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The VLCM receives constant signals from the A/C pressure sensor which monitors the refrigerant system high side (discharge) pressure. The signal from the A/C pressure sensor is sent to the powertrain control module through the data link connector (DLC):

When the high side pressure is less than 1034 kPa (150 psi), the cooling fan motor speed is 1000 rpm.
As pressure increases above 1034 kPa (150 psi), the VLCM begins to increase the cooling fan motor speed.
At 2034 kPa (295 psi), the cooling fan motor speed is 1800 rpm (full speed).
If the high side pressure reaches 2930 kPa (425 psi), the VLCM interrupts the A/C compressor circuit to stop A/C compressor operation while the fan control continues to operate.
At 1379 kPa (200 psi), the VLCM closes the A/C clutch circuit to resume A/C compressor operation.
The cooling fan motor is controlled by the VLCM and control signals from the powertrain control module, which are sent on the data link connector (DLC). Another feature of the VLCM is the ability to stop fan control operation at vehicle speeds of approximately 76 km/h (47 mph) and:

Cooling fan motor operation resumes when vehicle speed drops to approximately 68 km/h (42 mph).
One exception is when the high-side pressure is above 2069 kPa (300 psi).
The fan control does not shut off when the high-side pressure is above 2069 kPa (300 psi).
If an A/C pressure sensor failure occurs, the fan control continues to operate.

Variable Load Control Module Applications
The Mark VIII uses a variable speed cooling fan, which is controlled by the PCM and Variable Load Control Module (VLCM). The PCM monitors certain engine parameters (ECT sensor, vehicle speed, A/c demand, A/C head pressure, etc.) to determine engine cooling fan needs. The PCM will send the desired fan speed to the VLCM through the BUS(+) and BUS(-) circuits. The VLCM then adjusts the fan speed according to vehicle battey voltage and outputs the appropriate voltage through the fan output driver with a pulse width modulated duty cycle. The following cooling fan information is sent from the VLCM to the PCM on the BUS(+) and BUS(-) circuits: Confirmation of cooling fan operation; any cooling fan secondary circuit faults that are detected.