72 C-10 fuel and tach guage questions

[rjfoy]

My gas guage either doesn't work at all or reads 3/4 regardless of how much fuel is in the tank. There's only one wire to the sending unit - straight from the gas guage block on the fuse box. I replaced that, but it didn't change the problem.

The tach works but reads about 1000-2000 higher than the engine is running.

???? Thanks.

[old dawg]

You need to make sure you have a good ground from the sending unit plate (at the gas tank) directly to the frame. If that doesn't fix it, then you should be able to disconnect the wire to the gauge, and then ground it, while watching the needle. It should show full and then empty (or vice-versa). If it does, the problem is likely your sending unit (in the tank). If not, then the dash unit is probably bad.
Dunno about the tach. If it has settings for the number of cylinders, make sure it's set to match your engine.

[rjfoy]

Thanks Dawg, the ground was the problem. I took the sending unit out just to make sure the mechanical part was working (it was), cleaned around the opening , put it back together and it works.