Tach stopped working?

Here are some trouble shooting tips:
  The red wire is battery voltage (~13+ volts), the black wire is ground, and yellow is the tach signal which derives either directly from the computer or one of the coils. The best tool for checking the yellow wire is an oscilloscope, second best would be a voltmeter with a built in bar graph. The bar should jump back on forth. A conventional voltmeter will also work. No volts with the key off. At idle it may read as low as 500 millivolts and creep up as you raise the rpms. The voltage isn't actually rising but the voltmeter will integrate the ratio of on to off time as the pulses get closer together. Exactly what the tach itself is supposed to do. There is no way to test if the meter coil is broken without opening the case as the coil is driven by an integrated circuit and not directly connected to the yellow terminal.

Pete Serrino