A friend of mine has a 90 GL with a non-functioning temperature gauge.  Is it likely that replacement of the gauge will resolve the issue?  Can the gauge from an 83 1/2 to 85 be substituted?  Could the problem be caused by the temperature sender, the connecting wires, or the printed dash circuit?

Budd Premack 
91 GL, 86 Weekender
Still the Land of Sky-Blue Waters