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      Generally the tacho reads as I would expect at different speeds. On a couple of occasions recently the needle has swung round to the 6 o’clock position after running for sometime even when cruising at a constant speed.

      I believe the reading is taken from the alternator not a sensor. My questions are

      1 has anyone experienced similar and how did you fix it?
      2 does anyone know if it is the alternator or the tachometer?
      3 if there is a test I can perform to find out the problem what is it and would the fault only be detectable when the problem is being displayed?

      Happy to a tackle measurements and replacements etc but could do with some advice I anyone can help.

      The engine is a Volvo D-40 2006 vintage




        It sounds like the sense wire is not providing the signal to the tacho or providing the wrong signal ie its grounding.
        I cant remember exactly the circuit, so I may stand corrected; the tacho has a + and – feed with a third wire which comes from the alt. I seem to remember this is a white cable from the W terminal (see pic) I would check that you have 12v at the Tacho as a supply voltage. This will prove you have a good power supply.

        Track the cable from the alt sense, and disconnect it from tacho. With a volt meter check that you are getting the same reading at the alt and the at the end of the wire at the tacho. Again from memory its about 3 volts at tickover and a couple of volts more when the engine is reved up.
        Its AC from the windings so start on a high setting, say 50 volt scale and step down until you get a good scale (most digital volt meters will read still but with a .0 in front of the reading or indeed a .00)
        From this will prove you have a good signal from the alt and at the end of the wire and that the alt is sending a pulse voltage. If its different the wire is damaged or broken.
        If not its the alt.

        if it ok, QED the tacho is faulty.

        If you find its the tacho post again and I will run through another set of tests.

        Hopefully, this will be of use to you and others in the future 8)

        Best regards
        [attachment=0:25hx3d4f]Alt Volvo.JPG[/attachment:25hx3d4f]


          Thanks Ant, very comprehensive response and greatly appreciated.

          One for Saturday morning I think and if any queries I’ll post a response




            No problem, written in a red wine haze so forgive the typos 😳

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