Cooling System: Description and Operation

CIRCUIT OPERATION

The Powertrain Control Module (PCM) controls both Cooling Fans by controlling three relays through two outputs. There are two modes of cooling fan operation, low speed and high speed. In the low speed mode the cooling fans are powered in series, Cooling Fan 1 Relay is energized by the PCM and both cooling fans run at low speed. In the high speed mode the cooling fans are powered in parallel which allows the fans to run at high speed. Cooling fans operate only when the ignition is in RUN.

Low Speed Conditions (4.9L)
The PCM will turn on the cooling fans at low speed when the engine coolant temperature exceeds 106 degrees C. The cooling fans will be turned off when the engine coolant temperature falls below 98 degrees C. The PCM monitors engine coolant temperature using the Engine Coolant Temperature Sensor.

The Heater and A/C Programmer will request the PCM to turn on the cooling fans at low speed through the UART data line when the A/C high side refrigerant temperature exceeds 60 degrees C. The HVAC Programmer will request that the cooling fans be turned off once the A/C high side refrigerant temperature falls below 50 degrees C.

The Heater and A/C Programmer also will request low cooling fan speed operation when all of the following conditions are met:

1. Vehicle speed is below 20 mph.
2. A/C Compressor Clutch engagement is possible.
3. Outside air temperature is greater than 7 degrees C (45 degrees F).

Low Speed Conditions (Northstar)
The PCM will turn on the cooling fans at low speed when the engine coolant temperature exceeds 106 degrees C. The cooling fans will be turned off when the engine coolant temperature falls below 102 degrees C. The PCM monitors engine coolant temperature using the Engine Coolant Temperature Sensor input.

The PCM will also activate low speed operation if the transaxle temperature exceeds 115 degrees C. The cooling fans will be turned off when the transaxle temperature falls below 110 degrees C. The PCM monitors transaxle temperature using the Transaxle Temperature Sensor.

The Heater and A/C Programmer can request the PCM to turn on the cooling fans at low speed through the UART data line.

The PCM will turn on the cooling fans at low speed if a low coolant level is detected and the engine coolant temperature exceeds 80 degrees C. The cooling fans will be turned off when the engine coolant temperature drops below 76 degrees C. The PCM monitors coolant level using the coolant level switch input.

Low Speed Operation (4.9L)
Low speed cooling fan operation will occur when the PCM grounds Cooling Fan 1 Relay's coil. With only Cooling Fan 1 Relay energized, voltage is applied to both the Pusher and Puller Cooling Fan Motors. The voltage path to the motors is through the Cooling Fan 1 Relay's contacts, and the Cooling Fan 2 Relay's contacts. The two motors are connected to each other through a series circuit. This causes the voltage being applied by MAXI Fuse 6 to be divided between the two motors. The division of the voltage causes the motors to operate at a lower speed. In the series mode the fans are grounded through ground G104.

Low Speed Operation (Northstar)
Low speed cooling fan operation will occur when the PCM grounds Cooling Fan 1 Relay's coil. With only Cooling Fan 1 Relay energized, voltage is applied to both the LH and RH Cooling Fan Motors. The voltage path to the motors is through the Cooling Fan 1 Relay's contacts, and the Cooling Fan 2 Relay's contacts. The two motors are connected to each other through a series circuit. This causes the voltage being applied by MAXI Fuse 6 to be divided between the two motors. The division of the voltage causes the motors to operate at a lower speed. In the series mode the fans are grounded through G107.

High Speed Conditions (4.9L)
The PCM will turn on the cooling fans at high speed when the engine coolant temperature exceeds 108 degrees C or the Engine Coolant Temperature Sensor circuit has failed. The PCM will switch the cooling fans to low speed operation once the engine coolant temperature falls below 106 degrees C.

The Heater and A/C Programmer will request the PCM to turn on the cooling fans at high speed through the UART data line if the A/C high side refrigerant temperature exceeds 70 degrees C or an A/C High Side Temperature Sensor circuit failure occurs. The Heater and A/C Programmer will request that the cooling fans be switched to low speed operation when the A/C high side refrigerant temperature falls below 59 degrees C.

High Speed Conditions (Northstar)
The PCM will turn on the cooling fans at high speed when the engine coolant temperature exceeds 110 degrees C or the Engine Coolant Temperature Sensor circuit has failed. The PCM will switch the cooling fans to low speed operation once the engine coolant temperature falls below 105 degrees C.

The PCM will also activate high speed operation if the transaxle temperature exceeds 120 degrees C. The PCM will switch to low speed operation once the transaxle temperature falls below 115 degrees C.

The PCM will turn on the cooling fans at high speed if a low coolant level is detected and engine coolant temperature exceeds 85 degrees C. The PCM will switch to low speed operation once the engine coolant temperature drops below 80 degrees C.

The Heater and A/C Programmer can request the PCM to turn on the cooling fans at high speed through the UART data line.

High Speed Operation (4.9L)
The cooling fans run at high speed when Cooling Fan 1, 2 and 3 Relays are energized. The PCM energizes the relays by grounding the relay coils. With Cooling Fan 1 Relay energized, battery voltage is applied to the Pusher Cooling Fan Motor through the relay contacts. The Pusher Cooling Fan Motor runs at high speed because now the fan is grounded directly through the contacts of the Cooling Fan 2 Relay.

Since Cooling Fan 3 Relay is also energized during high speed operation, battery voltage is applied directly to the Puller Cooling Fan Motor through the closed contacts of the relay. The Puller Cooling Fan Motor will run at high speed since it is no longer in series with the Pusher Cooling Fan Motor.

High Speed Operation (Northstar)
The cooling fans run at high speed when Cooling Fan 1, 2, and 3 Relays are grounded by the PCM. With Cooling Fan 1 Relay energized, battery voltage is applied to the LH Cooling Fan Motor through the relay contacts. The LH Cooling Fan Motor runs at high speed because now the fan is grounded directly through the contacts of cooling fan 2 relay and ground G109.

Since Cooling Fan 3 Relay is also energized during high speed operation, battery voltage is applied directly to the RH Cooling Fan Motor through the closed contacts of the relay. The fan will run at high speed since it is no longer in series with the LH Cooling Fan Motor.