Hyundai Repair: coolant temperature sensor, coolant temperature sensor, coolant level

Question
I current don't have the manual to my 1996 sonata and are currently having a new thermostat and new hoses installed, with flushing the radiator. After installing the new thermostat and coolant and new hoses, I am getting overflow with the car running hot. See no leaks at the radiator, or cap or anywhere else. There is no antifreeze or fluid in the the oil or leaking anywhere else. It only comes out through the over flow bottle. On the top of the thermostat housing is a senor. I am trying to find out how that senor works in conjunction to the thermostat opening and closing. I have a new senor on order, but would like some input on exactly how it works.

Answer
The sensor on top of the thermostat housing turns off the air conditioner compressor when the car is overheating.  It doesn't control anything in relation to the cooling system.  The cooling fans are controlled by a cooling fan control module which is located behind the right kick panel inside the car.  The module receives input from the sensors in the bottom of the radiator.

Right now, it's probably best to step back and take an audit of your cooling system operation.  

First, verify coolant level and flow.  Remove the radiator cap and verify the radiator is full.  Then start the engine and observe the coolant flow by looking into the filler neck.  The coolant should start to flow before the coolant begins to boil out of the radiator.  If you're unable to see whether the coolant is flowing, you can determine this by feeling the radiator hoses.  If only one hose is very hot, the coolant is not flowing.  If both are hot, the coolant is flowing.  Similarly, if the fan begins to run but only blows lukewarm or cool air off the radiator, then the coolant is not flowing.  If the coolant begins to boil out prior to flow occurring, there's a blockage somewhere in the system.  A stuck closed thermostat is the most likely culprit.  Since you've just replaced the thermostat, we'll assume it's okay.  But you might check to be sure it was properly installed.  The side with the spring on it should be toward the engine.  It's also possible the radiator is clogged, but this is much less frequent than thermostat problems.  And a last possible issue is that the water pump impeller is corroded away or broken off.  I've never seen a car where this occurred, but I have heard of it.

If the coolant begins to flow properly, the next step is to check fan operation.  Once again, prior to the coolant boiling, the fans should begin to cycle on and off.  If they don't you'll need to diagnose the fan circuits.  This includes the fan control module, the sensors in the radiator, the fan motors, the fuse, and the wiring.  

You can view the factory service manual for your car at www.hmaservice.com.  You'll need to register for a free account, and the site requires Internet Explorer.