The Stovebolt Site Forums The Shop Area Interiors [Solved/Updated] Speedometer Re-Assembly....

(1949 3600) My speedometer quit on me yesterday, started by the needle not returning and responding "slowly" then eventually quit altogether.

I had a parts speedometer which seems to have the exact same issue; after cleaning out the nest of a VERY tiny spider (see my separate post on that in the General forum), I thought maybe I'd put it together wrong but now I don't think so.

- Clocksprings on both units are good.

- I've made sure there is no oil or grease of any form in the "clutch" on each. Dry and clean

- Everything turns freely, but the "clutch" isn't engaging on either.

I'm tempted to try some thin shimstock in there, if it's just wear? I've ordered a replacement but it's weeks out and yup, I can estimate the speed just fine off the tach but I'd love to have one working even as a temp fix.

Update - and I'll post this separately as it's probably easier for someone to follow along, if you've got a similar issue...

I haven't solved the root cause, but seem to have managed the symptom by swapping the "clutch drum" I'll call it from the spare unit to the one that I had in the truck already (both used & I believe, oem). I still **think** this is caused by wear, although I don't see any filings or wear marks in either unit.

Some observations:

1) Clockspring actually doesn't need to be wound. Just set it to where it's resting on the stop, and as long as the spring has some spring, it should return. But if you try and increase the tension then (and especially if there's any wear/loss of grip in the "clutch") there won't be enough force in the "clutch" to move the needle.
2) One important lesson learned - DO NOT TRY AND SPREAD THE MAGNET... I had the "brilliant" idea to tighten it up a bit by spreading the "ears" - see the result below. Took no real force either, that bad boy ain't steel and it does not flex. So now that one is strictly for parts.
3) For the sake of argument, I tried some mineral oil in case it actually needed some. It didn't. Causes hydraulic action/suction of sorts and guarantees it won't work. Keep things dry in there.
4) One recurring problem I'd have was that if I spun the brass by hand, things would appear to work normally. Hook up a drill and it would not work at all. I didn't figure that one out, you'd think the more inertia, the easier things would overcome any wear.

Outstanding question for myself is, why the needle "dragged" before the unit failed - as the spring seems to have a lot of energy. Didn't really solve that, unless it was simply from some old crud in there. Certainly after cleaning, it would whip back with energy (but the clutch wouldn't move it ahead).

I also include, for reference, a picture of the location of the clockspring for anyone who removes it and forgets.

Don't know if that will help anyone, but...

hi NorthCoast3800
the thing you refer to as a clutch is actually a magnet in a metal enclosure, there should be zero contact between that magnet (the part you broke) and the drum enclosure
the magnet spins and induces a current into the drum that causes it to get dragged with the rotating magnetic field

any disturbance of the clock spring will result in the speedometer being off, the clock spring is calibrated to read true on the dial when the drum is being dragged by that magnet, it's K value sets the angular displacement of the needle and as the clock spring wears (rusts and ages) the speedo will error on displaying a speed faster than what you are actually going
but this time increasing error will be more than overcome by the magnet losing it's magnetism which results in the speedo displaying a value slower than what you are actually going

the clock spring is very delicate, if you massaged it too much cleaning it then it might be no good already, but can't hurt to fix it all up and test it

the magnet can be re-magnetized so that the speedometer is true to correct reading again assuming all the other bits are clean and lubricated just where they need to be
do not get oil on the clock spring
spinning the input of the speedometer in the direction of needle deflection at 1000 rpm should result in a reading of 60 mph

there are other posts in this forum about speedos and how to set up the magnets and make it work again, i have fixed a couple of them to date, they are delicate but not impossible to fix
i bet i could even fix the broken magnet one if you have most of the pieces, the magnetic field would not care if the bits were crazy glued back together then re-magnetized in unison.
best of luck
-s

2ManyTrucks - thanks that's great information; I searched but clearly didn't search enough. Just from what you say there, sounds like the issue is likely they need re-magnetizing? I'll have another search here and read up more on this.

Well, neither worked before I tore into 'em so if this doesn't work then I'm no farther behind . If it quits pouring rain I'll try it out. I have a new unit on the way regardless but it takes weeks to get anything up here.


I do have the broken piece, I'll try and epoxy it back together. Nice to have some spares, if they can be salvaged.

Edit: Rain stopped, I test drove it - and it's no change, even with the swapped drum. 2ManyTrucks I found one of your other posts now on how to re-magnetize, wish I'd seen that earlier before I got carried away. I'm sure I've tweaked both those springs now. Another lesson learned the hard way

try looking for this thread:
Forums The Shop Area Interiors '51 speedo needle removal

it's like 4 pages of posts with many photos, should help you out
-s

Much obliged sir, I've pulled it out again but this time, reading first... probably wishful thinking but might as well give'r an honest and better informed try. I have some rare earth magnets here too as far as the magnetizing goes.

Certainly makes all the symptoms I've seen now make sense - I was looking at the design all wrong.

That’s okay
Smart people learn from their own mistakes
Wise people learn from observing other peoples mistakes
If you fall into either category you are beating the average by far
-s

2ManyTrucks; I read some good info, refreshed my memory on how to remagnetize (had to dig out my compass from the old forestry days), figured out which poles were which on the speedometer & on the rare earth magnet I used - and SUCCESS. I couldn't even believe it.

Apparently I hadn't flexed the hairspring in the one I'm running - purely LUCK nothing more, I better buy a lottery ticket. Just took it out for a 15 mile run with the GPS and it's absolutely perfect. I will keep the new speedometer as a backup when it gets here as a just in case, but I'll run this one happily.

Thanks!!!

NC3800