Speedo issue

[Jamie]

My speedo has always been a bit erratic, it would be fine up to about 40mph then it would flicker around the speed you were doing as the speed increased above 40. I always presumed this would be my speedo cable being a bit long in the tooth.

This week though, it will start flickering and then wind right round to full speed 90mph+ if I blip or ease up on the throttle it seems to reset itself for a little while then winds back round to full speed.

I have taken the inner out of the cable and regreased it, but that hasn't made a difference, have just taken the speedo drive out of the gearbox and the teeth on it look fine.

Has anyone experienced anything simmilar? Or have any ideas?

Many thanks

Jamie

[Daz1968]

I had the same with my speedo, tried a different speedo and was fine, I did take it apart to have a look and couldn't really see anything obvious but was going to get it rebuilt at speedy cables but then another good used one turned up so I have opted for this route, I can only presume there is something wrong with the drum or magnets inside. I did strip it down and degrease it all I case it was oil causing it but it made no difference.

[Jamie]

Thanks for that...sounds like I need to give speedy cables a call tomorrow.

[minicentie]

It could be a tooth problem in the gearbox.
One of the gears is from plastic and can get loose on the axle. If the tension on the cable and bends in it get too high,
the plastic gear can slip over the axle.

[Jamie]

Hi minicentie, thanks for the advice.
In the end I "borrowed" a mk2 speedo off a pal, I've fitted that and all is working as it should. So have sent the mk1 speedo off to speedy cables for an overhaul and I can use the mk2 one until it returns.
Thanks
Jamie

[1071 S]

I've had similar issues.. and I have a friend who trained as an instrument fitter.

His diagnosis was debris between the drive and driven plates in the speedo. His solution was the making of a special tool made by slimming down a hacksaw blade (and cutting off the teeth) - it looked like a small stiletto - which was then wiped carefully around between the plates. If you could imagine the speedo needle being stuck between the plates at, say, 6 o'clock and being rotated around past 12 back to 6...

The speedo is calibrated by changing the distance between the plates.... so be careful...

Cheers, Ian

[Kiwi Craig]

I have an issue with the speedo in my Cooper S.
It will read up to about 50 - 55 mph but no more whatever the speed increases to.
I removed it and had it checked by a instrument repair business. He connected it to a machine and wound the knob, right up to 80, then 100 mph, no problems, so the speedo itself seems ok. I also connected it to my cordless drill and it went up to 80 mph with no issues.
The speedo was checked over several years ago.
I have had the speedo drive gear out (the easy one to get at) and there is no sign of damage to the nylon gear.
Any ideas on where to next?? New cable??

[timmy201]

I would guess either of the speedo drive gears in the gearbox are coming apart from the spindle and spinning after a certain amount of load/speed

[Toby]

A speedo works by the little flywheel in it being driven. This works based on magnetism. The parts can loose their magnatism which would result in it not having a strong enough pull if it needs to move fast. If you disassemble the speedo and re-magnetise it then you stand a good chance of solving the issue

[mk1]

From the main MK1 Site.

https://mk1-performance-conversions.co. ... nst-67.pdf

[Peter Laidler]

Been here before...... After my early 60's apprenticeship I was working in a RAEME instrument workshop and instruments such as speedometers etc etc, including wristwatches, clocks, optical instruments, theodolites and compasses were our everyday bread and butter from all over the Far East.Everyone knows that a speedo consists of two parts.
The milleometer which works by gears and cams from the cable. And
The speed meter which operates directly from the cable via the magnetic drive and receptor disc........ controlled by the flat coil hairspring. These two discs work totally independently and are NOT connected in any way except my the constant magnetic field imparted by the drive disc magnetic field causing the receptor disc to follow it......... rotating the pointer across the screen.

There is NO mechanical interference whatsoever between the driver and receptor disc. And if there is, this is what will cause the problem, together with dust or contamination of some sort. That's why our instrument workshops were dust free and thank heavens in the far east, air conditioned. You don't want any dust in your Omega or Longines - nor your calibrated speedo. The pointer should swing freely throughout its 300 degree or so pathway.

Increasing (to reduce the speed) or decreasing the air gap and therefore magnetic field between the driver and receptor plates ain't something a home hobbyist should be trying, believe me. Nor is the alternative of shortening the throw of the mainspring (to tighten it up and therefore increase the spring tension to slow the speedo part down. We had drive cabinets - also calibrated - that allowed the speeds AND distances to be verified

All the average bloke can do is to fine calibrate the speedo to an exact, say, 70mph, by using your GPS to tell you the TRUE speed versus the speed shown on the screen. REMEMBER THIS. Then, if you are game (we had brass jigs for this), hold the receptor disc (cotton gloves please) and pointer to read the number you remembered earlier. Using your two hardwood levers, pop the pointer off its tapered seat on the receptor disc and carefully replace it on the 70 mark.

If you really want to take it any further in your home workshop, by stripping it down or just dunking the 'works' in a bowl of isopropyl alcohol or methylated spirit. just make sure that you have balls of steel.

That's all there is to it. Don't worry, I never really understood it all either but just sufficient to satisfy my examiner, Sgt Tony Bowman and better mention Sgt Dennis Aslett for good measure