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Factory Tach Calibration Issues

Discussion in 'The Garage' started by mosesburb, Oct 16, 2003.

  1. mosesburb

    mosesburb For Rent Premium Member GMOTM Winner

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    I have a factory tach for a 76ish truck that reads higher than what the engine is actually spinning. At about 850 rpm, the tach reads about 2000 rpm. Took it to the local speedo/tach service facility and they said there is nothing that can be done with it. A person needs a dumptruck full of money to afford a new one, so I was wondering if anyone had ever calibrated/fixed one before??
     
  2. kennyw

    kennyw N9PHW Premium Member

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    There was a factory tach in the for sale forum on this site for $40 or something like that. jjlaughter's post I believe.
     
  3. kennyw

    kennyw N9PHW Premium Member

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    Oh, make sure you have a good ground and connector on the HEI as well before you do anything else. /forums/images/graemlins/thumb.gif
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    I have one for my truck, and my dad has one in his truck. (both the 80's style)

    Our tachs aren't as far off as yours, but I can't come up with the reason his and mine are both off. It's not a simple difference in actual vs. advertised tire size, or incorrect assumption of gear ratio, the tachs are simply incorrect, which is kind of odd IMO.

    Does your tach "zero out" when you turn the key to "run", before you actually start it?
     
  5. Don

    Don 1/2 ton status

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    Hate to be a smart azz, but take it out, and bend the needle, so it correspond's with the real rpm. /forums/images/graemlins/eek.gif /forums/images/graemlins/rotfl.gif /forums/images/graemlins/grin.gif

    Seriously, take it out, remove the needle off the shaft, and rotate counterclockwise, and put it back on. /forums/images/graemlins/grin.gif
     
  6. ericguy320

    ericguy320 Registered Member

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    like kennyv said make sure you have a good ground thats what mine was dooing and i found a corroded wire
     
  7. mosesburb

    mosesburb For Rent Premium Member GMOTM Winner

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    Yes, all the wires are good. When it was installed I had made a new harness for the engine compartment and under the dash because of previous owners butchery. No problems with grounds. I even hooked up a raggedy old Sun Tach to it and it read right around the same as the Snap-On timing light/tach.
     
  8. Redfrog

    Redfrog 1/2 ton status

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    I maybe be thinking of something else but.. In the factory harness isn't there a resistor or something in-line with the tach wire??
     

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