If there is a sending unit, then this is the electric gauge.

I had this problem on a 280SL. The way it seems to work is that 12v is supplied to one side of the gauge and the sending unit varies the resistance between the gauge and ground. The gauge is basically a voltmeter. Higher voltage, higher reading. If the wire to the sender is touching ground at any point, though, you get full voltage and the gauge pegs.

If you have a meter, hook it up to a 12V source and ground it at the sending unit and see what voltage you are getting with Key On. If you read 12v, you probably have a wiring problem as described above or a bad sender. If you see 0v, start the engine and watch what happens to the voltage. It should start out high and drop as the engine warms up. If it stays high, it is probably the sending unit. If it drops, but the gauge stays up, then you either have the wrong sender or the gauge istself is bad.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe