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]