Tach just bounces between 0-2000 RPM, is it bad or is it wiring?

[cywelchjr]

I can't say for sure, but I have 3 alternators that all read closer to .8, and they all work great and charge just like the spec says, keeping the battery fully charged and all. That said, I've not checked the one on my spare engine yet, I probably ought to, but the one in the engine in the bike now, in the original engine and in my XS400 (pretty much the same charging system just lower capacity and has the same testing specs) all read out at about .8 ohms on the white wires. Everything else in the charging system reads to spec, just the stator wires (which is what the white wires are) read just that little bit over spec). If it weren't for the fact that it works, I would be worried, but they ARE over 30 years old you know.

[JeremyL]

Well I was thinking that a .2ohm difference may not be a big deal and may just be from 32 yo wiring.

I have the tank off my bike right now so I couldn't run the motor very long but when I checked the voltage with the bike off it was around 12v and when I started it I remember seeing a 13.xx voltage before my multimeter fell and hit the ground. I don't have the aligator clip leads on mine, just the pointed type so I was trying to rev and hold the leads to the battery at the same time.

I'm starting to think (and kinda hope) it's a problem with my tach itself or a connection issue inside of the tach. I cleaned some of the connectors but could clean them better so I'll try that again and maybe clean every wire and connector I can find just to be safe. None of my speedo/tach backlight bulbs work either so I don't know if they're just dead or don't work for a specific reason but I'll worry about that later. I think I'll pull the tach off the bike tonight and see if I can look around and notice anything obvious like a nasty connector or anything. Might eventually open it up if I run out of options and try resoldering some connections. I don't have anything to lose if I end up getting another tach on eBay anyways.

[JeremyL]

Well I decided to pull my tach and see if I could find any loose or broken connections. Ended up prying the ring off and taking it completely apart. The needle swings freely from one side to the other when I move it by hand but when I connected a 9v 500ma power supply (didn't have a 9v batt) it didn't move an inch. I did get some light arcing when I brushed the 9v wire across the terminals so I know power is there but it's just not working. I tried touching the solder joints directly and even tried touching the wires directly to the metal that the wires are soldered to. Nothing.

I didn't see any springs broken or anything like that inside... until I tried to take it even further apart and one of the springs came unwound. So I guess I killed that tach.

I'm gonna start looking at eBay for another one and just hope its my tach that is the problem and not something else.

Is there any way to test the wiring going to the tach to see if its outputting a signal?