My 914 tach is acting weird. When the ignition is turned on the tach instantly reads 3000 rpm. When the engine is running it still reads 3000 rpm regardless of the engine speed until it is above 3000 rpm and then it appears to be reading accurately at 3000 rpm and above.

I pulled the tach out and disconnected the black/purple wire and the tach still reads 3000 rpm with engine not running. If I disconnect the red wire it goes to zero. I checked the tach ground and it is good.

I then connected another tach and it does exactly the same thing.

Any ideas what is going on?

Black purple on the coil get swapped ?

I think your tach has Bluetooth capabilities and your exhaust system is super quiet.
3000rpm is 100hz, I can't see anything generating this signal in your car other than the black/purple wire. Sounds like you have some bad tachs.