You don't need a diagram. You do need a voltmeter to check voltages. Every gauge needs the foillowing:
"I" terminal: 12 volts when the key is ON. Comes from the "I" terminal on the ignition switch.
GND terminal: Must have a good connection to ground (negative terminal on the battery)
"L" terminal: This is the internal light. Comes from the "I" terminal on the ignition switch or the NAV/ANC switch. If from the ignition switch the light will be on whenever the key is ON. If from the NAV/ANC the light will be on only when the switch is in the NAV position.
"S" terminal: This is the "sender" terminal for fuel, water pressure, oil pressure, etc. Speedometer does not typically have a sending unit since most are pitot operated. Plastic tube from engine or separate pitot at stern.
Most gauge problems, especially if more than one is not working, is due to a bad ground or lack of +12 volts. Gauges with senders can be checked by MOMENTARILY jumping the "S" terminal to ground. If the gauge pegs, it's a sender wiring or sender problem. If it doesn't move, it is a power and/or ground problem.
