Before you mess with the sending unit double check to see if the circuit from the sending unit to the gauge is good. There are two wires on the top of the tank where the sending unit is located. Shorting (grounding) one wire should cause the reserve light to go on, grounding the other should pin your fuel gauge (I believe on full). If that is not happening then start checking the circuit for problems. If you do get the reserve light and gauge to work by grounding then it is probably the sending unit that's bad.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold