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Thread: Tach and Speedo off

  1. #1
    Join Date
    Mar 2012

    Default Tach and Speedo off

    Can anyone tell me why my tach is off 500 to 600 rpms depending on speed . The speedo is off 6 mph at 60. the faster you go the more they are off. Note: the overdrive is hooking up and my gas mileage has not changed. The tires are 265/75. The truck is a 1996 K2500 Suburban with a 2.73 rear. Engine is the F code diesel. At 65 mph the tach reads 2700 when it should read about 2250.

  2. #2
    Join Date
    Feb 2007
    Baja California Sur Mexico


    Second try to reply, Walt, the site has been mostly unavailable for weeks, and lost my post after I wrote it. Tach revs come from an alternator pulse, if the pulley has been changed to a different size, the tach will read wrong. The speedo gets it's signal from a module near the ECM. Yours is 10% wrong, the difference between a 3.73 diff and a 4.10. Someone may have changed the module, GOOGLE how to connect the right resistors in it, to correct the speed.
    Grant H.
    1995 6.5 turbo GMC K2500 extra cab 4x4 pickup, PMD under bumper, mechanical wastegate, 228,000 miles, rebuilt engine, 4.10 diff, 16 mpg
    1994 6.5 turbo GMC K2500 4x4 suburban "F" VIN #9 resistor, PMD in the bumper, mechanical waste gate, 226,000 miles, Positrac 3.73 diff, 17 mpg highway.
    1982 4x4 shortbed 6.2 Chev 260,000 miles untouched motor 17 MPG 4.27 diff. Best thing I ever owned, 21 years.

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