Digital tachometer

Discussion in 'Mech Tech' started by grandmst, May 2, 2019.

  1. Hey all,

    Before I take I out and bin it I thought I would ask if anyone knows how to clibrate / fix my rpm guage (one on the left)

    It's only a cheap unit so thought I would give it a go, set to 4 cylinder mode wired as they normally are to the - of the coil, but it always reads far to high.

    I've added a inline 10k 1/2w resistor to the signal wire.

    But I've found conflicting info

    Is the resistor suppose to be inline with the signal or bridged between the power and signal wire.

    Cheers all. [​IMG]
     
    Last edited: May 2, 2019
  2. Lot of waffle in that link, but it implied the resistor should be used as a pull-up, so wired between the tach signal line and ignition-switched +12V
     
  3. PS you’re wiring the tach input to the -ve/“1” terminal of the coil, yes?
     
  4. mikedjames

    mikedjames Supporter

    It may need a capacitor to ground as well as the resistor at the coil end in the signal wire.
    Its the metres of wire connected to the coil that cause problems.

    I will think on this some more and make some suggestions tomorrow.
     
    grandmst likes this.
  5. Sure is dude.
     
  6. Cheers dude, I put the resistor up the top end, might do some testing with it closer to the coil.
     
  7. Best to have it closer to the gauge. I think the problem is there’s no internal pull up in the gauge, so the input will “float” when the breakers (or transistor in any electronic ignition unit) are open. A pull-up yanks it up to +12 when nothing’s happening.
     
    grandmst likes this.
  8. Fair enough, so at least that's in the right place
     
    snotty likes this.
  9. I had bizarre readings on my VDO tachometer so I fitted it with an inline diode (1N4007) which cured my problem
     
    F_Pantos and grandmst like this.
  10. Diode in mine like @Valveandy
    The diode is if the gauge is getting a double signal because of the length of cable...don't know if it affects the digital versions...
     
    F_Pantos and grandmst like this.
  11. Cheers bud will give this a go any preference on were it gets located closer to cool or tach.
     
  12. MorkC68

    MorkC68 Administrator

    Same here with the diode, I seem to recall popping ours down towards the coil
     
    grandmst likes this.
  13. Does this imply you're seeing reflections on the long length of wire and the diode is preventing the reflected signals? Whatever the reason, the diode will need to be the right way around.
     
  14. PS I am not sure why you would need a 7 amp diode, I am just guessing that's what was in @Valveandy 's bits box.
     
  15. The VDOs don’t like negative-going voltage spikes below the 0V rail for some reason. Adding a series diode chops any spikes off, or at least clamps them. A 1N4007 is a 1A diode, just with a higher (400V?) rating.
     
  16. Sorry, yes, had to look that up, 1000v PRR, 700 VRev (rms), which is presumably where the 7 comes from.
     
    grandmst likes this.
  17. Just testing you ;)...
     
    F_Pantos likes this.
  18. Yes I used a 1N4007 because I have a drawer full conveniently in the workshop:)
     
    snotty and grandmst like this.
  19. And a drawer of 1N4148s ;)?
     
    Last edited: May 3, 2019
    Valveandy likes this.
  20. Yep:)
     
    snotty likes this.

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