'88 90q temp sensor?

[DK]

Check your coolant level in the reservoir/expansion tank, thats where
the sensor is located that will trigger the light (low coolant and
high temp/over heat). Even if it looks full, a small amount off will
trigger the light, as the sensor ball is either not floating/stuck or
its just a bit low. Disconnect it and see if it goes away.
Otherwise, maybe some wiring issues, grounds...but check the simple
things first. Not sure if has a MFTS, should have a two prong sensor.
You can check the resistance and see if it is close to the Bentley
specs. Usually there is one sensor for the gauge, and the other one
goes to the ECU.  The switch on the radiator is for the radiator fan
control.