Cannon Plug, Tach Generator
You wont find us saying anything bad about the design of Yak airframes or their engines. Even so, the Cannon plug between the Tach Generator and the airframe was not the design Bureau's finest hour.
Three pins in the Tach Gen Male side of the Cannon plug slide into brass female sleeves in the female ( airframe harness).
As pictured, more often than not, long term engine vibration will sever at least one of the wires or engine dirt will get in, reducing signal reliability and giving you funny tach readings in the cockpit.
If you're taking yours apart to explore, take the time to color code or mark wires and their original positions in the female Cannon shell because it's likely that vibration has worn everything to the point of falling out, including the little tab in the Bakelite insulating guides which polarizes the connectors.
Mix these up and you will be playing permutations with the three pairs until you get it right.
The Tach Generator puts out a three phase signal which the Tach head decodes as something between zero and 100% engine rpm. So to test it, no ship power is required. With the Tach Generator removed from the engine, you can carefully turn its shaft using a hand drill to verify appropriate rpm readout.
Cannon plug replacement eliminates reliability questions about wear in the brass connector, wear in the insulators, and overall cleanliness.
It's the weak point of the chain, whereas the Generator and Gauge fail much less frequently.