XB - Testing Notes

NOTE: You should enter this Pinpoint Test only when directed here from Quick Test or Pinpoint Test C or PC.

REMEMBER

This pinpoint test is intended to diagnose only the following:

- Harness circuits: All associated with VCRM.

- Variable Control Relay Module (VCRM) (12B581)

- Powertrain Control Module (PCM) (12A650)

DESCRIPTION

The Variable Control Relay Module (VCRM) is used to perform the following functions: fuel pump control; engine cooling fan control; A/C clutch control; A/C head pressure control; supply VPWR to the EEC-IV system and Powertrain Control Module (PCM). In addition to these functions, the VCRM performs diagnostic checks of its system. Information on any failure that is detected by the VCRM is sent via DCL (DATA+ and DATA- circuits) to the Powertrain Control Module (PCM). This failure information is output as Diagnostic Trouble Codes (DTC) during EEC-IV Self-Test. Each function is described in more detail below.

Fuel Pump Control

The Mark VIII uses a two speed fuel pump control strategy. The fuel pump receives either B(+) voltage (High Speed Fuel Pump), or a voltage less than B(+) (Low Speed Fuel Pump). The vehicle normally runs on the Low Speed Fuel Pump (FP), with the High Speed Fuel Pump (HFP) being activated during engine start-up and during certain high load conditions.

For low speed pump operation, the PCM grounds the Fuel Pump (FP) circuit. The VCRM receives this request for the low speed pump and activates the fuel pump output driver, sending voltage to the pump through the power-to-pump circuit. The power-to-pump circuit from the VCRM is designed so that the full B(+) voltage will not reach the pump, resulting in low speed pump operation.

For high speed pump operation, the PCM grounds the FP circuit and also sends a message, through DCL, indicating that the high speed pump is requested. With the FP circuit grounded, the VCRM will activate the fuel pump output driver, the same as is done in low speed operation. With the high speed pump requested the VCRM will also ground its HFP output, which closes the normally open contacts of the HFP relay. This sends full B(+) voltage to the pump, resulting in high speed pump operation.

Engine Cooling Fan Control

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 VCRM via DCL. The VCRM then adjusts the fan speed according to vehicle battery voltage, and outputs the appropriate voltage through the fan output driver with a pulse width modulated duty cycle.

A/C Head Pressure Control

The VCRM receives the A/C pressure reading from the A/C Head Pressure (ACP) sensor. This information is supplied to the PCM through the DCL, and is used to help determine the desired engine cooling fan speed. The PCM also has the ability to turn off A/C (through WAC) if the A/C head pressure has, or could, exceed the safe limits of the system. The A/C Cyclic Pressure Switch is also used to help control A/C pressure.

Vehicle Power

When the ignition switch is turned to the start or run position, the VCRM supplies Vehicle Power (VPWR) to the EEC system and the PCM.