Speedo & Speedo Gear

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Boink - 25 Jun 2003 01:29 GMT

Getting around to something I've been putting off......... I'm trying to get
my speedo working again on my 1992 reshelled Mini Cooper (with a 1293cc
<1300> engine and a MarkIV tranny). It was working before someone fooled
with it just before I got it (to try to get the speedo more accurate). This
person changed the cable and the gear... but it's not working now.

I've got a RED gear (currently in the tranny) and a GREEN gear as a spare of
sorts. I have the original cable which has a more traditional square-shaped
center wire/cable and I have the new one (in the car) that is more of a
bladed thing. As per the original owner, I've now tried both gears with this
new cable (that's supposed to work). Before I go to the trouble to replace
the damn gear again (which is a pain to remove) and put the old cable back
in, I'm wondering if there is something else to try.

I HATE Mini speedometer cable work (even though this newer one has a
relatively easy access at the speedo end from under the dash)! Thoughts or
sympathies kindly accepted.

Oh, and while I'm at it, any thoughts on producing a modestly accurate
speedo (allowing for being off from tires, etc.)? I'm not sure that changing
this gear will matter if something isn't modified in the speedo itself.

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Michael Heinzelmann - 25 Jun 2003 13:51 GMT

Hi,

What's your problem now?

> with it just before I got it (to try to get the speedo more accurate). This
> person changed the cable and the gear... but it's not working now.

The speedo isn't working at all...

> Oh, and while I'm at it, any thoughts on producing a modestly accurate
> speedo (allowing for being off from tires, etc.)? I'm not sure that changing
> this gear will matter if something isn't modified in the speedo itself.

...or the speedo is just reading wrong?

In the first case make sure that the speedo drive in the box is turning, if
yes then make sure that the speedo cable is turning. If all those is true
and the speedo is not working, then it's defective. You can dismantle and
try to repair it.

In the second case, you need to make sure that the speedo drive ratio
matches your tyres, diff ratio and speedo.

It's not only changing the "output" gear, the "input" gear is also avaible
in - AFAIR - 3 different types (5,6 and 7 teeth??)

First you need to known how many turns your speedo will get per mile. On the
centre speedo, there's a number written below the odometer. On side mounted
speedos, this is somewhere at the back of the speedo, AFAIK.

Then calculate how many turns the gearbox will make when your wheel doesn
enough turns for one mile. The difference between those numbers is the ratio
for the speedo drive.

Sounds complicated, but there's a little program avaible on the internet,
which calculates that for you. You can find it in the download section of
www.minidevils.de Search for "Gearboxmanager"

Basically you can't make any changes in the speedo itself for adjustment.
Usually this is done via speedo drive ratio or some gears within the speedo
cable.

BUT - if you change the distance between the magnetic rotor and the needle
in the speedo, it'll read different. I guess, there's only one chance to
find that out: compare the odometer to the speedo needle. If they show
different readings, i.e. after driving 10min with 80mph or so than it
*might* be possible that your speedo is wrong. But this would be the last
possibility that I would think of.

...oh yes, another point: It *might* also be that the needle is misaligned
on it's axis. Usually there's a mark somewhere on the speedo where the
needle should rest at "idle"

HTH
Michael

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Boink - 25 Jun 2003 16:12 GMT

> In the first case make sure that the speedo drive in the box is turning, if
> yes then make sure that the speedo cable is turning. If all those is true
> and the speedo is not working, then it's defective. You can dismantle and
> try to repair it.

No no... thes speed is fine (and I've verified - thought I said that).

> In the second case, you need to make sure that the speedo drive ratio
> matches your tyres, diff ratio and speedo.

Right. I'm currently, finally, determining that.

> It's not only changing the "output" gear, the "input" gear is also avaible
> in - AFAIR - 3 different types (5,6 and 7 teeth??)

Yes, I follow.

> First you need to known how many turns your speedo will get per mile. On the
> centre speedo, there's a number written below the odometer. On side mounted
> speedos, this is somewhere at the back of the speedo, AFAIK.

My gauge has 6057 A1 on it.

> Then calculate how many turns the gearbox will make when your wheel doesn
> enough turns for one mile. The difference between those numbers is the ratio
> for the speedo drive.
> Sounds complicated, but there's a little program avaible on the internet,
> which calculates that for you. You can find it in the download section of
> www.minidevils.de Search for "Gearboxmanager"

Thanks! I'm on it. :-) (actually, I'd found a couple pages to this
effect)

> Basically you can't make any changes in the speedo itself for adjustment.
> Usually this is done via speedo drive ratio or some gears within the speedo
[quoted text clipped - 5 lines]
> *might* be possible that your speedo is wrong. But this would be the last
> possibility that I would think of.

I understand that a few shops will do this. At this point, I'm going to put
the original cable back in and leave this "red" output gear in there. Then,
I may have a shop calibrate the speedo to that set up.

> ...oh yes, another point: It *might* also be that the needle is misaligned
> on it's axis. Usually there's a mark somewhere on the speedo where the
> needle should rest at "idle"

I *think* that's OK.

Thanks, Michael

- MARK

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Graham - 27 Jun 2003 01:23 GMT

> First you need to known how many turns your speedo will get per mile.
> On the centre speedo, there's a number written below the odometer.
> On side mounted speedos, this is somewhere at the back of the speedo,
> AFAIK.

To give you an idea of value of the number you're looking for, one
common speedo in Australia has a calibration of 1425 turns per mile. A
metric speedo for the same car would obviously require a calibration of
about 890 turns per kilometer. (1425/890 = 1.6km/1km).

Obviously if you find a number of say 3000, that's too big to be a
speedo calibration, likewise anything less than about 500 would have to
be too small.

> Basically you can't make any changes in the speedo itself for
> adjustment.

In fact adjusting the speed reading of the speedometer is relatively
straightforward (and cheap) for an instrument specialist, but the
odometer is geared and cannot be changed without putting different gears
in it.

> BUT - if you change the distance between the magnetic rotor and the
> needle in the speedo, it'll read different.

I think they also increase or decrease the magnetisation to alter the
speedo response.

> I guess, there's only one chance to find that out: compare the
> odometer to the speedo needle. If they show different readings, i.e.
> after driving 10min with 80mph or so than it *might* be possible
> that your speedo is wrong.

This provides a check that the speedo agrees with it's own odometer. If
it doesn't then you will never be able to get that speedo AND odometer
both to read correctly. Basically the idea is that if it takes 1425
turns to register a mile, then if you spin the cable at 1425 turns per
minute the speedo must read a mile a minute, or 60mph. If the speedo
reads 30MPH then any change you make (other than adjusting the speedo
mechanism as described above) which makes the speedo read 60mph will
also make the odometer read 2 miles for every true mile-on-the-ground.

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Boink - 27 Jun 2003 02:40 GMT

> In fact adjusting the speed reading of the speedometer is relatively
> straightforward (and cheap) for an instrument specialist, but the
> odometer is geared and cannot be changed without putting different gears
> in it.

I finallly figured that out (or understood it). I'm not now sure what
combination I need. I'm told I need a particular tranny gear (which I have)
and a cable that matches it nicely... but I can't seem to get it to work.

> > I guess, there's only one chance to find that out: compare the
> > odometer to the speedo needle. If they show different readings, i.e.
[quoted text clipped - 9 lines]
> mechanism as described above) which makes the speedo read 60mph will
> also make the odometer read 2 miles for every true mile-on-the-ground.

Thanks Graham. This was helpful in my understanding it better.
- MARK

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