I pulled the nonfunctional tachometer out of my 1963 Avanti and here's what I found: 6V applied to the brass and steel terminals does nothing; there is no restoration force from the internal springs, so the pointer will stay wherever I put it. (I have tested the sending unit and it puts out RPM-dependent voltage, so the problem does not lie there.) Any thoughts other than buy a new tachometer?
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Have you opened up the Tach? . From the pictures I have seen of the inside it looks like there is a selenium diode and a resistor of around 38 ohms besides the coil that moves along the iron bar. I would be interested in how they are wired. From the pictures it looks like one side of the diode is connected to one of the terminals and the resistor is connected to the other. Placing both in series with the movable coil.
Ron -
Hi Ron,
I have not opened the tachometer and I believe the problem may be mechanical, rather than electrical. I recall reading an article about the ring that is attached to the pointer dragging on the circular magnet, and I see that mine is doing that. That could account for the lack of restoring force by the spring - the force must be miniscule, but then, so is the electromagnetic force moving the ring. There is a brass threaded screw of some type that appears to affix the magnet ring to the housing and if I could find some way to turn that - and knew which way to turn it - I might be able to recenter the circular magnet in the pointer ring. Any thoughts on that?
TomComment
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OK, I figured it out. The magnetic ring was displaced so it fouled the moving coil attached to the pointer. I was able to back off the nut that clamped the ring so that I could adjust its position. I was able to do that through the holes in the tachometer can where the lights attach. It wasn't easy. Now I have to get the tach - and the two lights - back into the dash.Comment
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