Check that the connector at the sender is OK (Yellow, red wire). If that is okay, check the wiring harness at the back of the gauge cluster.
The resistance reading for my RB30 sender with the wire off, is 500 ohms cold and around 34 ohms at about a third way up the temp scale.
With the car running and the multimeter on volts and the wire connected it should be a constant voltage signal that drops as the engine warms up.
The photo shows the Temp sender wire (Yellow red) and the sender input terminal behind the gauge. I made a 70 cm test lead by stripping the ends of the wire and inserting one end under the input terminal (make sure no copper is touching any other terminals).
I taped the wire to the positive multimeter alligator clip and connected the negative to earth. With the meter set on volts, get an assistant to drive and see if the signal is erratic as the car bounces around. If the signal is steady, and no other gauges are flucuating it will be a faulty gauge.
The black earth wire that supplies the temp and speedo is directly under the Yellow red wire at the connector.
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