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Series 1 speedo error

Discussion in '"Stock" Alpine' started by beamdream, Jul 26, 2020.

  1. Tom H

    Tom H Platinum Level Sponsor

    Mike, beamdream, Just FYI, changing the gearing inside the speedo will only change the Odometer readings. There is no gearing involved in the actual speedo MPH indicator, just magnets and springs acting on the the needle movement.
  2. hartmandm

    hartmandm Moderator Platinum Level Sponsor


    Good to know. With speedos varying on the revs per mile, e.g. from 980 to 1504, do they just use different magnets and springs to deal with the different cable spin rate?

  3. Mike O'D

    Mike O'D Gold Level Sponsor

    I wasn't suggesting that a weak return spring was the problem, just stating that even if you have the correct speedo for your set-up it still may not be accurate anymore. If both speed and odometer are reading the same percent error, then the spring seems fine. The 2 that I checked had speed reading higher than the odometer.
  4. Tom H

    Tom H Platinum Level Sponsor

    Mike, I'm not sure. But the coupling between the spinning cable is simply a magnetic coupling. I assume they magnetize the spinning "rotor" to a specific strength. That spinning rotor sits inside a shallow aluminum cup with a pointer on the end of its shaft. And there are springs pulling the cup and needle back toward zero. I think the springs position is adjusted to achieve calibration, but it seems to me that the springs lose some strength over time, which can cause the needle to read a bit high. I assume the speedo can be calibrated by moving the spring position, but I have no idea how much. On my own speedo, after I installed an OD and changed rear axle ratio, my speedo was reading low by about 30% . I swapped some gears on an extra speedo I got from Jeff Howarth to get the odometer correct, and then I glued one or two tiny rare-earth magnets to the "rotor" to get the coupling increased and the speedo correct. First I did some road tests to establish how far off the speed was, say 27%, then I used an electric drill to spin the rotor. I experimented with different locations for the magnet, using just the magnet's holding power. Once I found the magnet position that came pretty close to increasing the reading, using the drill, by about 27%, I used super glue to permanently anchor the magnet in place. Amazingly, the speedo reads about right on the road. The odometer reads low by 2.5 % and I use that factor to correct my trip mileage and MPG calculations.
  5. beamdream

    beamdream Gold Level Sponsor

    Thanks Tom, I was actually referring to the speedo drive pinion at the gearbox, sorry I
    Thanks Jim

    I got the od unit with other Alpine parts some years back, my memory (sadly lacking) is that the owner had fitted it to his Alpine at some point having sourced it from a Sprite or Triumph owner. There are ways of identifying it off the data plate I believe, but in situ it's difficult to read, nevertheless I will try and identify it and agree it could be useful info for others.

    I do recall that there was a similar od fitted to Volvos which Healey or Triumph owners frequently use(d) and readily available to you guyss
  6. beamdream

    beamdream Gold Level Sponsor

    Hmmm I'm having finger trouble here.

    Thanks to all for your inputs.

    In summary
    . the od unit I'm pretty sure is not from an Alpine

    . as Michael says the (d) series unit is fairly common amongst Brit cars of our era and with the correct adaptor plate will go behind an Alpine from a different make, pretty sure that a similar unit was fitted to Volvo (s) in the 80s

    . given that other makes had different trans ratios then od units were supplied with compatible ratios and therefore speedo drive pinions.

    .my problem, I think, will only be fixed with the correct ratio speedo drive pinion (at the od end) I'm probably going to now catch up with a UK based Laycock parts guru and see what advice he has to offer.

    As previously mentioned there's ways to bodge the speed reading with spring and/or magnet fiddling but this won't fix the odometer reading which is gear related.

    Phew ! have I covered it all? Welcome further inputs or info.
  7. beamdream

    beamdream Gold Level Sponsor

    Just a follow up re the origins of od unit, photos show its a 25% reduction unit from a Humber Sceptre 1971/72.

    So having removed the od from that gearbox I fitted to a non 1st syncro box which happened to have the correct main shaft for od, and, subsequently fitted to my S1.

    Knowing I have a 4.2 diff I now need to measure the tpm output and see if I can find a speedo to match or or a suitable pinion output gear from the od thats compatible with my speedo

    Many years ago, in an early version of the Alpine guide, I seem to remember that there were references for speedo drive pinions from various other Rootes transmissions, can anyone shed light on that ?

    Attached Files:

  8. beamdream

    beamdream Gold Level Sponsor

    So an update on my quest to get the Speedo reading right, took the speedo apart and guess what it's been modified, see the first photo with details scribed on the rear of the dial face, the housing also has 1683 scribed on the back so assuming that's the actual tpm rather than the original 980 tpm as stated on the dial face, which would be correct for S1 with od.

    In the second photo is the gearing you would expect to find in 980 tpm speedo, vastly different to that taken out of the car.

    Now here's where it gets messy, having done the measured 100ft exercise to get the actual cable tpm I find that it's actually 2138 tpm so goodness only knows what the od output pinion ratio is. In researching most of the Rootes products and the various speedo application's, the highest tpm number I've come across is 1580.

    I might cast my search for this type of speedo a little wider and look at MG,s, Triumphs and the like.

    20200817_160841.jpg 20200817_161153.jpg 20200817_160841.jpg 20200817_161153.jpg
  9. husky drvr

    husky drvr Gold Level Sponsor


    Please recheck your TPM number. If your speedo needs 980, or even 1683, TPM's and the OD is supplying 2138 TPM's, the speedo should read HIGHER than actual MPH. 2138/1683 = 1.27 or 27% too high.

    I'm guessing you have a tire diameter near 24 inches (sorry, I'm not metric). That would give about 840 rotations per mile.

    840 * 4.22 = 3545 driveshaft rotations per mile

    100 / 5280 = 0.0189

    3545 * 0.0189 = 67 driveshaft rotations in 100 ft.

    To give 2138 TPM's, your cable needs to rotate 40.41 times in 100 feet of travel.

    For 1638 TPM's, your cable should rotate 30.96 times in 100 feet of travel.

    For 980 TPM's, your cable should rotate 18.52 times in 100 feet of travel.

    All numbers subject to change depending on real world conditions.
  10. beamdream

    beamdream Gold Level Sponsor

    Thanks Don, yes the calculated 2183 tpm seemed such a bizarre number, it bothered me so much that I did as you said and went over the whole measuring process again.

    And so I have a more meaningful result with a calculated 1100 tpm, this means that I will probably find a speedo with a compromise reading (say a 1050 tpm) which will put me in the ball park.

    And just for interest, the tyres fitted are 175/70R13 82H , thanks again for your input.

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