While troubleshooting the tachometer there was a lot of info out there about the system and it's components but nothing on testing the tach amp with something basic like a digital multimeter or any other realistic way to get an idea of it's condition beyond dissecting it and looking at it or buying some high end test equipment. This is not for the guys with oscilloscopes and the like. That equipment is not on the list of home auto mechanic tools. I could always get such data but it still wouldn't be much use to most of us.

After the gauge itself tested ok and after replacing an absolutely shredded crank pickup sensor my last component was the tach amp. There's always a chance that there's a break in a wire somewhere but the odds were on the tach amp. I bought a used working one and decided to still get the info I was after for future reference. For what it's worth these are resistance readings from every pin combination. I compared it to the bad tach amp and not one single reading matched this good one.

The +++ symbol is to indicate the fact that the readings on some of the pin combinations would steadily climb. It would be ideal to isolate every component inside the tach amp but here it's not possible. I suspect that the variable readings have to do with the interaction of other components on the same circuit.

This is off the car with a room temp tach amp and an auto-ranging DMM. I don't know min/max working ranges or which readings might indicate impending doom. I just know that this tach amp works and these are it's readings.

Pins ______ Ohms
2-3 _______ 40 KΩ +++
2-6 _______ 8 MΩ and dropped to 6 MΩ
2-7 _______ .2 Ω
2-8 _______ .2 Ω
2-9 _______ 3.625 MΩ
3-6 _______ 27.10 kΩ
3-7 _______ .850 MΩ +++
3-8 _______ 1 MΩ +++
3-9 _______ 3 MΩ jumped to 20 MΩ
6-7 _______ 1.25 MΩ +++
6-8 _______ 1.3 MΩ +++
6-9 _______ 0L(beyond meter's range)
7-8 _______ .2 Ω
7-9 _______ 14 MΩ +++
8-9 _______ 25 MΩ +++