Alfa Romeo Repair: Water temp overheat light on., ground cable, bolds

Question
I have a 1991 164S. The water over temp light comes on immediately upon start.  I removed the sensor and checked it.  Seems to be OK. (i.e. the temp sensor is closed when cold, around 3 ohms. (Grounding the sensor wire turns the light off). I checked the ground of the center thermostat housing and it does not seem to be at ground potential.  Is there a seperate ground wire that attachs to it somewhere?  The three bolds that attach the three pieces of the thermostat together do not seem to be providing a solid ground connection to the engine or chassis.  Also, I was told that the new sensors are from the later 164Qs and operate opposite for the warning light.  I bought one and it seems to be open at cold instead of closed.  Thanks, Stephen  

Answer
Do run a 16 awg or 1mm ground wire to the chassis.  Also, do this: check for volt drop on the negative side when cranking the starter.  To do this attache the negative lead of the voltmeter to the negative terminal of the battery and the positve lead to the engine block.  While cranking observe the voltage at the meter--it should be half a volt or less. If the reading is more than half a volt clean the connections of the ground cable from engine to battery or replace it.

You may find a need to clean all the grounds on the vehicle.
This is not uncommon on older vehicles