To add to what Dave said at the rear of the tach there should be a yellow and green wire screwed to the rear of the tach. Unscrew annd disconnect those two wires. Use a good high end meter only as the yellow wire goes to the ECM and you should have a high impedance meter to protect the ECM. If you have a meter that measures frequency, dwell or DC volts low range will work. With the meter wired with the red probe to yellow and the black wire to the green. Now start the engine. the reading is not important but as you accelerate the engine the meter must respond. If it responds the signal is present and the meter is bad. If there is no signal take the black meter probe off the green wire and ground it directly to the bike ground and set the meter to ohms. There should be virtually no resistance if the ground is good.You need to check for a signal at the back of the instrument panel, yellow/green wire in the 20 pin white connector. If no signal there then the 13 pin black connector behind the right fairing pocket.
Did it completely quit working? Or will it start working 2 - 3 minutes after starting the engine?The tach just stopped working on my 2000 Goldwing SE