Cooling fans

Tiny
DUSTIN GOAD
  • 2002 CHEVROLET IMPALA

Six cylinder front wheel drive automatic 137,000 miles.

Hello, I am having a problem with my cooling fans. They Will not come on unless I unplug the temperature sensor, Unplug it fans come right on no problem but of course the temperature gauge does not work. I replaced the temp sensor hoping this would be the problem, but no such luck. Also, I checked all the three relays that a labeled fon cont. And fuses but this checked out as while. I am thinking that the computer reads the signal from the temperature 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 previous owner had some cooling problems and had in the shop several times.

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Tuesday, March 23rd, 2010 AT 10:37 PM

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Tiny
FACTORYJACK
  • EXPERT
  • 4,238 POSTS

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 power-train 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 temperature/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 power-train control module (PCM). The BCM sends a Class two message to the PCM requesting cooling fan operation. If there is a Class two 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 two electrical cooling fans and three fan relays. The relays are arranged in a series/parallel configuration that allows the PCM to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the engine wiring harness junction block. The ground path 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 one relay coil, closes the relay contacts, and supplies battery positive voltage from the FAN CONT #1 fuse through the cooling fan motor supply voltage circuit to the right cooling fan. The ground path for the right cooling fan is through the cooling fan two 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 one relay through the low speed cooling fan relay control circuit. After a three second delay, the PCM supplies a ground path for the cooling fan two relay and the cooling fan three relay through the high speed cooling fan relay control circuit. This energizes the cooling fan two relay coil, closes the relay contacts, and provides a ground path for the right cooling fan. At the same time the cooling fan three relay coil is energized closing the relay contacts and provides battery positive voltage from the FAN CONT #2 fuse on the cooling fan motor supply 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 more than twelve 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 power-train control module (PCM). The BCM sends a class two message to the PCM requesting cooling fan operation. If there is a class two 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 two electrical cooling fans and three fan relays. The relays are arranged in a series/parallel configuration that allows the PCM to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the engine wiring harness junction block. The ground path 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 one relay coil, closes the relay contacts, and supplies battery positive voltage from the FAN CONT #1 fuse through the cooling fan motor supply voltage circuit to the right cooling fan. The ground path for the right cooling fan is through the cooling fan two 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 one relay through the low speed cooling fan relay control circuit. After a three second delay, the PCM supplies a ground path for the cooling fan two relay and the cooling fan three relay through the high speed cooling fan relay control circuit. This energizes the cooling fan two relay coil, closes the relay contacts, and provides a ground path for the right cooling fan. At the same time the cooling fan three relay coil is energized closing the relay contacts and provides battery positive voltage from the FAN CONT #2 fuse on the cooling fan motor supply 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 more than 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 data-line 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 two), the cooling fans operate normally with the BCM disconnected (in step three) and the charging system is operating as it should.

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Tuesday, March 23rd, 2010 AT 11:32 PM

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