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I am working on a 85 Fiero GT which has had its tachometer go bad. I have managed find the signal wire coming from the Ignition coil producing AC voltage bottoming out a 7v while idling, increasing with the engine speed.

How would I go about finding an after market replacement that would work?

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  • Are you sure that's the signal wire? I don't know for sure, but voltage readings would be far to inaccurate to give a good signal to a tachometer ... at least to my thinking. Usually, when it's a "signal" it's an on/off kind of thing. Dunno for sure, though. May 31 '19 at 0:19
  • I could just lack understanding then. The diagrams I have found label the wire as the tach wire and the AC voltage over that wire changes with throttle position. May 31 '19 at 0:39
  • Fair enough ... sure as heck could be. I don't know why anything in the system would be throwing back an AC voltage ... the only place I'm aware of which has AC voltage in a vehicle is in the alternator, prior to the juice being rectified to DC ... I could be way off base. I really don't have any idea how to find what you're looking for ... hopefully someone can chime in. The ONLY thing I could even suggest would be to find a rebuilt or used tach as replacement. May 31 '19 at 0:43
  • Again It could be my lack of understanding, I am saying AC as that is the setting I have my multi meter on. It could be DC that is changing too fast to be measured by a multimeter. I do know that the line is coming from the ignition coil and through a tach filter if that give any more context. May 31 '19 at 0:56
  • If you know how the tach works then you will know if that signal is relevant...
    – Solar Mike
    Jun 1 '19 at 6:54

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