914 Tach Pickup

As  the usual gurus seem to have abandoned/not answered you, I will give it my best shot;

Tachos usually respond to ignition pulses, either spark or signal (from sensor) to spark. 

Clearly your TCU pin 26 (no idea what that is) is not delivering the pulses your Dynon/tacho need to give you a reading.

Seems to me your instruments are not connecting with the system you wish to get a read out from.

Possible causes;

Wrong pin (TCU26) - double check the wiring diagram.

Broken wire/connection from TCU 26 - test for continuity by physically inspecting connections making sure they are secure and there is no corrosion. Then use a continuity tester (multimeter or light) as a broken wire may be hidden by undamaged insulation.

High resistance from TCU 26 -  using multimeter test of high/low resistance (part of above testing)

Poor Earth/Ground return - often overlooked and the cause of many a 12V system failure - make sure all related connections are secure &  free from corrosion. You can also use a continuity tester (multi meter or light) as above.

Have patience and work through the potential problems/solutions in small  progressive steps.

Good luck

See TACHO OUTPUT is:

Pin 26 = 0 Volt

Pin 13 = Output