Variable Control Relay Module Operation Functions for 1996 Lincoln Mark VIII

Variable Control Relay Module

The variable control relay module is used to perform the following functions:

fuel pump control
engine cooling fan control
A/C clutch control
A/C head pressure control
supply vehicle power to the EEC-IV system and powertrain control module (PCM) (12A650)

These components interrelate through electronics as follows:

The variable control relay module controls fan operation when required and will increase or decrease the fan speed depending on the refrigerant system high-side pressure.
The variable control relay module also can turn the A/C on or off if the system high-side pressure exceeds 2930 kPa (425 psi) as indicated by the A/C pressure cut-off switch (19D594).

In addition to these functions:

The variable control relay module performs diagnostic checks of its system.
Information on any failure that is detected by the variable control relay module is sent through the data link connectors (14489) (Data+ and Data- circuits) to the powertrain control module.
This failure data is output as diagnostic trouble codes (DTCs) during EEC-IV Key On/Engine Off On-Board Diagnostic. Refer to Powertrain Control Emissions/Diagnosis Manual for EEC-IV Key On/Engine Off On-Board Diagnostic (Self-Test).
The variable control relay module is connected electrically to the powertrain control module, the A/C pressure cut-off switch, the A/C clutch field coil circuit and the fan control circuit.
Variable control relay module receives the A/C pressure reading from the A/C pressure cut-off switch.
This information is supplied to the powertrain control module through the data link connector and is used to help determine the desired engine cooling fan control speed.
The powertrain control module can also turn off A/C through the wide-open throttle A/C cutout signal, if the A/C head pressure has exceeded the safe limits of the system or if engine temperature is excessive.
The powertrain control module monitors certain engine functions (engine coolant temperature sensor (ECT sensor) (12A648), vehicle speed, A/C demand, A/C head pressure and other functions) to determine fan control needs.
The powertrain control module will send the desired fan speed to the variable control relay module through the data link connector.
The variable control relay module then adjusts the fan speed and outputs the appropriate voltage to the fan control (from approximately six volts to 12 volts depending on battery state of charge) through the fan control output driver with a pulse width-modulated duty cycle.
If the engine temperature exceeds the normal operating limit, the fan control speed modulates to try to reduce temperature and the A/C is shut off. If the vehicle speed exceeds 76 km/h (47 mph), the fan operation will be shut off.
The variable control relay module receives constant signals from the A/C pressure cut-off switch which monitors the refrigerant system high-side (discharge) pressure.
When the high side pressure is less than 1034 kPa (150 psi), the fan control speed is 1000 rpm.
As the pressure increases above 1034 kPa (150 psi), the variable control relay module begins to increase the fan control speed.
At 2034 kPa (295 psi), the fan control speed is 1800 rpm (full speed). If the high-side pressure reaches 2860-3068 kPa (415-445 psi), the variable control relay module will interrupt the A/C compressor circuit to stop A/C compressor operation while the fan control will continue to operate.
At 1517-1930 kPa (220-280 psi), the variable control relay module will close the A/C clutch circuit to resume A/C compressor operation.
The variable control relay module has the ability to stop fan control operation at vehicle speeds in excess of approximately 47 mph (76 km/h).
Fan operation will resume when vehicle speed drops to approximately 42 mph (68 km/h).
One exception is when the high-side pressure is above 2070 kPa (300 psi). The fan control will not shut off when the high side pressure is above 2070 kPa (300 psi).
In the event of an A/C pressure cut-off switch failure, the fan control will continue to operate.
In the event of a variable control relay module failure, the A/C pressure cut-off switch sensor will open at approximately 2860-3068 kPa (415-445 psi) to prevent damage to the A/C system components.