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factory tachs with HEI

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by Chris Demartini, Jun 29, 2001.

  1. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>It is possible, just thought I'd pass this on. I had my dash apart yesterday to see why my left dash lights were dim and when I looked at the tach I saw a small screw in the top of it. Plugged up the 12+ and the coil +, grounded the body of the tach and hooked up a timing light, then adjusted the screw until the tach matched the light. And yes, its acurate throughout the range. I have a picture of the screw if anyone wants it. Does anyone know how I can test the printed circut board for the guage cluster?</font color=blue>
     
  2. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Please tell me more about this....

    I have a '72 with HEI and I'm convinced that the tach reads wrong! With 3.73 gears and 35" tires the tach says i"m turning 3500 RPM at 65MPH. I find that VERY hard to believe.

    If you can, please send me the photo and any description about what you did. I'd be interested to try it on my own rig. My e-mail is gregblanchette@netscape.net

    Thanks,

    -Greg72
     
  3. ErikH

    ErikH 1/2 ton status

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  4. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>I took the dash out, made extension wires for the coil + and B + (so I can pull the dash foward enough to adjust it) then I hooked up a timing light to read what the RPM should be. You need a small phillips head screwdriver to adjust the screw. I found my tach was also reading too high of an RPM.
    Heres the picture, pen marks the screw and the two screws on the bottom are the coil + and B +</font color=blue>

    <A target="_blank" HREF=http://71blzer.tripod.com/tach_screw.jpe>http://71blzer.tripod.com/tach_screw.jpe</A>
     
  5. 70Dave

    70Dave Registered Member

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    What about a factory tach and a MSD box? Will it need to be calibrated? Previous owner never hooked it up, and I haven't tried yet.
     
  6. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>I dont know but I'll find out soon. I have an MSD box I never got around to installing.</font color=blue>
     
  7. Steve_Chin

    Steve_Chin 1/2 ton status

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    Most factory tachometers do not read correctly with electronic ignitions (whether they're Hy-Fires, MSDs, HEIs, or whatever) because the output signal from a points distributor has a slower slope to it than the output signal from an electronic ignition (electronic ignition output is closer to a square wave). This makes the early GM tachometers read erroneously high. A filter circuit can be made to correct the problem, and later factory tachometers include such a circuit (or the cars have the circuit bolted to the side of the coil).

    I suppose that a potentiometer adjustment might be able to correct the misreading, but I don't know whether it'd have enough range or make it accurate at all RPM of interest. ANother caveat of using the factory tachometer is that it only reads to 5K RPM, so if you're cammed to make power up to 5K, you'd likely need more tachometer anyhow.
     

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