Rallye cluster gauge issues - 67 Barracuda

Mattax said

How does the tach visibly get power but not respond to the signal?

Also was the aftermarket tach working directly off the coil negative or some other arrangement?

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I can tell the tach is getting power because when it is unpowered, the needle rests below zero. When it powers up, the needle moves to zero. It just no longer reads above zero when the engine revs, so I am assuming it is defective. And yes, it is being fed the tach signal from coil negative. I need to verify with another device that the signal is still reaching the tach before I pull the tach and try to return it.

Incidentally, I drove the car to work today (~25 miles) and (knock wood) all gauge operation (other than the tach) appears to have returned to normal. The fuel needle is stable again. It is really starting to look like the failing/failed tach was interfering with the other gauges somehow. When I install a replacement tach, I will keep the power feed off that blue/white wire just in case.