Speedo intermittent reading

[Gunner]

Hi all, some time since last post but am struggling with an intermittent speedometer reading. It started when I noticed the needle acting like a rev counter ie dropping when off the gas. I’d read about frayed / stuck cables so fitted a new one. No issues after a couple of miles test run but now after about 8 miles on a new test run where at one stage it was showing just slightly faster than a roadside digital speed sign and ran smoothly up to a registered 60mph it has reverted to dropping when off the gas but registering when back on it. Old cable was free in outer and new cable slotted into speedo and gearbox smoothly. Could this be related to the cable warming up or should I be looking at the speedo and / or gearbox pinion?

Car is 57 with 1275 midget engine.

Any thoughts appreciated

[svenedin]

My speedo started reading intermittently (it has been rebuilt professionally about 2 years before). At the beginning of the journey it didn't work at all and then miraculously would start working again. Then it stopped working altogether.

I changed the cable for a new one and it seems fine for a couple of months now.

Make sure the cable is routed in nice gentle bends and also that it is restrained gently by the clips in the engine bay.

To test whether the speedo and cable are OK, disconnect the gearbox end of the cable and secure the end of the cable into an electric drill chuck. Spin the drill backwards (in reverse to normal rotation of a drill) and the speedo should give a reading. If it does not, the problem is cable or speedo. If it does give a reading, the problem is gearbox end.

I did both ends of the cable up by hand, making sure first that the square end of the inner cable seated properly. Then I tightened just enough to nip up with pliers.

Good luck!

Stephen

[Gunner]

Thanks for the reply. Mine appears okay when I set of on a run but then it goes downhill where it seems to show lower than I think I am doing when coasting then jumps up by a fair degree when I go on the gas which would be great if the acceleration matched the increase in revs. Then as the run gets longer it starts registering 0mph until I go on the gas again I will double check the routing but it doesn’t appear tightly curved.

[svenedin]

Thanks for the reply. Mine appears okay when I set of on a run but then it goes downhill where it seems to show lower than I think I am doing when coasting then jumps up by a fair degree when I go on the gas which would be great if the acceleration matched the increase in revs. Then as the run gets longer it starts registering 0mph until I go on the gas again I will double check the routing but it doesn’t appear tightly curved.

Sounds like sticking as the cable warms up. The inner cable spins rather fast.

Stephen

[Gunner]

Did the drill test and speedo was very smooth. Cable didn’t appear too tightly bent so will look st the gearbox pinion next. Thanks for your advice.

[don58van]

Could it be a faulty speedo drive gear? I vaguely recall someone having this sort of problem when the gear was slipping on its shaft. I suppose the amount of power going through the gearbox could have some influence on the behaviour of a faulty drive gear.

Don

[myoldjalopy]

Could it be a faulty speedo drive gear? I vaguely recall someone having this sort of problem when the gear was slipping on its shaft. I suppose the amount of power going through the gearbox could have some influence on the behaviour of a faulty drive gear.

Don

Maybe. This guy had some issues with the drive gear slipping. May be worth watching, 'Gunner': https://www.youtube.com/watch?v=x5ivMk1iggM

[Gunner]

Thanks for sharing the video. I’ll be checking the drive tomorrow and will update with the results.

[Gunner]

So, yesterday I pulled the speedo drive pinion out and found the gear had been rubbing on the housing - possibly due to the copper washer shown on the Moss website being absent thereby reducing any free play that would reduce any friction. After cleaning up the scuffed face of the housing and placing a spare rubber o-ring from a Suzuki Bandit carb service kit and connecting up, it appeared okay over a few miles. Today 50-60 miles later and everything appears good.

So, following the comments in responses to my original post and the powers of elimination I now, for the time being, have a perfectly functioning original speedo.

Thanks to myoldjalopy and don58van for their comments

[svenedin]

Good news! I don't why it had never occurred to me but until I tested my speedo and cable with the drill method I hadn't realised quite how fast the cable spins in normal operation. I had assumed it was geared down to something quite slow.

Stephen

[geoberni]

Good news! I don't why it had never occurred to me but until I tested my speedo and cable with the drill method I hadn't realised quite how fast the cable spins in normal operation. I had assumed it was geared down to something quite slow.

Stephen

It needs all that speed to enable the magnet to drag the disc around against the tension spring, plus having the smoothness of needle movement whilst doing so. It was a really clever concept when it was invented, to use magnetic flux and eddy currents in that way.

Speedo internal.JPG (46.23 KiB) Viewed 542 times

[svenedin]

Good news! I don't why it had never occurred to me but until I tested my speedo and cable with the drill method I hadn't realised quite how fast the cable spins in normal operation. I had assumed it was geared down to something quite slow.

Stephen

It needs all that speed to enable the magnet to drag the disc around against the tension spring, plus having the smoothness of needle movement whilst doing so. It was a really clever concept when it was invented, to use magnetic flux and eddy currents in that way.
Speedo internal.JPG

Interesting Berni thank you. I shall read a bit more about it. My grandfather was a watchmaker but during the war he made instruments for aircraft. The watchmaking/clockmaking skills were readily transferable to other small, high-precision mechanical devices.

Stephen

[geoberni]

Interesting Berni thank you. I shall read a bit more about it. My grandfather was a watchmaker but during the war he made instruments for aircraft. The watchmaking/clockmaking skills were readily transferable to other small, high-precision mechanical devices.

Stephen

This is quite a good explanation. it's detailed but still easy to follow. Just scroll down past the introductory blurb about Speed Guns to the bit where is says

.....back in 1902, when German engineer Otto Schulze invented the first practical solution: the eddy-current speedometer.

Mechanical (eddy-current) speedometers

https://www.explainthatstuff.com/how-sp ... mechanical

[myoldjalopy]

So, yesterday I pulled the speedo drive pinion out and found the gear had been rubbing on the housing - possibly due to the copper washer shown on the Moss website being absent thereby reducing any free play that would reduce any friction. After cleaning up the scuffed face of the housing and placing a spare rubber o-ring from a Suzuki Bandit carb service kit and connecting up, it appeared okay over a few miles. Today 50-60 miles later and everything appears good.

So, following the comments in responses to my original post and the powers of elimination I now, for the time being, have a perfectly functioning original speedo.

Thanks to myoldjalopy and don58van for their comments

Good to hear its fixed