Tach issues

[brighton911]

The coil on my 86 arced to the primary side and now the tach is pinned past the red line. Car still runs like a new one. Thinking it could be either the DME driver circuit or the tach, can anyone tell me if there is a check I can do to verify the tach signal from the DME is OK? I do not have an oscilloscope, only a voltmeter. Thanks

[JJ 911SC]

I think that to check the signal from the ECU, you need a O Scope.

You do know someone that got a scope... Start with an A and finish with a X (4 letters)

My bet is on a less then a $ diode in the gauge

[brighton911]

Thanks for the reminder JJ Can anyone tell me what kind of trace etc the DME tach output will produce? Also, if anyone knows if a multimeter can be used to verify the DME tach signal, that would be great.

[ischmitz]

Take the tach out and disconnect its signal line but keep power and GND connected. If it's still pegging you have a bad tach.

The DME puts out a square wave and you will need an o-scope to properly measure that

[brighton911]

Thanks Ischmitz, that's exactly what I needed to know. I'll be on that this weekend.