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old school tachometer check

Discussion in 'Peterbilt Forum' started by roamaround, Jun 25, 2015.

  1. Jun 25, 2015 #1
    roamaround

    roamaround Bobtail Member

    14
    3
    Jun 7, 2012
    las vegas nv
    0
    Does anyone here know how to verify a mechanical tach is reading accurate? I have a 1984 Pete 359 with a Cat 3406B. The truck has 370 rears with a 13 speed. My RPM is 1800 @ 65mph and 11R 24.5. The Eaton speed graph states with the transmission I have I should be going 73MPH @1800 RPM. Well I'm not. Short of replacing the Transmission for a verified Double over drive I want to check the tach first. I doubt it's wrong but need to know. This is a mechanical Tach with a cable.
     
    roamaround, Jun 25, 2015
    roamaround, Jun 25, 2015
    #1
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  3. Jun 25, 2015 #2
    uncleal13

    uncleal13 Road Train Member

    3,757
    6,199
    Apr 9, 2009
    Humboldt, Sk
    0
    Your speedometer probably has a row of micro switches on the back of it that have to be set to your driveline specs.
    They make a photo tachometer that you'd have to get ahold of. You put a chalk line on the main pulley on the crank, and this gizmo counts the rpm.
     
    uncleal13, Jun 25, 2015
    uncleal13, Jun 25, 2015
    #2
  4. Jun 25, 2015 #3
    beemergary

    beemergary Light Load Member

    229
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    Apr 11, 2014
    0
    I have the same truck only with 3.90 rears. Has the 14613 trans. Mine goes 68 mph @ 1800. I ve did the math-rear end-tire circumference-rpm-trans. gear .87. Also 60 mph=1 min (60 sec.) mile marker too mile marker. Mines correct and Eatons speed graph sounds right for 3.73 gears.
     
    beemergary, Jun 25, 2015
    beemergary, Jun 25, 2015
    #3
  5. Jun 25, 2015 #4
    beemergary

    beemergary Light Load Member

    229
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    Apr 11, 2014
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    Do the 60 mph in 60 sec. for 1 mile. If that's correct then check the tach. If I wasn't so tired tonight I'd do the math for ya. Gotta have the final gear of trans.
     
    beemergary, Jun 25, 2015
    beemergary, Jun 25, 2015
    #4
  7. Oct 17, 2019 #5
    washiac

    washiac Bobtail Member

    1
    0
    Jun 19, 2019
    0
    Where does the the tachometer cable plugs into?
     
    washiac, Oct 17, 2019
    washiac, Oct 17, 2019
    #5
  8. Oct 17, 2019 #6
    Snow Monster

    Snow Monster Medium Load Member

    543
    1,296
    Feb 9, 2019
    0
    Most accurate way is to find a mechanic with a digital tach.

    They put a piece of reflective tape on the harmonic balancer and measure the crank's rpm's with the digital, only takes a few minutes.
     
    Snow Monster, Oct 17, 2019
    Snow Monster, Oct 17, 2019
    #6
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