Mechanics

CHEVY COOLING FANS

2002 Chevrolet Impala

Engine Cooling problem
2002 Chevy Impala 6 cyl Front Wheel Drive Automatic 137000 miles

Hello I am having a problem with my cooling fans. They Will not come on unless I unplug the temp sensor, Unplug it fans come right on no problem but of cource the temp guage does not work. I replaced the temp sensor hopeing this would be the problem, But no such luck. Also I checked all the 3 relays that a labeled fon cont. And fuses but this checked out as while. Im thinking that the computer reads the signal from the temp sensor and once it sees 210 or how ever many oms it sends a signal to the relays to turn the fans on. I know that the perv. Owner had some cooling problems and had in the shop sevrel times.
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Dustin Goad
March 23, 2010.




Is there an overheating problem, and the fans were discovered to be non-functional? The fan relays are controlled by the body control module, and the powertrain control module. The fact that they come on, is due to default when the PCM sets a code. There are other situations that would command fans(one or both), and that would be a/c compressor engagement, and/or refrigerant temp/pressure sensors. Is the vehicle getting hot enough for fans to come on? Does one or both come on, when a/c is engaged? There are two strategies depending on engine size however, they are somewhat similar. Here is the description.
Cooling System Description and Operation
Cooling Fan Control 3.4L (LA1)
The engine cooling fans are controlled by the body control module (BCM) and the powertrain control module (PCM). The BCM sends a Class 2 message to the PCM requesting cooling fan operation. If there is a Class 2 communication malfunction between the BCM and the PCM. The PCM will operate the engine cooling fans independently of the BCM.

The engine cooling fan system consists of 2 electrical cooling fans and 3 fan relays. The relays are arranged in a series/parallel configurationthat allows the PCM to operate both fans togetherat low or high speeds. The cooling fans and fan relays receive batterypositive voltage from the engine wiring harness junction block. The groundpath is provided at G101.

During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage from the FAN CONT #1 fusethrough the cooling fan motor supply voltage circuit to the right coolingfan. The ground path for the right cooling fan is through the cooling fan 2 relay and the left cooling fan. The result is a series circuit with both fans running at low speed.

During high speed operation the PCM supplies the ground path for the cooling fan 1 relay through the low speed cooling fan relay controlcircuit. After a 3-second delay, the PCM supplies a ground path for thecooling fan 2 relay and the cooling fan 3 relay through the high speed cooling fan relay control circuit. This energizes the coolingfan 2 relay coil, closes the relay contacts, and provides a groundpath for the right cooling fan. At the same time the cooling fan 3relay coil is energized closing the relay contacts and provides battery positive voltage from the FAN CONT #2 fuse on the cooling fan motorsupply voltage circuit to the left cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.

The BCM will request low speed cooling fan operation from the PCM under the following conditions: • Engine coolant temperature exceeds approximately 103 °C (217 °F).

• When A/C is requested and the ambient temperature is more than 50 °C (122 °F).

• A/C refrigerant pressure exceeds 1310 kPa (190 psi).

• After the vehicle is shut off if the engine coolant temperature at key-off is more than 117 °C (243 °F) and system voltage is morethan 12 volts. The fans will stay on for approximately 3 minutes.

The BCM will request high speed cooling fan operation from the PCM under the following conditions: • Engine coolant temperature reaches 110 °C (230 °F).

• A/C refrigerant pressure exceeds 1655 kPa (240 psi).

• When certain DTCs set.

Cooling Fan Control 3.8L (L36)
The engine cooling fans are controlled by the body control module (BCM) and the powertrain control module (PCM). The BCM sends a Class 2 message to the PCM requesting cooling fan operation. If there is a Class 2 communication malfunction between the BCM and the PCM. The PCM will operate the engine cooling fans independently of the BCM.

The engine cooling fan system consists of 2 electrical cooling fans and 3 fan relays. The relays are arranged in a series/parallel configurationthat allows the PCM to operate both fans togetherat low or high speeds. The cooling fans and fan relays receive batterypositive voltage from the engine wiring harness junction block. The groundpath is provided at G101.

During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage from the FAN CONT #1 fusethrough the cooling fan motor supply voltage circuit to the right coolingfan. The ground path for the right cooling fan is through the cooling fan 2 relay and the left cooling fan. The result is a series circuit with both fans running at low speed.

During high speed operation the PCM supplies the ground path for the cooling fan 1 relay through the low speed cooling fan relay controlcircuit. After a 3-second delay, the PCM supplies a ground path for thecooling fan 2 relay and the cooling fan 3 relay through the high speed cooling fan relay control circuit. This energizes the coolingfan 2 relay coil, closes the relay contacts, and provides a groundpath for the right cooling fan. At the same time the cooling fan 3relay coil is energized closing the relay contacts and provides battery positive voltage from the FAN CONT #2 fuse on the cooling fan motorsupply voltage circuit to the left cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.

The BCM will request low speed cooling fan operation from the PCM under the following conditions: • Engine coolant temperature exceeds approximately 100 °C (212 °F).

• When A/C is requested and the ambient temperature is more than 10 °C (50 °F).

• A/C refrigerant pressure exceeds 1310 kPa (190 psi).

• After the vehicle is shut off if the engine coolant temperature at key-off is more than 117 °C (243 °F) and system voltage is morethan 12 volts. The fans will stay on for approximately 3 minutes.

The BCM will request high speed cooling fan operation from the PCM under the following conditions: • Engine coolant temperature reaches 105 °C (221 °F).

• A/C refrigerant pressure exceeds 1655 kPa (240 psi).

• When certain DTCs set.

If there are no overheating concerns, You would most likely need a scan tool to read what the coolant temp sensor is displaying, and determine if there is enough heat for the fans to be requested. There is one bulletin regarding this issue, it applies if you are seeing an overheat.
#PIC3045C: Engine Overheat And/Or Cooling Fans Inoperative Intermittently - (Feb 23, 2009)

Subject: Engine Overheat and, or Cooling Fans Inoperative Intermittently

Models: 2000-2005 Chevrolet Impala and Monte Carlo

This PI was superseded to update Recommendation/Instructions. Please discard PIC3045B.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern: Engine overheat and, or cooling fans inoperative intermittently.

Recommendation/Instructions: If, after following published diagnostics in SI2000, the condition still exists, the following procedure may lead to an effective resolution.

Operate the engine until it reaches operating temperature.
Check the load management status in BCM data display, under outputs.
If the status is active, with the engine running, disconnect the BCM from the dataline at the star connector (SP205) located at the base of the steering column.
If fans operate normally, check that the charging system is operating as it should.
Replace BCM if it shows load management status is active (In Step 2), the cooling fans operate normally with the BCM disconnected (In Step 3) and the charging system is operating as it should.

Factoryjack
Mar 23, 2010.


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