question about the coolant temperature sensor

[vincenzo masiello]

i’m an electrical engineer and been diagnosing, dealing with, and designing circuits and wiring and control systems for decades and have a good knowledge of car electronics.
my question is:
in an ignored car cooling system, a coolant sensor is usually covered with a layer of mud-like dirt inside the engine and the contacts in its connector (which are right under the thermostat housing and the sensor itself) and the wires get severely corroded through the years adding, probably, a few ohms to the resistance of the sensor that works with a negative temperature coefficient. the small increase would not be noticeable when the engine is cold and the sensor resistance is in the thousands of ohms, but when the engine reaches its standard operating temperature, the change should be significant (try adding 0.5 to 3000 and then 0.5 to 12, for instance). that should lead to the computer and the dash gauge, and even the fan to think that the car is cooler that it is. is my analysis correct?

i just inspected the sensor in my car. there is a wire that is about 5″ with a connector on one side to the sensor and another connector on the other side to another wire coming out of the main harness that goes everywhere (including the computer). i removed the little wire and cleaned all four sides with an electronic spray cleaner. then measured the resistance of the two conductors and it was two small for my fluke meter to recognize. then measured the resistance of the sensor (car was sitting for about 3 hrs) and it was about 850 ohms. then measured the voltage of the other side and it was only 1.2 volts when the key was at ON. that worries me because i expected something like 5 volts. should i dig into the harness?

i have a 2008 grand prix that probably never made it to the middle of the gauge (210 F) and the fan never ran without AC!
is it possible that the two factors i mentioned above is causing my gauge to show 185 average?
last weak i did a thorough flush and i’m about to put the manufacturer recommended coolant (dex-cool for GM) and replaced the thermostat and ordered an AC Delco radiator cap and did several trips with an obd2 temperature scanner connected and the temperature was always between 182 and 195 even though the new thermostat is of the 195 kind which is what the manufacturer recommends to keep the engine at 210 for optimum performance. do i need to replace/clean the sensor too?

Topic

[cj1]

“Forgot to mention that I put the 192 degrees OEM thermostat and a new rubber ring around it (the old ring was cracked and swollen).”
The cracked ring would allow coolant to bypass the thermostat. This would account for the low/erratic temp readings thus erratic fan operation.

[vincenzo masiello]

but I still get 180’s after hours of driving, see the articles I posted a link to in my last reply.

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