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How to test a tach? 200

posted by  Erling on  Wed Oct 6 21:53 CST 2004
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last visit: Wed May 22 00:57 CST 2013
[RELATED]

I have recently bought a GLT wagon as a parts car, and the large tach is not working. Hoping it is just a poor or broken connection from the coil, I want to install it in the son's GL wagon.

Is there a way I can test it with a multimeter after I remove it from the GLT? Or is the simplest way to just install it in the GL and start the engine?

Thanks,
Erling.
--
My 240 Page








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    How to test a tach? 200

    posted by  Volvodad on  Thu Oct 7 07:53 CST 2004
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    last visit: Thu Oct 23 16:18 CST 2014
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    Like JonB says. I've tested some junkyard tachs this way (they all were good, btw). If you look at the connections on the tach you'll see what is ground/earth, what is 12V+ and of course the ignition signal input to the male spade terminal on the back. Connect this to coil terminal 1.

    If you have a known accurate tach in the test car, you can calibrate your "new" tach by adjusting the little potentiometer visible at the side of the tach's inner workings. I use an automotive DMM, and set mine to be accurate at ~2500RPM.
    --
    Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)








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      How to test a tach? 200

      posted by  M on  Thu Oct 7 09:07 CST 2004
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      last visit: Sat May 1 10:12 CST 2010
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      When calibrated this way, the tach has got to be (power) supplied from the instrument cluster - not from unregulated voltage. If you calibrate at 13-14 volts (common idle rpm alternator voltage), the tachometer won't read right when supplied by the internal, lower (usually 10 volts) cluster voltage.

      Martin
      --
      '65 121, '73 165








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        How to test a tach? 200

        posted by  Volvodad on  Thu Oct 7 13:15 CST 2004
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        last visit: Thu Oct 23 16:18 CST 2014
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        Not so.

        The tach is supplied with unregulated (except by the alternator) battery voltage from the system. The only instruments supplied from the little voltage regulation transistor on the instrument panel are the fuel and coolant temp gauges.
        --
        Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)








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          How to test a tach? 200

          posted by  M on  Thu Oct 7 18:40 CST 2004
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          last visit: Sat May 1 10:12 CST 2010
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          Realized my mistake after an hour or so...I've been fiddling for too long with newer gauges/clusters. You're absolutely right, the 240 tach is supplied by 'unregulated' (system) voltage and should be calibrated at that level.

          Martin
          --
          '65 121, '73 165








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    How to test a tach? 200

    posted by  someone claiming to be JonB. on  Thu Oct 7 04:00 CST 2004 [RELATED]

    You could also make some leads (pos, neg, signal) and test the tach 'under the hood'. This would save you from pulling the instrument cluster just to find the tach is shot.

    -JonB. 83 245DL with tach upgrade








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    How to test a tach? 200

    posted by  M on  Wed Oct 6 23:30 CST 2004
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    last visit: Sat May 1 10:12 CST 2010
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    The simple way is to test in a car, yes. Tachometers generally require pretty strong signals (pulses) and therefore are a bit tricky to test on the workbench with common pulse generators. You can't verify the function with a multimeter alone.

    Martin
    --
    '65 121, '73 165








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      How to test a tach? 200

      uid 18083's pic
      posted by  lucid on  Thu Oct 7 10:34 CST 2004
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      last visit: Fri Oct 24 12:31 CST 2014
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      I believe only the Fuel and Temperature gauges use the 10v regulated supply. Bentley WD shows tach power from ign sw or fused Key On source depending on year.
      --
      Bruce Young
      '93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.







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