Home away from home
From Melbourne Australia (and likely under the car)
Most (and I haven't pulled apart a datsun one to be up front here) mech speedos are a simple enough gadget. There's a magnet attached to where the cable slots in. when the cable spins, so does the magnet. It is surrounded by a circular 'cage' that is connected to the speedo guage 'needle' or pointer. The magnet never touches the cage, but as it spins, it tries to pull the cage with it, and the needle/pointer swings around the guage. the pointer is spring loaded, so it will return to 0 when there's no movement of the cable/magnet, adn the faster the thing spins, the further it will manage to 'encourage' the pointer around the dial.
And that works pretty well. But the reason I bring it up is that over the years things will wear a little bit, and the magnet (it could also be reversed - the cage spins and the magnet is attached to the pointer, I suppose) - well anyway , the magnet no longer just spins, there's enough play in there (worsened by the cable itself wearing/getting old and trying to magnify this issue) for it to 'wobble' just a little whilst it is spinning. Then instead of just magnetically moving the cage/needle/pointer, it will actualy scrape against it, and it will pull it further than if it was prevented from actual contact with the cage. Then it wobbles back, lets go and it continues.
If it is just a mild wobble, it'll probably smooth out mostly and just give a high reading, but if it gets more severe, you'll experience it grabbing and springing back , and, at higher speeds especially, the speedo needle will jump forward and backward like one of those lie detector needle things..
If it is down to wear (and by this stage - being close to 40 years old, it's a strong possibility) then the speedo will tend to be somewhat accurate at lower speeds, but worsen as the cable rpm and 'wobble' get worse, and read higher and higher.
What this means is that the corrector box might 'fix' it for 100km/h but it would no longer be accurate at 60km/h (or whatever roadspeeds you choose, and for km/h or mph, whichever is being displayed).
To 'fix' this it's likely possible that an instrument maker/repairer (and there are a couple in Melbourne, or used to be, and they mostly do resto work for rarer musclecar era fords and various other old british cars and so forthe) could make/fit a plastic or bronze bush to have the magnet/cage spinning without any wobble (but still spinning freely without binding of course). The problem I suspect would be the price - even if you remove the speedo from the dash and give it to them, so there's no extra fitting charges, it still won't be cheap.
As to exactly how much - nfi, but if you happened to have a spare speedo, you could probably take it apart and see how it works, and if a diy re-bushing might be possible. If at all possible, find a completley broken speedo, and pull that apart, that way you won't lose any sleep if you break/damage it further whilst learning/finding out how they are taken apart 'properly' and what can be done to fix them.
After all that, the pointer/cage is spring loaded, and it will probably be a 'clock' type wound flat band of spring steel. For sure over the years it will have lost some tension, so even once you fixed the 'wobble' issue (if it _is_ in fact an issue, which isn't 100% certain) - well then you might also need to increase the spring preload just a little to bring the indicated speed back to where it should be. As to how easy that would be on this particular speedo, I don't know, but it _should_ be doable.
Posted on: 2012/7/4 9:41