Hello, thought I'd share this in case someone else is having a similar problem and wasn't sure about opening up their tach, or what they would find inside. The mechanical tach on my '82 Suzuki GS550 just recently started working erratically.
It was slow to respond to changes in engine speed, then one day it just froze on the 2k line.
First thing I checked was the cable, but with the engine running, the inner part of the cable was spinning just fine. Took it out anyway to check for frays, and to lube it up. All okay there.
Then I turned to the tach itself. Removed two cap nuts holding on the back cover, pulled out the light bulb holders, and the whole unit comes off the bike. I'd read elsewhere on here that they are hard to open up for servicing. Sure enough, the front trim ring is crimped over the main case of the tach. However, in my case, the previous owner seems to have already had mine apart, judging by all the dig marks around the back side of the trim...
Anyway, I took my time working a small screwdriver around the ring until it was loose enough to pull off. Two small phillips head screws next to the cable connection were all that was left holding the guts into the case.
With the mechanism out, this is what we have inside.
It's actually a pretty simple design. The cable from the engine spins a circular magnet. The magnet is at an angle to, and partially inside of a bell shaped part that is connected to the indicator needle. As the magnet spins faster it's magnetic field causes the bell to move further in a circular motion. There is a clock spring behind the needle that keeps the bell from spinning all the way around continuously, and to return the needle to the lower numbers as the engine speed decreases.
On my tach the whole bell assembly seemed to be stuck. I squirted some alcohol onto the bearing in the photo, and inside the bell. I'm not sure, but the might be a bearing inside the bell. But anyway, after it soaked for a little while I was able to start working the bell and shaft back and forth, and in and out a little until there was no resistance on it other than the spring tension. Even after that it still felt a little gritty a few times, so I flushed it out several more times until I was sure everything was freed up. Then I blew it all out with a little blast of air and hit all the bearings with some light oil.
I left it apart all night on my bench just to see if it froze up again. In the morning I stuck a screwdriver tip into the cable drive hole and spun it between my fingers. I could see right away that the magnet's motion was making the needle jump every time I tried it, and the spring returned it back to the zero peg each time. Just to be sure, I placed a hex drive bit into my drill and used that as a test drive to see if the tach worked properly and held a steady rpm reading. All looked good, so I put the tach back together and reinstalled on the bike for a real test drive...
Got a little bit ahead of myself here. The trim ring had to be worked back onto the body of the unit. Once it was on all the way, I placed it face down on a cloth on my workbench, and used a flat blade screwdriver to crimp the edge of the ring back down the best I could. Mine was really dinged up by the P-O (bike was dropped), but once the outer black shell of the tach went back on it really wasn't that bad looking. Just have to be patient prying it off. Also, be careful not to break the lens. I think it is real glass and might break if pried on to harshly.
I'll update if anything goes bad again, but so far this is the best I've seen this tach work since I got this bike 8 months ago.