Bench testing a points tachometer

Discussion in 'Sparky's corner' started by Dan Hach, Mar 4, 2021.

  1. Dan Hach

    Dan Hach Well-Known Member

    Hi everybody, I found a 62 era tachometer for cheap. I'm trying to figure out a way to bench test it. My engine is out of the car at the moment and I don't want to bother installing this thing if it doesn't work. I tired putting afunction generator on it, but I don't think it has enough juice to drive the tachometer. Anyone?
     
  2. rkammer

    rkammer Gold Level Contributor

    Do you have a buddy with a running car with a points distributor? Try it on his car. Otherwise you'll need a signal generator with enough power to drive it.
     
  3. 6455spd

    6455spd Silver Level contributor

    You can test it with a 60 hz battery charger. I believe if it is set for 8 cylinders it should read 3600 rpm.
     
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  5. Dan Hach

    Dan Hach Well-Known Member

    Thanks everyone, I have a function generator, but I'm thinking it doesn't have the horsepower to drive the signal. I know coil output isn't really square but I think the circuitry is set up to approximate a square wave input. I think the issue is that the function generator can't deliver enough amperage. Or the tach is dead.
     
  6. 12lives

    12lives Control the controllable, let the rest go

    Kurt - how would you hook up the battery charger?
     
  7. 6455spd

    6455spd Silver Level contributor

    I did it a few years ago and it worked in my custom made tachometer using an Autometer tachometer parts.

    I copied and pasted this info below from a z/28 forum-

    How to Check Your Tachometer

    I was doubting my tach and was wondering how to test it to see if it is actually accurate. On the alt.autos.rod-n-custom newsgroup from 2001 (used www.dejanews.com to search), I found a discussion that talked about using a 12 volt battery charger to check the tach. So what the heck, I tried it.

    The theory goes that a 120 Vac 60 Hz battery charger puts out an imperfect DC voltage. It is actually a pulsating DC voltage that corresponds to

    * 1800 rpm for an 8 cylinder,
    * 2400 for a 6 cylinder
    * 3600 for a 4 cylinder

    Well it works! When I hooked it up to my tach's input (+ve to tach +ve and -ve to gnd), it showed exactly 1800 rpm. The technical theory goes like this. The battery charger doesn't put out a DC signal, it is a 120 Hz full rectified waveform. When a battery charger is connected to a battery, the battery acts like a huge capacitor and smoothes out the waveform (simple explanation). I used a frequency counter to verify that the battery charger was putting out 120 Hz.

    Here's the math:

    120 Hz = 120 cycles/sec

    120 cycles/sec * 60 = 7200 cycles/min

    There are 8 cylinders, so 7200 / 8 = 900 cycles/min

    but it is a 4 stroke engine which only fires on 1 firing stroke (combustion)

    It fires once every 2 cycles

    So 900 cycles/min *( 4 strokes/1 firing stroke) * (1 firing stroke/2 cycles) = 1800 cycles/min or 1800 rpm
     
    69GS430/TKX and Max Damage like this.
  8. Dan Hach

    Dan Hach Well-Known Member

    Very interesting. So I went home for lunch today and decided to try this. It didn't work. No signs of life from the tach. I think the tach is probably dead. I may try to stick a servo in there with some sort of Arduino and call it good.
     
    Last edited: Mar 5, 2021
  9. 87GN_70GS

    87GN_70GS Well-Known Member

    More than likely it's the circuit board. I have some custom ones made for me that work with points or electronic ignition. Not real expensive either
     
  10. 12lives

    12lives Control the controllable, let the rest go

    "When I hooked it up to my tach's input (+ve to tach +ve and -ve to gnd), "

    So battery charger(bc) + to red or power, bc - to black or ground, and bc + to the trigger wire (green)?
     
  11. NZ GS 400

    NZ GS 400 Gold Level Contributor

    I would like to hear more about this if you don't mind. I am thinking I might need to do this.
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    NZ GS 400 likes this.
  13. 6455spd

    6455spd Silver Level contributor

    Sorry I should have mentioned that a battery is needed
    Here is the tach hook up to test rpm
    Tachometer red to battery positive
    Tach black to battery negative
    Charger black to battery negative
    Charger red to tach green
    Here's a picture for a v8 at 1800 rpm
     

    Attached Files:

    Last edited: Mar 7, 2021
    69GS430/TKX likes this.
  14. 12lives

    12lives Control the controllable, let the rest go

    Cool - thanks!
     
  15. charlierogers

    charlierogers GSX stage 1 4 speed #149

    very nice!!!!!!
     

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