1995 Buick Century 3.1 Cooling fans not running. I have replaced the cooling temp sensor and the ECM the fans and relays are all working i have jumped out relays and both fans run. Check the coil of the relay ok just never getting power to the relay coil from the ECM i have checked the wiring from the sensor and the relays back to the ECM also tested good. WHY WONT MY FANS COME ON!!

Check the wiring harness to the fan they sometimes put a fuseable link in the wire and it might of blown.

Your vehicle cooling fan's are controlled by three relay's , couple of fuse's an the PCM - engine computer . The engine coolant temp. sensor is a input to the PCM , when engine temp get's to 212 the PCM will turn on the cooling fans in low speed . Checking to see what the actual temp is with a scan tool an a infrared thermometer at the thermostat housing ,an looking up the specification's in a service repair manual is how to diagnose . Not guessing an replacing parts . Plus learning how the cooling fan's work .
Relay Driven Dual Cooling Fan Diagnostics Reading factory service info as to how they work also would be a good idea .

Cooling Fan Control - Two Fan System
The engine cooling fan system consists of 2 puller type electrical cooling fans and 3 fan relays. The relays are arranged in a series parallel (S/P) configuration that allows the engine control module (ECM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood fuse block. The ground path is provided at G104.
During low speed operation, the ECM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the low speed fan relay coil, closes the relay contacts, and supplies battery positive voltage from the low fan fuse through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the cooling fan S/P relay and the right cooling fan. The result is a series circuit with both fans running at low speed.
During high speed operation the ECM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. After a 3 second delay, the ECM supplies a ground path for the high speed fan relay and the cooling fan S/P relay through the high speed cooling fan relay control circuit. This energizes the cooling fan S/P relay coil, closes the relay contacts, and provides a ground path for the left cooling fan. At the same time, the high speed fan relay coil is energized closing the relay contacts, and provides battery positive voltage from the high fan fuse on the cooling fan motor supply voltage circuit, to the right cooling fan. During high speed fan operation, both engine cooling fans have their own ground path. The result is a parallel circuit with both fans running at high speed.
The ECM commands the low speed cooling fans ON under the following conditions:
• Engine coolant temperature exceeds approximately 94.5°C (202°F).
• A/C refrigerant pressure exceeds 1447 kPa (210 psi).
• After the vehicle is shut OFF, if the engine coolant temperature at key-off is greater than 101°C (214°F), the low speed fans will run for a minimum of 60 seconds. After 60 seconds, if the coolant temperature drops below 101°C (214°F), the fans will shut OFF. The fans will automatically shut OFF after 3 minutes, regardless of coolant temperature.
The ECM commands the high speed fans ON under the following conditions:
• Engine coolant temperature exceeds approximately 104.25°C (220°F).
• A/C refrigerant pressure exceeds approximately 1824 kPa (265 psi).
• When certain DTCs set
At idle and very low vehicle speeds the cooling fans are only allowed to increase in speed, if required. This ensures idle stability by preventing the fans from cycling between high and low speed.

I have the 1999 Century and have finally found the Head Gasket is leaky coming out the exhaust. I have to pray for the temperature gauge to come down. The mechanic I go to finally put in nearly a quart of Head Gasket sealant and she ran fine for a couple months and I popped a hose. From here I asked him to put some more sealant in, yet it did not work. I needed coolant and the only way to put it in was to open a relief valve when it was getting hot while running and it took almost a whole gallon. Without knowing of this relief valve I would be dead. I've heard of a way to put a resistor in the coolant fan harness to turn it on earlier, does anyone out there have an answer to this. Thanks a million. [email protected]

The engine temp sensor sends a signal to the computer and the computer grounds the fan relay when it is needed.
I'm showing 6 relays on the firewall under the windshield.
The two on the right side are high and low radiator fan relays.

if the cooling fan didn`t come on then you need to find out why.is the cooling fan motor seized or is the fan not being sent a signal from the ecm(computer) to turn on the fan through the cooling fan relay.if you take and unplug the cooling fan and run a power wire and ground wire to the connectors at the fan and it won`t work then the fan is seized and needs replacing.this is the most common problem with these.the other is the relay.if the ecm isn`t receiving a signal from the cooling system sensor then it may or may not turn on the check engine light.then you may have to have it checked with a scan tool that will show you what the ecm is seeing,to see if the temp is registering with the ecm.

most time coolant temperature sensor faulty.but check cooling fan fuse and relay.then hot wire the cooling fan if fan runs.problem in the fan wiring or pcm.if every thing else okay.if cooling fan dont run when hot wired the cooling fan need replacing fan motor has internal open winding circuit.

FIRST IS CHECK ENGINE LIGHT ON....YOU SHOULD BE ABLE TO UNPLUG THE ECT... ELECTRONIC COOLING TEMP SENSOR.....A TWO WIRE SENSOR THAT TELLS THE COMPUTER THE TEMP OF MOTOR....WHEN THE MOTOR TEMP GETS HIGH THE COMPUTER GROUNDS THE COOLING FAN RELAY TO TURN FANS ON..IF FANS DON'T RUN WITH SENSOR UNPLUGGED THE GET A SMALL PIECE OF WIRE AND FOR A SHORT TIME SHORT THE TWO WIRES ON THE PLUG FOR SENSOR...IF IT DOES COME ON REPLACE SENSOR IF IT DOES NOT COME ON SUSPECT COMPUTER...ONLY IF ALL FUSES ARE OK LET ME KNOW

If you have changed out the relay, and disconnected the temp cooling sensor, make sure you have replaced the correct relay; I believe there is one bank of four relays located by the passenger side head lamp. If the fan is running continuously, then I would say the relay is stuck in the closed position, resulting in completing the electrical circuit, powering the fan. Have the new relay tested. You can find a wiring schematic in the Hanes manual for your make and model.

Most GMs use a relay package to turn on the fans. That is often controlled through a processor package. Very complicated...if yours is one of them. You will need Mitchell's manual or equal to trace this to see if you have all the signals.