zKars

Need a tooth count. Vehicle Speed sensor calibration underway!

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    Crazy question. In a 4 or 5 speed trans, there is a plastic gear on the output shaft that drives the speedo cog. I need to know the number of teeth on that shaft gear.  I've had several trannies apart in the past and recycled more of these shafts that I can remember, but never kept one!

    Why you ask? I have a electronic vehicle speed sensor on my 5 speed now, attached to the speedo cog output housing, and need to compute it's output frequency vs wheel speed. To get a ballpark number, I need the tooth count on the shaft gear to get the speed vs RPM data to enter into my Haltech tuning software. (I get my MPH from a GPS speedometer)

    If someone just happens to have a tranny apart and can count, it will save me some time at the dyno.

    Just to record this, I'm using a very special little dohicky made by a Toyota 4x4 offroad support company that came up with a lovely little low cost custom VSS that plugs directly onto the speedo cog housing of many Toyota's and quite conveniently many early Nissans's as well. Outputs the most perfect little 5v square wave. In case there are any others of you that are crazy enough to go DIY EFI in the future. Picture below. 

    IMG_3901.PNG

    Edited by zKars
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    Side note, is it not interesting, how, given the number of speedo cog tooth counts there are out there for all the various diff ratio's, (yellow, blue, white, etc), that they all mesh properly with a single toothed gear on the transmission output shaft? Crazy how they made that work....

    Or are all the different tooth count colored cogs different diameters as well, so the tooth spacing stays the same? The mysteries of the universe never end. 

     

    Edited by zKars

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    I do not have an answer for the teeth count, but I think you are correct as too all of the different tooth count colored cogs being different diameters as well .
    I am sure they compensated for the different diameters by off-setting the housing to bring the gear back in full contact with the main gear.
    DSC_0340_1khs_ol1b.jpg

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    Jim: Pulled the speedo gear out of a spare 4 speed A tranny to see if I could see anything. It is a worm gear with 6 "threads" from the start to finish.
    speedo gear.jpg

    Edited by 240dkw
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    Oh that's amazing! How did I not think of that? (One track mind, that's how...) Thanks for that!

    Makes it a bit trickier to compute accurately with a worm. I don't think it matters how may turns of the screw the gear is made from, its got to do with the spacing or tooth angle or something too hard to compute or measure. 

    Guess I'll have to do the work and turn a tranny and count tail shaft vs speedo rotations. Not like I don't have a few trannies laying around... Now if I can just remember what color gear I put in my tranny.... white I think. 

     

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    Ok, things make more sense now. I pulled my little collection of speedo pinions, and found out what's what.

    The pinions are all the same diameter. What does vary, is the tooth count (we know that) and the ANGLE of the teeth across the gear. Fewer teeth, lower angle (compared to rotation axis). Pictures below show what I'm talking about.

    And, sigh, I found one of the plastic output shaft gears in my stash... 

    Yellow 3.9, blue 3.7, white 3.54.

    And finally the answer to initial question, using the white 19 tooth 3.90 gear, 20 output shaft turns = 6.25 pinion turns. Now I have my VSS conversion. 

    5D4E5F85-4009-4B0E-AE9E-493FEE25CCAD.JPG

    4F6B7B97-30D2-4390-8A48-09D5F974AEEF.JPG

    3289F183-80AE-48B7-8D86-078F74CFDCF7.JPG

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    Crazy, I look at the photo that I posted from ZParts and see different sizes. I would have never guessed that they are the same size.

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