions?
Any ideas appreciated.
Dougal.

[john.p.clegg]

Dougal

I think you have got it, that is exactly what I would expect if the speedometer drive gear was loose and slipping on the output shaft.., could also be in the angle drive?

John

[elansprint]

Dougal remove cable at speedo end and roll inner cable between fingers to make sure it is located in angle drive gearbox end so should not spin continuously the cable can just catch on the end of the drive giving the symptoms you describe if the square end is not located correctly
Ian

[dougal9887]

Thanks for the prompt replies.
Ian, that's a simple check that I don't recall doing, so first up.
John, which parts are you describing as drive gear and output shaft?
Dougal.

[john.p.clegg]

Dougal
Parts 28 and 29.

http://www.lotuselan.net/wiki/FE_-_Gear ... rnals_%2B2

John

[prezoom]

I had a problem with a loose spindle that goes into the driven gear. Off the gearbox, I could spin the spindle and watch the output turn without a problem, When all hooked up, the spindle would slip. A new spindle fixed the problem.

[dougal9887]

The workshop manual doesn't specify how the speedo gear engages with the output shaft, perhaps a key? Surely not just an interference fit? At any rate if the fault lies there it will have to wait for the engine to come out for some other reason!

Spindle slipping in angle drive sounds worth investigating so I need to get under the car and see if the angle drive can be removed with the engine still in. Anyone know?

Thanks
Dougal.

[john.p.clegg]

Dougal
I did it once ( or was it twice? ) you have to adopt the "Lotus Position" attack it from underneath and through the tunnel top....leave it till the box has to come out...

John

[prezoom]

If you have a child under the age of 10, their hands will fit. Otherwise, stock up on your favorite libation, you will need it.

[mbell]

Sorry for delay, Had a look at the Speedo drive on my box while it was out.

I think it's a very similar setup to the four speed. Fault is probably the drive gear, rounding of the square end off the spindle or the elbow.

[dougal9887]

Thanks.
Just wish I had looked at it much more closely before putting it all back in. I always assumed the fault was with the speedo.
Dougal.

[dougal9887]

Happy New year all!
Well the holidays seemed like a good time to re-visit this problem.
I cut a triangle of glassfibre from the top of the tunnel, matching the top flange of the chassis. I will probably replace this with an aluminium panel and self tappers although it could be glassed back in.
This gave excellent access to the speedo drive angle and also allowed me to remove the aluminium bung, spindle and driven gear.
All these were in perfect order as expected.
So that leaves the question of the drive gear slipping on the output shaft. I had struggled to find out how/if this was possible, so was happy to find this old post.
lotus-gearbox-f37/tips-for-replacing-the-speedo-drive-speed-t20782.html
Peterako decribes the gear is an interference fit, and that his problem was solved by tightening the output shaft nut. So perhaps the gear is more of a slide on fit relying on compression from the nut to keep it from turning which doesn't sound a great engineering solution. It would however fit perfectly with the above mentioned behaviour of the speedometer.
Clearly tightening the output shaft nut is an engine out job which seems a bit extreme for a wavering needle.
However as things stand, I am in the middle of EFI conversion so have the exhaust off, carbs off, alternator off interior stripped and now the gearlever out so no time is likely to be easier.
Before I bite the bullet, any further insights into the problem would be welcome. And it would interesting to know if the OP Craig Elliot went ahead with Peterako's fix and the results. I see Craig last posted earlier last year so he may still be about?
Dougal.

[dougal9887]

Well the engine's coming out today.
I'll post back findings. I should find that I can hold the speedo drive, turn the output shaft and feel the speedo drive gear slipping. If not I'll be disappointed! Although it may just slip when the drive is loaded - that's what the needle indicates.
Dougal.

[dougal9887]

That's the engine now out and the problem verified!
I can gently hold the spindle and merrily turn the shaft.
Now to see if tightening the nut fixes it.
Many thanks to Peterako, if he reads this, for that pearl of wisdom.
Dougal.

[dougal9887]

The output shaft nyloc nut was about 1/2 a flat too loose. After testing that out I removed the nut and replaced it with some nut-lock.
Anyone replacing an engine with a 5 speed gearbox would be well advise to check this nut!
Dougal.