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Strange gearbox on the speedometercable

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by ankarback, Mar 18, 2003.

  1. ankarback

    ankarback 1/2 ton status

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    [​IMG]

    Hi

    Does anybody know wath this thing does. It´s located at the tranfercase and the speedometercable is attached to it. When I bought my car the speedometercable was broke and when I replaced it it still didn´t work untill I removed this little gearbox. I asked around but nobody knows where to get a new one. Without it the speedometer works but way too slow.
    Hope someone can help me with this problem.
    /forums/images/graemlins/1zhelp.gif

    Thanks in advance

    /Johan
     
  2. TX Mudder

    TX Mudder 1/2 ton status

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    It is a speedometer gear add-on box - it uses the gears inside to correct for bigger tiers and/or deeper gears. You can change just the little gear inside the tcase, but sometimes your nonstock tires or gears are too far out of range and you simply cannot buy a different gear for inside. So, these add extra gear reduction (or speed it up, whatever your need) so that hte speedo will work correctly.

    Chances are that the speedo box is not right for you now anyway - who knows what tires and gears it was meant for.
    Sounds like you need a new one.
    GOod luck on figuring out the math. My best advise is to find a speedoemeter shop (yes I have seen just a speedoemeter shop before - there's one in Houston that I know of) and they can do the math for you and order you the correct one.
    Good luck.
    -- Mike
     
  3. HarryH3

    HarryH3 1 ton status Author

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    It's called a ratio adapter. There are interchangeable gears inside the main unit that provide a change in the speed of the cable. There are MANY different gear sets inside the adapter to allow to the adjust the speed over a very wide range. Here in the US there are shops that specialize in speedometer calibration and repair. If you can find such a shop in your country they should be able to help you. Here's a link to an online speedometer shop in the US: http://www.speedometerservice.com
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    *IF* you haven't changed tire size, gears, and so on, you can probably get the ratio off of that adapter and probably order it from GM or something. All the adapters I have are slightly different than that setup, however, once I cleaned them off, there was a ratio like ".78:1" stamped into it.

    These were also used on cars for some time, but I've only seen them on the transmissions after they were pulled, never while in the donor vehicles.
     
  5. ankarback

    ankarback 1/2 ton status

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    Thanks for the help guys!!

    I suspected it had something to do with the car having larger tires. When I bought it it had 35´s.
    The problem is that with stock size tires the speedometer should work correctly but it doesn´t. Now I have the speedometercable connected directly to the tranfercase without the box and the car has stock 235/75-15 tires on it. The speedometer still shows about half the real speed but I think it should be correct now. Perhaps it´s some other problem with it too.
    I´ve bought 33" tires but with them the speedometer probably wouldn´t show anything at all.

    Any suggestions?

    /Johan
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    I wouldn't assume that with stock size tires the speedometer will read correctly. These were *stock* units, and the application section/variety listed in the '85/86 parts manual is enormous.
     

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