I have a 99 Buick Century with a 3.1 V6. I have checked the relays all three check good, I have checked the fuses they are all good. I have jumped the fans and they both work. I feel there is a thermostat controled swith that sends a signal when the fans need to come on but I cannot find it. can anyone tell me where it may be located?
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Re: Cooling Fans Not Working
Thermo sensor switch in cylinder head might be in thermostat housing or in the cyl head some place in that area look closely you will see it usually 2-3 wire plug it is brass with black plastic type plug on end good luck.John
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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.
You have done well so far, but this is a hard problem. Have you checked the reference voltage (usually 5vdc) that is going to the temp sensor? It might be getting pulled down by one of the other sensors on that wire. A way to check is to get a baseline voltage on the coolant temp sensor black wire. Then unpulg the map sensor--does the voltage on the black wire increase? If yes, replace the map sensor. If not, next unplug the A/C pressure sensor. Same process. Finally, unplug the transmission temp sensor. If unplugging any of these sensors increases the voltage on the black wire by say a volt, replace the bad sensor and retest.
Both radiator fan relay's ????? There are three relay's !
DTC P0481Cooling Fan Relay 2 Control Circuit
Output Driver Modules (ODMs) are used by the PCM to turn on many of the current-driven devices that are needed to control various engine and Transaxle functions. Each ODM is capable of controlling up to 7 separate outputs by applying ground to the device which the PCM is commanding on. DTC P0481set indicates an improper voltage level has been detected, which controls the Cool Fan 2 and Cool Fan 3 Relays.
Check cooling fan maxifuse 15 amp. This fuse supply's B+ voltage too cool fan relay's 2 & 3 control side . The PCM supply's a ground to energize these two relay's , the PCM is not seeing the correct voltage that it should be . The fuse could be the problem , an open circuit , the transistor or driver as GM calls it output driver module . The number 1 relay is working fine , with A/C on # 1 relay is turned on or energized by the PCM for low speed operation .
There is a good video on youtube on how this three relay cooling fan system works an how to trouble shoot . Relay Driven Dual Cooling Fan Diagnostics
The ECM turns on the fans when the engine tempurature gets to around 210 degrees. You could have a bad coolant tempurature sensor, not telling the ECM that the engine is hot enough to turn on the fans. I would check to see if the fans come on when the A/C is turned on. Or there is a problem with the ECM. The ECM supplies a ground to the cooling fan relay to turn on the fans.
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