The flashing light is an inidcator that your water level is below the two electrical probes in the coolant expansion tank (not the refill tank) or the electronics that control this function could have gone out.  This is a classical problem.  There are many things to try.
1.  check the water level in the expansion tank.  There should not be any air in the tank, or very little.  Excessive air may indicate that you have a blown head gasket and that exhaust is blowing into the cooling system.  This is the worse case scenario.  Water cools have a bad habit of blowing head gaskets.  It seems that the wrong kind of antifreeze eats the metal under the head gaskets breaking the seal.  There is no solution to this except to replace the heads.
2.  check to see if the water antifreeze ratio is 50%  Some people report that they have solved the problem by establishing the correct ratio.  I personally haven't had any luck with this fix.
3.  check to see if the o-ring under the two electrical probes is leaking to the outside and into the connecter plug
4.  check to see if the connection to the probes is a good electrical connection.
5.  If all of the above are OK, there are three electrical components which may have become defective.
    The instrument voltage stabilizer ( a 10v voltage regulator that is located on the harness under the instrument panel
    The temp. gauge integrated circuit which is an integral part of the gauge.  This requires replacement of the gauge
    The coolant low level warning control unit is bad.  This is a plug in that on an 84 is located on the left wall beneath the front dash.
 Hope this helps.  Please let me know what you find to be the problem.
George 84

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