Since you've just had the cooling system broken and filled, try this (I did read right and you have an 87, right?)
Look at the driver rear of the cylinder head for a small copper-coloured plug with a wire. Unscrew the plug, and see if you get a water trickle. If there is no water over the sender, it won't read even if you do overheat.
Second, was the thermostat changed as well? Was the cooling system "burped" afterward? I suppose one could get a large air bubble - maybe large enough to make the sensor in the head for the gage not work!
Since you are having overheat problems anyhow, change the thermostat. Before you put it in, drill two 1/16" holes in the metal flange on the new thermostat directly opposite each other. Install the thermostat with one hole at 12:00 and the other at 6:00, and the system will purge itself of air as it runs. This works fine for me on an 87, and 88, and an 89, so I'll swear by it.
With the early "closed" systems, it is all to easy to get an air bubble, and the "closed" system is fairly rare in automotive circles until very recently, so not all mechanics know an easy way to deal with it.
5-90