Tony,
The first thing to do is get the -12 Operaters TM and read it. If you do not have all the TM's, you can get them all in the TM forum.
There is no switch to change the output of the gen set. That's the funtion of the TB-1. The TB-1 is the Voltage Rweconnection Board. Normally, the board is set in 3 phase position. Read the TM, check the TB1. It's fool proof. Then, in the operators TM it describes how to position the switches so you can read the proper voltages and amperage, on each phase.
There are test procedures in the TM's for testing gages. The first thing I would do is check the wire diagram to insure that I he wires are correctly hooked up. Also, that the little metal brackets is not loose, (that's what gives you gage ground) and that they are not over tight or bent, that sort of thing.
The fuel gage is most likely stuck, due to the fuel level sending unit being stuck all the way at the top, or defective, or the ground wire, (very important if the fuel tank is plastic) is loose or missing. Take it out, then hooks the plug back up. Then ground out the metal housing on something metal. Then raise and lower the arm. Have someone else look at the meter to see if it moves. There are test procedures in the TM's.
Notice anything yet? If you do not have lots of experance on this kind of equipment, the TM's are your best friend. Work one problem at a time. Read first all the info in both the -12 AND the -34 TM's. Don't just look at the table of contents, look also in the index at the rear of the TM's. There are often several places in the books with info you need. This is an easy set to work on, and the books are well writen. Take your time and if you need help, write us. Good luck!
