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Hoping someone has some advice... I have two speedometers with the same problem. At low MPH the needle is relatively steady but once above 30mph or so they start flinging all over the place, even going right around and hitting the stopper. I have opened them up and can't really see what I can do to fix them.

Any ideas?

If not, could someone post the contact details of a well respected gauge restorer in the UK that will be able to sort out the problem?

Has the speedo cable got an even route from the gearbox to the back of the speedo.?

As posted above, it sounds like a speedo cable problem. I'd replace the cable as it could be the inner cable is starting to fray and is catching on the outer.

Thanks for the replies. The one car has a new cable and the other car's cable works fine with a different speedo. Cables are not the problem.

One of the speedos is an S speedo 3.44 final drive. Would it work if I swapped the internals with another (non -S) speedo with the same final drive? That might help avoid having to mail speedos halfway around the world to get restored by someone who knows what they're doing.

Hi Abri,

the odometer is gear driven, so both should have the same gears. If you count the number of teeth on the little yellow/black/white gear on the right hand side of the odometer numbered wheels and multiply it by the worm pinion teeth, you should have 1280 which is the TPM (turns per mile) of a MPH 3.44:1 normal S speedo. If both do 1280 TPM, then you can swap them and know they count miles the same way.

As for the speedometer, I'm not sure of to what extent (and also would like to know) parts are different between an S speedo and an 850 speedo. Common sense would say they are different as the speedometer scale is different in both speedos... but are they?
The speedometer is calibrated by 2 magnets attached to a 'balancing' disc and a wire spring. Differences, if existent should be in these parts.

Once you remove the needle, if you don't take notes to mount it back in the same exact position, you will lose calibration. This is a much more sensible part than the odometer. Calibrating it back could be hard to do at home. You can set the needle to the little dot below the 0 MPH, but you will not know at what speed it is closer to actual speed. Using a GPS may be a good idea to correct it. Just remember the speedo will not be calibrated during the whole speed range.

My speedo had the wire spring and the worm pinion broken. I will repair it at home as I don't want to send an S speedo in KPH by mail, so I'm also interested to know if the wire spring/magnets combination is different from the 850 speedo.

Sorry, I guess I got out of the track in the previous reply. Check if the balancing disc is fouling anywhere and check if the wire spring has lost its strength. The problem, if it really isn't the cable, then seems to be related to the magnet wheel, I would say...

My S speedo's odometer works fine (or appears to).

The spring that returns the needle to the 0 mark also seems fine. The turning magnet part seems to catch on the "balancing disk" which is attached to the needle. If I understand the workings of this part right, the turning magnet (which is turned directly by the speedo cable) creates "stress" (rotating magnetic field?) on the balancing disk as it turns and this moves the needle in the opposite direction as the needle spring. When you decellerate, the magnet turns slower and the spring forces the needle back to 0. The wild fluctuations are caused by the disc and rotating magnet either touching eachother or somehow exercising inconsistent energy on eachother .

I agree with you that its possible the 850 and S spring tension etc might be different, however, is it not possible that the scale (0-130mph is just larger because the S can turn the magnet faster?

Hope that makes sense.

The turning magnet part seems to catch on the "balancing disk" which is attached to the needle.

That is the reason for your problem.

REMEMBER! Once speedo's are dismantled by someone who doesn't REALLY know what they are doing with them they very, very rarely work as they should afterwards.

I hope I can help with a little background information. I am however all the way around the other side of the world where I repair speedometers many of which are smiths for a living.
First of all I agree with Mark implicitly. We all have our own little specialty in life. For some it is better to concentrate on being the best accountant, solicitor, storeman etc that they can be and pay someone else to be an instrument maker. But for those who insist....
The principle that previous posts referred to is the eddy current applied to instruments for the first time by otto schultz in 1902.
The hairspring and the magnetic field are the two elements that may be varied to create different calibrations both from a span perspective ( 90 mph, 120 mph etc) as well as the turns per mile due to diff ratio1280 (3.4) 1408 (3.7) etc.
It is not necessary to change the hairspring between an 850 (90 mph 140km/h) and Cooper S (120mph etc) the calibration can and is taken up in charging or discharging the magnet. The reference to gears made earlier are with respect to the eccentric gears at the base of the instrument in a mini this is normally 32 teeth which matches with the worm on the magnet shaft. The variation is with the ratchet gear which sits at the end of the odometer assembly. In the case of a 3.4 diff mini with 1280 turns per mile it is by mathmatics a 40 tooth gear.
To the problem at hand you have
The height of the speedcup shaft with respect to the speedcup.
The condition of the bearing below this within the magnet.
the amount of endplay in the magnet shaft
the amount of sideplay in the magnet shaft
the height and level of the magnet
Did I mention I agree with Mark implicitly.
There are very few of us repairing instruments these days. I only discussed this with my 67 year old father who has been repairing Smiths instruments for 41 years this month. I know of Speedy Cables, John Marks & Richfield. All of these guys are getting old as well.
In the past week I have been lucky enough to complete the restoration of the instruments from LRX 829E and today I started to repair the Weston tachometer from Graham Hills Speedwell demonstrator which has not worked since at least Mini 40 celebrations.For a mini owning instrument maker it is much more interesting than resetting a digital odometer in a post 2000 car which i also did today

mini129 wrote:things

very interesting - im one of those people for whom taking an speedo to bits is a gauranteed lesson in futitily!

its what happens when you have big farmers hands

Yes, I guess for some it is a necessity to fiddle with their delicate parts themselves

Seriously though, I don't know of anyone in South Africa where I live who repairs these speedometers. If it was just an 850 speedo, I'd have tossed it, but it is a SN4417/4418A 1280 130MPH speedo.

Mini129, going by what I said in previous posts, how big do you think the damage to my wallet will be for a repair? (if you'd be willing to take on the job, that is).

Thanks
Abri

Theres a guy over here in Aus in the Moke Club who gave a series of really interesting talks at the last Moke Muster about speedo's.

He had perfected the are of re calibrating the units by re magnetising the disc, I recall he stroked the disc a number of times against another magnet (or something) then re assembled the unit and tested.

He had a standard electric motor running on a precise (oscilloscope controled / monitored) variable voltage, he then knew the speed of the cable and hence could check the calibration. If still wrong, out came the magnet again for more stroking!

Was very interesting! he also noted that the jumping effect (if not caused by the cable) can be the disc and magnet catching. There should always be an air gap.

I'll try and find his details.....

After a lot of looking around locally, I can't seem to find any speedos with 1280 turns per mile/3.44 final drive internals.

If anyone has a 90mph speedo for a 3.44 fd car for sale, please let me know. It doesn't need to be a nice one, I will strip it for the parts I need.

my speedo :

but i don' t understand to set the cable in my oval board.

Premier Instruments in Canberra (Oz) do Smiths speedo repairs and calibrations. A calibration is not that expensive. They did mine years ago and its still fine - although I now need to do the mental math following the change from a 3.76 to 3.1 diff. They also do tach calibrations...

If you decide you have nothing to lose and choose a bit of self medication for a wandering needle, you might try this...

Take a standard hacksaw blade and cut off a piece about three inches long. Now cut this in half lengthwise and cut file one end into a rounded point. You will now have small stiletto 3 inches long, perhaps 3/16th wide and a standard blade thick with a point at one end. Now wipe the blade gently between the two discs that drive the mechanism. The aim is to remove any guff and debris that has accumulated between the plates. The guy who showed me this started (his working) life as an instrument fitter

Done gently you should not damage anything and may be surprised at the amount of "stuff" that comes out. And my speedo needle was certainly much steadier afterwards....

Good luck, Ian