Hi roncichon. Welcome to the forum. If you measure the coolant temperature sensor, you will find it's not a switch. It's a thermistor, or temperature dependent resistor. As its temperature goes up, it's resistance goes down.
From here on, I have to defer to my previous experience with Chryslers, but it sounds like the same circuit. On the Chrysler engines, as the coolant temperature goes up, the resistance of the sensor goes down. The Engine Computer sends 5.0 volts to it through another internal resistor. The only way to see the full 5.0 volts is to unplug the connector. When it is reconnected, the current flow results in some of the 5.0 volts being dropped across the internal resistor and the rest is dropped across the sensor. That voltage across the sensor is interpretted by the computer as temperature.
Under normal conditions, that signal voltage will always be between 0.5 and 4.5 volts. Anything outside that range is detected by the computer as a problem and it sets a diagnostic fault code in memory. Grounding the signal wire will result in 0.0 volts, and unplugging the connector results in 5.0 volts, both unacceptable conditions.
When 5.0 volts is seen, the computer turns on the radiator fan because it knows it can't rely on that sensor to tell it if the engine is overheating. Running the fan is a precaution. On the older cars with distributors, unplugging the sensor was also a fast way to kill the electronic spark timing advance so you could adjust base timing.
Your coolant temperature sensor should have a second wire in the connector for the ground. That wire will not go to the engine or body. It goes to ground through the Engine Computer. The computer monitors current flow through a small resistor for part of its diagnostics. You will typically measure around 0.2 volts on it. The two wire connector is an easy way to differentiate the sensor from the one for the temperature gauge on the dash. That sensor typically has only one wire.
If the coolant is getting too hot and the fan is not turning on, you have already performed the first quick check; that is disconnecting the sensor. That proves the relay, fan motor, and high-current wiring are ok, and the computer has control of them. The next step is to find a mechanic with a hand-held computer, called a scanner, that will read live sensor data. That will show the temperature the computer thinks the coolant is at. Expect the Engine Computer to turn on the fan at around 210 degrees and off at 198 degrees. If the engine appears to be overheating before the fan turns on, the sensor could be defective, the head gasket could be leaking, or there could be an air pocket in the system.
If you see bubbling in the overflow reservoir, use a thermometer to measure the actual temperature of the coolant. If it's higher than what's displayed on the scanner, replace the sensor. Except for Fords, they give very little trouble. If the temperature is below 210 degrees but you see bubbling, have the cooling system checked for combustion gases. That involves using a glass cylinder filled with two chambers of a special dark blue liquid. Air is drawn from the radiator. If combustion gases are sneaking in through the head gasket, the liquid will turn bright yellow. If that air forms a pocket around the temperature sensor, it won't work properly. They only respond to hot liquid, not hot air.
Friday, April 23rd, 2010 AT 3:01 AM