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Re: Electric fans for radiator do not turn on. Tested
Temp sending sensor may be bad. If it is bad the fans don't have something "saying it's hot or overheating, need to turn on fans to cool down" The fans are not getting the signal it needs to turn on. If you have checked the motors and relay then this is the only other problem I have seen,
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Unless you have a check engine light on then the sensor (called the ECT sensor) in question is not the problem, do you have a C.E.L. on in the dash display? Have you tested the fan motor, it is ok? Is the cooling fan motor getting power when you jumper the fan relay socket?
A mechanic would use a scanner to check the engine temp sensor. The temp sensor tells the computer what the engine temp is and the computer activates the fan relay when the temp reaches about 230 degrees. Since you said relay I am assuming your Jeep has an electric radiator fan.
The car model was not included in the question. In general if the radiator fan is not starting the first thing to check are the fuses. Depending on the car's model there is one or two fuses in line with the cooling fan circuit. If there are two fuses, then one is in the fuse plate inside the car, the other one is under the hood.
The next part to test is the thermostat. The thermostat housing is located on the refrigerant fluid lines, either near to the radiator, or near to the engine block. The thermostat can be tested using VOM tool. Read either Ohms or volts to find out if the thermostat is closing the fan circuit when the engine is hot.
Most fan circuits use also relay. Variable speed fans have often more than one relay. The fan relay(s) is often located under the hood. Relays can be tested reading volts at their contacts or swapping the relay with one of the same kind.
If fuses, relay and thermostat are OK, then the fan motor is often the next test. The motor can be tested energizing it directly. Variable speed fans have more than two wires powering the motor. To avoid risk of damaging the fan motor the repairer should refer to specific wiring diagrams before jumping fan cables. A different way of testing the motor is reading volts at fan motor while engine is hot. If there is volts at motor contacts but motor does not come on, then the fan motor is defective.
It could be the radiator fan motor if all sensors and relays have been changed.......unplug the coolant temperature sensor the fan should come on and stay on unless the motor is bad or it is not getting electrical current.
What year is the vehicle? Your coolant temperature sensor may be bad which indicates the system when to turn the electric fan on and off with engine temperature and also a a/c clutch switch that turns the fan on with a/c usage. If the temperature sensor is not operating properly your fan will not come on. Depending on the year, you may have TWO sensors; one is for gauge operation and the other for the PCM and the fan, also known as the ECT (engine coolant temp). It should be a fairly cheap part.
By pass it. remove wires from fan leaving enough wire to splice onto.Run 2 wires 1 to ground and 1 to a power source in fuse panel that only has power when ignition is on such as running lights . this by passes fuses and relays and assures your fan is running if your vehicle is running.