The coolant does not only have a level sensor. There is also a pressure sensor hidden close to the passenger side fender.
It is not really a "sensor" but actually an on/off switch. getting stuck means alarm will go off.
This switch was add starting with the 1984 models, and is explained (not too
clearly) in the booklet for the changes by year for 1984.
If the coolant level of a cold engine is too low, this will switch on the coolant level warning light after a delay of approximately 20 seconds. Coolant will expand as the engine warms up and the coolant level will rise. If the engine is stopped and started again the warning will go out. This happens even though the cold coolant level is too low.
To avoid unnecessary troubleshooting, starting in 1984 models, the central warning system has a pressure switch located in the return line to the expansion tank which now works in conjunction with the coolant level switch. The pressure switch senses the pressure in the cooling system and sends this information to the central warning unit.
Now, if there had first been a warning from the coolant level switch and afterwards from the pressure switch, the central warning system will store this signal sequence and maintain the warning, even after the coolant has expanded and the coolant level switch has an opened circuit.
In other words, when starting the engine again, the warning lamp remains on, even if the coolant level appears correct.
If the cap at the expansion tank is removed (to add coolant), pressure escapes and the pressure switch circuit opens. The central warning system, with this indication, assumes coolant has been added, and turns the warning lamp out.