The fan should turn on between 210 and 212 degrees. It is controlled by the engine computer which gets its reading from the coolant temperature sensor. This is not the same sensor that is used for the gauge on the dash. There is most likely nothing wrong with the sensor. As the coolant temperature increases, the voltage on the sensor will drop, but it will always be between.5 and 4.5 volts. Anything outside that range will set a diagnostic trouble code and turn on the "Check Engine" light. If the light isn't on, the sensor is working.
You'll find the two sensors near the thermostat housing on the passenger side of the engine. The gauge sensor will have a single wire plugged into it, usually purple. Unplugging that wire will cause your gauge to read full cold, nothing else. You need to find the computer's sensor in that same area. It will have two wires in the plug. When you unplug that sensor while the engine is running, the fan should come on and the Check Engine light should be on. Because of the default 5.0 volts that will appear on one of the wires, the computer knows it can not believe that sensor, so just in case the engine is overheating, it commands the radiator fan to run. The fan will turn off a few seconds after reconnecting the sensor's plug. If the fan runs when doing this test, the system is working. I suppose the sensor could be reading lower than the real temperature, but it would be more likely the gauge sensor is inaccurate. Chrysler had very little trouble with either sensor.
If the fan does not run with the sensor unplugged, check to be sure the Check Engine light is on. In the very rare instance it is not, suspect a problem with the wiring harness. Most likely someone cobbled in a resistor because they didn't know how to properly diagnose the system. That would also explain why the fan isn't turning on in response to high engine temperatures. The computer thinks it is staying at one constant temperature. Remember, the two wires being shorted together or one of the wires being broken open will be detected by the computer. A makeshift resistor will not be detected.
More likely you will find the Check Engine light is on indicating the unplugged sensor has been detected and the circuit is working. When you unplug the sensor, you should hear the radiator fan relay click within a couple of seconds. If you do, but the fan doesn't run, unplug the fan motor connector and probe the engine side of the connector with a test light. One of the two wires must have 12 volts. This is one of the few instances where a cheap test light is more accurate than an expensive voltmeter. I'll explain later. If neither wire has 12 volts, the possibilities include a defective relay coil, (it won't click), rusted-off relay contacts, (you might hear it click, you might not), or a burned open fuse link wire, (typically caused by a shorted fan motor).
If you do find 12 volts on one wire in the connector, put the ground clip of your test light on the battery positive post, then probe the second wire in the connector, (engine side of the harness). If the test fails to light, the ground wire is corroded off or it was left off during a previous repair. If the light is nice and bright, it indicates the ground wire is good.
If the 12 volt and the ground wire are both good, that only leaves the fan motor. If you want to double-check yourself, you can run a pair of wires from the battery terminals to the two wires on the motor. Which wire goes to which battery terminal isn't important. One way the fan will run backwards, but at least it will run.
Many mechanics no longer use test lights because they draw so much current they can damage computers. In this case though, digital voltmeters don't draw enough current to accurately test this high-current circuit. A coworker drove himself nuts on a K-car many years ago. The fan ran when he jumped it to the battery so he knew it was good. He had 12 volts and he had a good ground. Nothing made sense and he got even more confused when I told him to use a test light instead of his voltmeter. The test light did not light up where the voltmeter showed 12 volts. The fuse link was blown because even though the fan would run when jumped, it was tight and that caused it to draw very heavy current.
The fuse link is one of a bunch of them that run along side the left strut tower. It is smaller in diameter than all the rest of the wiring in that circuit, so it's the weak link in the chain. It also has special insulation that will not burn when the wire burns open. To test them, just pull on 'em. It they act like a wire, they're good. If they act like a rubber band, they're bad. New fuse link wire is purchased according to the color of the insulation from any parts store. One wire is long enough for two repairs. (You only need half of what they will give you).
What happened on the K-car is when the fuse wire burned open, it left a carbon track behind, similar to what used to happen in distributor caps when they got water inside from a heavy rain storm. That carbon track cold pass only a tiny tickle of current but that is enough to be detected by a digital voltmeter. A test light needs a lot more current to light up. That higher current can't get through the carbon track.
The same thing happens if you would put a pressure gauge on the end of a garden hose. If the nozzle is turned off, the gauge will read full pressure even if you're standing on the hose. Open the nozzle and you'll just get a little trickle; not enough to run the gauge, (not enough voltage to run a test light).
Once you find the problem and correct it, the fault code will still be in the engine computer but the Check Engine light will go off. I can't remember if it will go off as soon as you plug the sensor in or if you have to turn the ignition switch off and restart the engine. If nothing else happens to the circuit, the fault code will be erased automatically after starting the engine about 50 times. You can also erase it by disconnecting the battery for half a minute, but that will also erase the radio presets and clock and more importantly, the long and short-term fuel trim data stored in the engine computer's memory. You probably won't even notice a change in engine performance as the computer rebuilds the lost data over the next few hours of driving, but it really isn't necessary to do anything. Just drive the car like normal and within a month the code will be gone from memory.
Sunday, January 10th, 2010 AT 2:43 AM