Smiths Speedo - SN 3273/04

[SENC]

Hoping the speedo on my Seven was also used on early Elans and that someone here may have some experience with them. Mine has stopped working (stopped moving), though the odometer continues to work fine so I know the cable is spinning. I'd noticed it was rather twitchy on earlier drives, but it is now stuck as seen in the picture below.

My inclination is to open it up and see if I can figure out what is going wrong and fix it - but a quick scan of the web last night and I couldn't find any like it (so not sure I could replace it if I screw something up) and though I found some generic guidance no specific resources.

Have any of you come across good resources, or rebuilt one of these yourself? Any guidance or tips? Thanks in advance!

Smiths Speedo - SN 3273/04

[nmauduit]

In the past I've serviced the magnetic type speedos without too much difficulty (on BMW bikes, swapping worn or broken parts taken from donor instruments), I suspect the chronometric type would be more delicate : once opened you should be able to figure that out before embarking too deep I suppose (one caution among others : to avoid scratching or soiling the face when removing the hand, try inserting a soft protection near the shaft and exerting an axial pressure to remove the hand with an ad'hoc tool, like a little fork or two small flat screwdrivers bent to suit)

[Craven]

Chronometric type are very complicated unless you have watchmaker level of skill leave well alone, basically it gets it /hour from an internal clock.
What may have happened is you have run the speedo up BUT not run it back down, chronometric type does not return to zero, it needs reducing speed of cable to return to zero.
http://www.mg-tc.de/articles/JaegerChro ... ments.html

[SENC]

What may have happened is you have run the speedo up BUT not run it back down, chronometric type does not return to zero, after say clockwise cable rotation if then stopped the pointer will stay upscale, it then needs anti- clockwise rotation of cable to return to zero.

I'm not sure I'm following you, Craven - the only thing that would reverse the speedo cable would be driving in reverse, right? I would have thought there would have to be some kind of spring or counterbalance in the speedo that brings it back down.

You've both identified it as chronometric, though, which is helpful as it gives me something else to research. Is there a "tell" that this is chronometric vs magnetic?

[nmauduit]

no, I just said I have no experience on opening up a chronometric instrument... I did a quick search using the SN you mentioned and did not find any specifics, so just mentioned the difference : when you open it up, you'll see which type it is (magnetic is for the way the hand is coupled to its position in relation to speed, using the Hall effect I suppose, with a spring balanced plate close to rotating magnets). It could be dust or filaments in between the two plates.
The odometer is a separate mechanism, just cog wheels (plus a reset if applicable).

[Craven]

Henry, sorry very poor on my part, in a chronometric speedo unlike a magnetic type there is a mechanical connection between the cable and the needle. Pointer is driven up scale but also driven down scale.
Magnetic type, cable spins a magnet over an aluminium disc attached to the pointer. Eddy currents generated cause disc to rotate. There is no mechanical connection between the cable and the needle.

[SENC]

Thanks again to both of you. On the notion that nothing ventured, nothing gained, I went ahead and opened it up. Looks to be magnetic. I'll go do some research before further disassembly and cleaning, but will share a series of photos for future information seekers.

[SENC]

[SENC]

[SENC]

I'll update further when/if I go further.

[SENC]

found a very useful website: https://www.magnetic-speedometer-repair.com/index.php

[lotusfan]

Henry

Attached .pdf on speedo repair that I think came from here and I also have a vague memory of some repair information culled from an MGB site in the USA. Try a search on MGB sites.

mgaguru.com also has information on speedo repair.

[john.p.clegg]

Starter for 10 , the disc arrowed BLACK is connected to the needle and should be spring loaded against the stop,,,gently, can you rotate it and does it spring back ?

Next , the part arrowed RED is connected to the input ( cable drive ) and you should see it rotate if you put a small screwdriver in the drive and rotate it...

1speedo1.jpg (148.44 KiB) Viewed 947 times

John

[joe7]

Is there a "tell" that this is chronometric vs magnetic? Generally if its a chronometic is clearly says as much on the instrument face and generally with a stated ratio, 4/1. That said I never heard of a chrono speedo.Speedos indicate tire rotation via the gearbox, where as a tach indicates engine RPMs via cam rotation, and some time affixed to the generator. Perhaps there is some confusion regarding a speedo rather than a tach, which you are inquiring about.

[john.p.clegg]

It's magnetic, as you can see between the RED magnets and Black disc and the 1040 ( on the ali. chassis and faceplate ) is the Turns Per Mile used in calibration.

John