Cooling fan pin out

builderpaul · July 27, 2005

On an 05 PCM. Do you know which pins operate the fans. I think one is for the AC on and one is for temp.

Also what is the stock on/off setting for temp.

Thanks

GM-Tech · July 27, 2005

The engine cooling fan system consists of 2 electrical cooling fans and 3 fan relays. The relays are arranged in a series/parallel configuration that allows the powertrain control module (PCM) 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.

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 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 PCM 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 PCM 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 there own ground path. The result is a parallel circuit with both fans running at high speed.

Important: The right and left cooling fan connectors are interchangeable. When servicing the fans be sure that the connectors are plugged into the correct fan.

The PCM 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 min. regardless of coolant temperature.

The PCM 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 insures idle stability by preventing the fans from cycling between high and low speed.

builderpaul · July 27, 2005

Thank you. Very informative. It clears up a lot. I thought there were times that only 1 fan worked but I see now it is not the case. Any idea why the 2 fans are different? Both work at the same time and the same speed.

I don't understand the last statement about at idle, only increasing fan speed if required. Does the PCM have a seperate set of Turn on temps/pressures for idle/low speed? Seems this is when high speed would be needed the most.

GM-Tech · July 27, 2005

I don't understand the last statement about at idle, only increasing fan speed if required. Does the PCM have a seperate set of Turn on temps/pressures for idle/low speed? Seems this is when high speed would be needed the most.

What you read up there is what I know. Don't forget, if the fans come on full, they pull quite a bit of current. Which means the alternator would have to ramp up its output, creating an additional load on the engine. They will go to high speed if it needs it. The PCM knows everything about the current conditions... engine temp, a/c pressures, engine rpms.

moregrip · August 1, 2005

I don't understand the last statement about at idle, only increasing fan speed if required. Does the PCM have a seperate set of Turn on temps/pressures for idle/low speed? Seems this is when high speed would be needed the most.

What you read up there is what I know. Don't forget, if the fans come on full, they pull quite a bit of current. Which means the alternator would have to ramp up its output, creating an additional load on the engine. They will go to high speed if it needs it. The PCM knows everything about the current conditions... engine temp, a/c pressures, engine rpms.

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Can these fans be used in an 05 that did not come with them? i.e. 2500HD?