You have a dead short to ground. Here's how to find it. Using a test light, leave the fuse out, and connect the two leads of the test light to the fuse contacts. With the key on, the test light will light. Now unplug the power wire to the tach if you can find it. If the light goes out, then the short is on the tach side. If the light stays on, the the lighter side is the short. Unplug the power feed on the back of the lighter socket, light out means the socket is grounded inside, light still on means the power wire is shorted. Or if you have a multimeter set it to ohms(disconnect both battery cables, key off), with the com lead to chassis ground, touch each fuse contact with the other lead. One should read OL or no reading, other will show very low ohms, probably 5 or less. That is the shorted side, follow the same as above but look at reading on the meter, when it shows OL or no reading it is the same as the light going out. If the light stays on or the meter still shows ohms with both the tach and lighter unpluged, then check the power wire from the fuse box to where you unpluged the tach and to the lighter. Enjoy!
"F'n" stuff up since 1990!
Some are engineers or fabricators for a living, I do it as a hobby.
Last edited by 69Goatee; 09-28-2011 at 11:06 PM.