A friend asked me to change his thermostat, after doing so he told me he had already changed the coolant temp sensor! Two days later he says the analog gauge was at zero and the digital readout was just a series of dashes and based on his conversation I believe there is a common ground issue between the analog readout & possibly the ECM? Has anyone had this same problem? Can someone direct me to a schematic?

 

This is usually a sign of a bad thermostat or using the wrong one, since it was just changed. What did you put in there?

 

Does the AC work? If not you have a bad t-stat or sensor.
ACDelco 131-158 is the ONLY t-stat that works correctly in these cars.

To begin; the sensor uses "low ref" (digital ground) not earth/ground. If somebody mucked the wires up possibly the problem.