I think I fried my tach - can I tell for sure?

[rallydiesel]

The "W" terminal on my old alternator grounded against the alternator housing and fried the alternator and starter. I have a new alternator and starter and everything is great but my tach doesn't work. It worked before the short but doesn't anymore.

Anybody know what sort of numbers I should be seeing from the "w" terminal and could I test the tach by giving it 12V? If the tach is toast is there anything that can be replaced to fix it or do I need a new cluster?

[Vincent Waldon]

Unfortunately we are talking about a frequency measuring system here so a voltmeter won't be of much help... you'd need an oscilloscope to see the waveform coming from the W terminal, and a frequency generator to test the tach.

If you have a multimeter with a frequency scale you might be able to detect the signal coming from the W terminal... never done it myself as I have a 'scope.  A voltmeter set to the AC setting might actually recognize the signal as well... never done it but it's worth trying now that I think of it.

Grounding the tach input should not have damaged it (although I suppose it might have gotten a voltage spike that did) so I'd be inclined to triple-check the wiring if you're convinced your newer alternator is in fact working... here's where your voltmeter will help you... got a good 12.5V-ish at the alternator with the engine off, that jumps a volt or more with the engine running ?

Failing that, part substitution is another way to go.

[rallydiesel]

The alt is working as far as I can tell. Battery is 12.something with engine off and 13.something while running. I guess I will have to break out the old bentley and check the wiring diagrams.