Hello I have a 1977 Yamaha DT400 and the tach stopped working after I replaced the bulbs in it. I didn't drop it or anything, just took the cable out and unmounted it to replace all the indicator bulbs that were burnt out. They were burnt out because the previous owner put a 12 volt battery on it. After I reinstalled it the rpm needle stays at zero, no movement at all while running. I checked the cable and I'm pretty sure it isn't broken. I ran the bike with the tach end of the cable out and was able to grab the spinning end with my fingers without it stopping. How could I have broke it by taking it off and putting it back on??

I wonder if the cable pulled up inside it's housing when you removed it from the tach, and disengaged from the lower drive point? Maybe sticking up too far inside the tach? (just guessing)

It's not easy to connect the cable on that bike, I had one...  it's very possible the inner cable simply didn't go into the gauge properly.  I've had that happen, 

Is there enough cable sticking out at the top.
Have you had the inner right out & put it in the wrong way.
I think they might have a limiting piece on the lower end of the cable.
Just wild guessing.

+1 to all previous replies.

I would check it by first completely removing the cable and tacho from the bike. Then fit the cable to the tacho - with the parts off the bike, you have lots of space to see that the cable has fitted correctly into the tacho. Finally, attach the engine end of the cable to an electric drill and spin the drill in reverse - the needle should move freely.

You can do all of the above with the parts in situ, but I find it easier with them removed.

Okay thanks for the advice, I will take the cable off and do some testing and adjusting.

With a drill attached to the engine side of the cable I made sure to push the inner part into the tach as much as possible and it still doesn't work! I don't know what else could be wrong besides something internal in the tach which I'm definitely not diving into. It baffles me that I broke it somehow.

Spinning in reverse?

Yeah with the drill spinning counter-clockwise

I assume that you're in the US? If yes, forum moderator DEET might be able to repair it for you.

There is a chap here in the UK who repairs speedos and tachos, but two way postage will kill that option.