Only Speedometer and Tachometer Consistently Read High?

Discussion in 'Maintenance' started by silver96bowtie, Dec 18, 2016.

  1. silver96bowtie

    silver96bowtie New Member

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    Hi everyone,

    I recently purchased a 96 K3500 (4wd, 7.4L, automatic, dually) with ~110,000 miles.

    It has needed a bit of work, but overall is in good shape mechanically.

    Details of my current issue:

    Instrument Cluster
    - My speedometer and tachometer always read high (e.g. reads ~90mph instead of 70mph/ reads ~4000rpm instead of 2800rpm.
    - The speedometer and tachometer needles do not bounce or circle back and forth.
    - The odometer gauge works and is accurate.
    - The other gauges appear to work just fine.

    Other Details
    - My readings for the vehicle speed and engine speed are correct through the OBDII connector and reflect how the engine sounds.
    - Cruise control works.
    - Installed a new input speed sensor on the transmission, no change.
    - Installed a new output VSS on the transfer case, no change.
    - Replaced bad ABS module
    - Replaced bad front right wheel speed sensor

    I'm tempted to purchase a 99/00 120mph Escalade cluster, but I'm not convinced the current stepper motors are bad considering that the Speedo/Tach are both consistently high. It's certainly possible, but I want to get everyones opinion before I go buying more parts.

    What is the likely hood that both stepper motors would read high in correlated fashion if they were bad? It would make more sense if a signal to each was incorrect. With that said I don't know what part would generate a bad signal only for the Speedo/Tach and not other parts.

    Note: The tires and gears are stock, which makes sense as the my vehicle speed/engine speed readings on the OBDII appear to be correct.

    I appreciate any thoughts or opinions you folks might have.

    - Mike


    Edit: Also, it appears there is no speed sensor buffer for my year gas vehicle.
     
    Last edited: Dec 18, 2016
  2. ron7000

    ron7000 GMFS Member

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    yeah it'll do that when you're speeding, don't push the gas pedal so far down...

    look up the transmission ratio and rear axle ratio for that year truck then do the math to figure out what engine rpm should be at 70 mph for the given transmission gear.

    and then use a gps on your phone, on a straight level stretch of highway see what 70mph on your speedometer matches to on the phone gps.

    the first thing that comes to mind is the rear axle ratio was changed and not accounted for in the engine computer. you say the rear gears are stock, how do you know ?
    And you say the vehicle speed reported by the odb2 on a handheld appears to be correct? What does "appears to be" mean?

    other thing is you purchased a used vehicle, so in addition to verifying what rear gear is actually in the axle i would also somehow verify the transmission is actually the oem transmission. for a 1996 with 100k + miles would not surprise me if the transmission could have been swapped out.
     
    Last edited: Dec 23, 2016
  3. GMCFanBoi

    GMCFanBoi New Member

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    That happened to me when i changed wheels
     
  4. Graystone

    Graystone Mmm Sandwich

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    Your tach reads higher because you changed your wheels? I highly doubt this.
     
  5. Phildirt

    Phildirt n. fil-dert.

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    But how do you know ?
     
  6. silver96bowtie

    silver96bowtie New Member

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    Thanks for the replies. I knew the tach and speedometer were off because I could see my true speed and rpm readings using the torque app.

    Fixed:

    I ordered a used Escalade cluster from eBay and swapped in my odometer and some led lights.

    The tach and speedometer are both accurate now. The escalade cluster has a failing stepper motor for the fuel gauge so I will swap parts this coming weekend.

    Also, I apologize for disappearing on the thread that I created. I didn't realize anyone responded. Working a startup firearms manufacturing business and replacing a heater core (had to remove the dash and drop the steering) on my car has tied me up a bit.

    Thanks again for the replies.
     

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