Either the switch that tells it when to turn on is bad or the fan itself is bad.
Either the fan itself is burnt out or the temperature control switch that tells it when to turn on is bad.
Relay or fan itself
Usually ECT or engine coolant temp sensor. That's what tells the engine computer to turn on the fans.
The coolant fan switch is located on the intake manifold near the thermostat. The fan should come on about 232 to 235 degrees and turn of at or near 195 degrees. Check on rear of radiator near top--should be plug with two wires
It is on the intake manifold near the thermostat.
Either the fan is burnt out or the sensor that tells the electric fan when to turn on is bad, you can wire the fan directly to the battery to see if it's good.
The (radiator) coolant cooling fan is activated by the coolant temp sensor which tells the computer to turn it on. There is also a cooling fan fuse and a cooling fan relay. Check the fan by connecting it straight to the battery,it must turn very fast and not just half fast. To check the relay (without getting it complicated) just swap it with the one next to it on the fuse box under the hood, and of course check the fuses. The fan should also operate when the A/C is turned on.
it tells the radiator fan when to turn on
On a 1999 Cadillac DeVille, the fan control sensor is typically located near the radiator or on the engine itself, often in the vicinity of the coolant temperature sensor. It can be found on the engine block or near the thermostat housing. For precise identification, consulting the vehicle's service manual or a repair guide is recommended.
check fuse , fuses located under dash left of steering column. second check heater fan switch finally the fan motor located under hood passenger side
I believe the trip sensor (switch) is bolted to the rear of the fan housing which tells the fan or relay in circuit to power on once it is heated by the air temperature from the radiator/engine.