Circuit Test XB - Variable Control Relay Module (VCRM)

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)

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) activated during engine start-up and 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 that the high speed pump is requested. With the FP circuit grounded, the VCRM activates the fuel pump output driver as in the low speed operation. With the high speed pump requested, the VCRM will also ground its HFP output closing 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.

A/C Clutch Operation 

When the A/C demand switch is turned on and the cyclic pressure switch is closed, voltage is supplied to the VCRM (Pin 9) and to the ACCS circuit (Pin 10). The voltage on the ACCS circuit indicates to the PCM that A/C is requested. The PCM will verify that A/C clutch operation is desireable (ex. engine not cranking or over heated, not at WOT, A/C head pressure OK, etc.). If A/C clutch operation is desireable, the PCM will adjust idle speed as necessary and unground the WAC output, which signals to the VCRM that the A/C clutch may be activated. The VCRM will then supply voltage to Pin 14 activating the A/C clutch. During A/C operation, if the PCM determines that A/C clutch operation is not desireable, the WAC output will be grounded and the VCRM will turn off the A/C clutch. Under normal conditions, with the A/C off, the PCM will ground the WAC output.

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

Pin	Pin Usage	Wire Color
1	Power Ground	BK
2	Power-to-Cooling Fan	LB
3	Power-to-Cooling Fan	LB
4	B(+)	Y
5	B(+)	Y
6	B(+)	Y
7	VPWR	R
8	Ground	BK/W
9	A/C Cyclic Switch	BK/Y
10	High Fuel Pump	BR/W
11	WOT A/C Cut-Off	PK/Y
12	ACP SIG RTN	LB/R

Pin	Pin Usage	Wire Color
13	A/C Clutch Return	GY/W
14	Power-to-A/C Clutch	P
15	A/C Clutch Ground	BK
16	Ground	BK/Y
17	IGN Start/Run	R/LG
18	Fuel Pump Circuit	LB/O
19	ACP Signal	DG/W
20	ACP VREF	O/LB
21	DATA(+)	T/O
22	Ground	BK/W
23	DATA(-)	PK/LB
24	Power-to-Fuel Pump	PK/BK

XB1 CHECK BATTERY VOLTAGE 

• Key on, engine off
• Measure voltage across battery terminals

Is voltage greater than 10.5 volts?

Yes	No
GO to XB2	SERVICE discharged battery. REFER to Charging/Electrical Group in Service Information

XB2 PCM GROUND FAULT ISOLATION 

• Key off
• Breakout box installed, PCM connected
• Measure voltage between B(+) post of the battery and SIG RTN (GY/R wire) in the Data Link Connector (DLC)

Is voltage greater than 10.5 volts and within 1.0 volt of the reading in XB1 ?

Yes	No
GO to XB10 (PCM ground circuits OK).	GO to XB3 (to check PCM ground circuits).

XB3 CHECK SIG RTN CONTINUITY 

• Key off
• Breakout box installed
• Disconnect PCM
• Measure resistance between Test Pin 46 and SIG RTN in DLC

Is resistance less than 5.0 ohms?

Yes	No
GO to XB4	SERVICE open in SIG RTN circuit. REMOVE breakout box, RECONNECT PCM. RERUN Quick Test

XB4 CHECK PCM POWER GROUND CIRCUITS 

• Key off
• Breakout box installed, PCM disconnected
• Measure resistance between the negative battery post, and Test Pins 40 and 60 at the breakout box

Is resistance less than 5.0 ohms?

Yes	No
GO to XB5	SERVICE open PWR GND circuit. REMOVE breakout box, RECONNECT PCM. RERUN Quick Test

XB5 CHECK INTERNAL PCM GROUND CIRCUITS 

• Key off
• Breakout box installed
• Reconnect PCM
• Measure resistance between Test Pin 46 and Test Pins 40 and 60 at the breakout box

Is resistance less than 5.0 ohms?

Yes	No
System Tests OK. VERIFY correct test results. REMOVE breakout box. RECONNECT PCM. RERUN Quick Test	REPLACE PCM. REMOVE breakout box. RERUN Quick Test

XB10 CHECK VPWR CIRCUIT CONTINUITY BETWEEN PCM AND VCRM 

• Key off
• Breakout box installed, PCM disconnected
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Measure resistance between Pin 7 of the VCRM vehicle harness connector and Test Pins 37 and 57 at the breakout box

Is resistance less than 5.0 ohms?

Yes	No
GO to XB11	SERVICE open in VPWR circuit between PCM and VCRM. REMOVE breakout box. RECONNECT all components. RERUN Quick Test

XB11 CHECK FOR B(+) AND IGN START/RUN VOLTAGE TO VCRM 

• Key off
• Breakout box installed, PCM disconnected
• VCRM disconnected
• Measure voltage between Pin 6 of the VCRM vehicle harness connector and the battery negative post
• Key on
• Measure voltage between Pin 17 of the VCRM vehicle harness connector and the battery negative post

Are both voltages greater than 10.5 volts?

Yes	No
Key off. GO to XB12	Key off. SERVICE open in B(+) or IGN START/RUN circuit. REMOVE breakout box. RECONNECT all components. RERUN Quick Test

XB12 CHECK VCRM GROUND (PIN 16) 

• Key off
• Breakout box installed, PCM disconnected
• VCRM disconnected
• Measure resistance between Pin 16 of the VCRM vehicle harness connector and the battery negative post

Is resistance less than 5.0 ohms?

Yes	No
REPLACE VCRM. REMOVE breakout box. RECONNECT all components. RERUN Quick Test	SERVICE open GND circuit. REMOVE breakout box. RECONNECT all components. RERUN Quick Test

XB15 CONTINUOUS MEMORY DIAGNOSTIC TROUBLE CODE (DTC) 584: CLEAR AND ATTEMPT TO REGENERATE DTC 

Continuous Memory DTC 584 indicates the VCRM PWR GND (Pin 1) is open. This could be due to a hard fault or an intermittent condition.

Possible causes:

• Open VCRM PWR GND circuit.
• Damaged VCRM

• Clear Continuous Memory
• Start engine
• Turn A/C on, wait 15 seconds
• A/C off
• Engine off
• Rerun Key On Engine Off (KOEO) Self-Test

Is Continuous Memory DTC 584 present?

Yes	No
DTC 584 is a hard fault. GO to XB16	DTC 584 is intermittent. GO to XB18

XB16 CHECK VCRM PWR GND CIRCUIT CONTINUITY 

• Key off
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Measure resistance between Pin 1 of the VCRM vehicle harness connector and the battery negative post

Is resistance less than 5.0 ohms?

Yes	No
REPLACE VCRM. RECONNECT all components. CLEAR Continuous Memory. START engine. A/C on, WAIT 15 seconds. Key off, A/C off. RERUN Quick Test	SERVICE open in VCRM PWR GND circuit. RECONNECT all components. CLEAR Continuous Memory. START engine. A/C on, WAIT 15 seconds. Key off, A/C off. RERUN Quick Test

XB18 CHECK VCRM PWR GND CIRCUIT 

• Key off
• Visually check the VCRM PWR GND circuit (VCRM Pin 1) from the VCRM to the circuit's ground point. The VCRM PWR GND circuit is grounded to the RH side radiator support to a weld nut
• Start engine
• A/C on (to turn cooling fan on).
  CAUTION: While performing this test, do not touch any moving/hot engine parts.
• Shake, wiggle, bend the VCRM PWR GND circuit, between the VCRM and the circuit's ground point. An open circuit will be indicated by the fan speed increasing to 100% (although the open must be present for at least 5 seconds).
• Lightly tap on VCRM to simulate road shock

Is a fault indicated?

Yes	No
ISOLATE fault and SERVICE as necessary. CLEAR Continuous memory (DTC 584 should have been set). START engine. A/C on, WAIT 15 seconds. Key off, A/C off. RERUN Quick Test	Key off, A/C off. GO to XB19

XB19 CHECK VCRM PWR GND CIRCUIT 

• Key off
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Connect a test lamp between Pin 5 and Pin 1 of the VCRM vehicle harness connector
• Observe test lamp for an indication of a fault while performing the following (the lamp will go out when a fault is found, indicating an open):
  • Shake, wiggle, bend the VCRM PWR GND circuit between the VCRM vehicle harness connector and the circuit's ground point

Is fault indicated?

Yes	No
ISOLATE fault and SERVICE as necessary. RECONNECT all components. START engine. A/C on, WAIT 15 seconds. A/C off, key off. RERUN Quick Test	Unable to isolate fault at this time. RECONNECT all components. For further diagnosis using the EEC-IV Monitor Box or Scan Tool, GO to appropriate article. For further diagnosis of this, or any other symptom, RETURN to appropriate article

XB20 CONTINUOUS MEMORY DIAGNOSTIC TROUBLE CODE (DTC) 581: CLEAR AND ATTEMPT TO REGENERATE DTC 

Continuous Memory DTC 581 indicates that when the cooling fan was activated, the Power-to-Cooling Fan circuit exceeded the normal current draw. This could be due to a hard fault or an intermittent condition.

Possible causes:

• Power-to-Cooling Fan circuit short to ground.
• Damaged cooling fan motor.
• Damaged VCRM.

• Clear Continuous Memory
• Start engine
• Turn A/C on, wait 15 seconds
• A/C off.
• Engine off
• Rerun KOEO Self-Test

Is Continuous Memory DTC 581 present?

Yes	No
DTC 581 is a hard fault. GO to XB21	DTC 581 is intermittent. GO to XB24

XB21 CHECK COOLING FAN MOTOR 

• Clear Continuous Memory again
• Disconnect cooling fan motor
• Start engine
• Turn A/C on, wait 15 seconds (but no more than 20 seconds).
• A/C off
• Engine off
• Rerun KOEO Self-Test

Is Continuous Memory DTC 581 present (disregard any other codes)?

Yes	No
GO to XB22	REPLACE cooling fan motor. RECONNECT all components. RERUN Quick Test

XB22 CHECK POWER-TO-COOLING FAN CIRCUIT FOR SHORT TO GROUND 

• Key off
• Cooling fan motor disconnected
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Measure resistance between the power-to-cooling fan circuit at the cooling fan vehicle harness connector and the battery negative post
• Is resistance greater than 10, 00 ohms?

Yes	No
REPLACE VCRM. RECONNECT all components. CLEAR Continuous Memory. START engine. TURN A/C on, wait 15 seconds. A/C off, key off. RERUN Quick Test	SERVICE power-to-cooling fan circuit short to ground. RECONNECT all components. CLEAR Continuous Memory. START engine. TURN A/C on, WAIT 15 seconds. A/C off, key off. RERUN Quick Test

XB24 CHECK POWER-TO-COOLING FAN CIRCUIT FOR INTERMITTENT SHORT TO GROUND 

• Key off
• Visually check the power-to-cooling fan circuit from the VCRM to the cooling fan
• Start engine
• A/C on (to turn cooling fan on).
  CAUTION: While performing this test, do not touch any moving/hot engine parts
• Shake, wiggle, bend the power-to-cooling fan circuit from the VCRM to the cooling fan (as close as practical). A short to ground will be indicated by the fan turning off (the VCRM FMEM will turn the fan off when an over current condition exists).
• Key off. A/C off

Is a fault indicated?

Yes	No
ISOLATE fault and SERVICE as necessary. CLEAR Continuous Memory (DTC 581 should have been set). START engine. A/C on, WAIT 5 seconds. A/C off, key off. RERUN Quick Test	GO to XB25

XB25 CHECK VCRM CONNECTOR AND POWER-TO-COOLING FAN CIRCUIT 

• Key off
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Connect a test lamp between Pin 2 and Pin 4 of the VCRM vehicle harness connector
• Observe test lamp for an indication of a fault while performing the following: (The lamp will be on, but not to full brightness. A short to ground will be indicated by the lamp getting brighter.)
  • Shake, wiggle, bend the power-to-cooling fan circuit near the cooling fan.
• Disconnect cooling fan. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary.
• Test lamp still connected between Pin 2 and Pin 4 of the VCRM vehicle harness connector
• Observe test lamp for an indication of a fault while performing the following (the lamp will turn on, indicating a short to ground):
  • Again, shake, wiggle, bend the power-to-cooling fan circuit from the VCRM vehicle harness connector to the cooling fan vehicle harness connector.

Is a fault indicated?

Yes	No
ISOLATE fault and SERVICE as necessary. RECONNECT all components. START engine. A/C on, WAIT 5 seconds. A/C off, key off. RERUN Quick Test.	Unable to isolate fault at this time. RECONNECT all components. For further diagnosis using the EEC-IV Monitor Box or Scan Tool, GO to appropriate article. For further diagnosis of this, or any other symptom, RETURN to appropriate article

XB30 KEY ON ENGINE OFF DIAGNOSTIC TROUBLE CODE (DTC) 582: VERIFY COOLING FAN IS NOT ALWAYS ON 

Key On Engine Off (KOEO) DTC 582 indicates an open or short to power in the power-to-cooling fan circuit.

Possible causes:

• Open or short to power in the power-to-cooling fan circuit (harness).
• Open cooling fan motor ground circuit
• Open circuit in cooling fan
• Damaged VCRM
  NOTE: If KOEO DTC 587 is also present, refer to the EEC-IV DTC charts to address the 587 first. Also verify that the cooling fan was not turning during KOEO Self-Test. If the fan was turning, back voltage could be sent to the VCRM through the power-to-cooling fan circuit resulting in an invalid 582 DTC

• Engine cooled down (so cooling fan would normally be off).
• A/C and defroster off
• Key on, engine off

Is cooling fan on?

Yes	No
Key off. GO to XB35 (to check for short to power).	Key off. GO to XB31 (to check for opens).

XB31 CHECK FOR VOLTAGE TO COOLING FAN 

• Key off
• Disconnect cooling fan
• Start engine
• Turn A/C on
• Measure voltage between the power-to-cooling fan circuit and ground circuit at the cooling fan vehicle harness connector

Is voltage greater than 6.0 volts?

Yes	No
Key off. REPLACE cooling fan. RERUN Quick Test	Key off. GO to XB32

XB32 CHECK COOLING FAN GROUND CIRCUIT CONTINUITY 

• Key off
• Cooling fan disconnected
• Measure resistance between the ground circuit at the cooling fan vehicle harness connector and the battery negative post

Is resistance less than 5.0 ohms?

Yes	No
GO to XB33	SERVICE open in cooling fan ground circuit. RECONNECT cooling fan. RERUN Quick Test

XB33 CHECK POWER-TO-COOLING FAN CIRCUIT CONTINUITY 

• Key off
• Cooling fan disconnected
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Measure resistance between the power-to-cooling fan circuit at the cooling fan vehicle harness connector and Pins 2 and 3 of the VCRM vehicle harness connector

Is resistance less than 5.0 ohms?

Yes	No
REPLACE VCRM. RECONNECT all components. RERUN Quick Test	SERVICE open in power-to-cooling fan circuit. RECONNECT all components. RERUN Quick Test

XB35 CHECK POWER-TO-COOLING FAN CIRCUIT FOR SHORT TO POWER 

• Key off
• Disconnect cooling fan
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Key on, engine off
• Measure voltage between the power-to-cooling fan circuit at the cooling fan vehicle harness connector and the battery negative post

Is voltage less than 1.0 volt?

Yes	No
REPLACE VCRM. RECONNECT all components. RERUN Quick Test	SERVICE short to power in the power-to-cooling fan circuit. RECONNECT all components. RERUN Quick Test

XB40 CONTINUOUS MEMORY DIAGNOSTIC TROUBLE CODE (DTC) 578: CLEAR AND ATTEMPT TO REGENERATE DTC 

Continuous Memory DTC 578 indicates the A/C Pressure (ACP) sensor VREF circuit is short to ground. This could be due to a hard fault or an intermittent condition

Possible causes:

• ACP sensor VREF circuit short to ground
• Damaged ACP sensor
• Damaged VCRM

• Clear Continuous Memory
• Start engine, wait 15 seconds
• Engine off
• Rerun Key On Engine Off (KOEO) Self-Test

Is Continuous Memory DTC 578 present?

Yes	No
DTC 578 is a hard fault. GO to XB41	DTC 578 is intermittent. GO to XB44

XB41 CHECK ACP SENSOR 

• Clear Continuous Memory again
• Key off
• Disconnect ACP sensor
• Start engine, wait 15 seconds
• Engine off
• Rerun KOEO Self-Test

Is Continuous Memory DTC 578 present (disregard any other codes)?

Yes	No
GO to XB42	REPLACE ACP sensor. RECONNECT all components. CLEAR Continuous Memory. START engine, WAIT 15 seconds. Key off. RERUN Quick Test

XB42 CHECK ACP VREF CIRCUIT FOR SHORT TO GND 

• Key off
• ACP sensor disconnected
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Measure resistance between the ACP VREF circuit at the ACP sensor vehicle harness connector and the battery ground post

Is resistance greater than 10, 00 ohms?

Yes	No
REPLACE VCRM. RECONNECT all components. CLEAR Continuous Memory. START engine, WAIT 15 seconds. Key off. RERUN Quick Test	SERVICE ACP VREF circuit short to ground. RECONNECT all components. CLEAR Continuous Memory. START engine, WAIT 15 seconds. Key off. RERUN Quick Test

XB44 CHECK ACP VREF FOR INTERMITTENT SHORT TO GROUND 

• Key off
• Visually check the ACP VREF circuit from the ACP sensor to the VCRM
• Disconnect the ACP sensor. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Disconnect VCRM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary
• Connect test lamp between Pin 6 and Pin 20 of the VCRM vehicle harness connector
• Observe test lamp for an indication of a fault while performing the following (the lamp will come on when a fault is found, indicating a short to ground):
  • Shake, wiggle, bend the ACP VREF circuit from the ACP sensor vehicle harness connector and the VCRM vehicle harness connector

Is a fault indicated?

Yes	No
ISOLATE fault and SERVICE as necessary. RECONNECT all components. START engine, WAIT 15 seconds. Key off. RERUN Quick Test	Unable to isolate fault at this time. RECONNECT all components. For further diagnosis using the EEC-IV Monitor Box or Scan Tool, GO to appropriate article. For further diagnosis of this, or any other symptom, RETURN to appropriate article

XB50 CONTINUOUS MEMORY DIAGNOSTIC TROUBLE CODE (DTC) 579: CHECK ACP VREF AND ACP SIG RTN CIRCUITS 

Continuous Memory DTC 579 indicates the A/C Pressure (ACP) sensor did not detect a sufficient change in A/C system pressure when the A/C was turned on or the ACP circuit is above maximum voltage. This could be due to a problem in the ACP sensor circuits or the A/C system, and could be a hard fault or an intermittent concern

Possible causes:

• Open or shorted ACP sensor circuit
• A/C system mechanical concern
• Damaged ACP sensor
• Damaged VCRM
• Power-to-A/C clutch circuit open (DTC 586 also present)

• Key off
• Disconnect ACP sensor
• Key on
• Measure voltage between the ACP VREF and the ACP SIG RTN circuits at the ACP sensor vehicle harness connector
• Key off

What was the voltage?

Greater Than 6.0 volts	Less Than 4.0 volts	Between 4.0 And 6.0 volts
GO to XB62 (to check ACP VREF for short to power).	GO to XB63 (to check ACP VREF and ACP SIG RTN circuits).	GO to XB51 (ACP VREF and ACP SIG RTN circuits OK to sensor).

XB51 CHECK IF A/C CLUTCH WILL ENGAGE (ACP SENSOR CONNECTED) 

• Key off
• Reconnect ACP sensor
• Start engine
• Turn A/C on, wait 30 seconds

Does the A/C clutch "click" on?

Yes	No
Key off, A/C off. GO to XB52 (ACP circuit is not open or short to power).	Key off, A/C off. GO to XB57 (to check if ACP circuit is OK).

XB52 CLEAR AND ATTEMPT TO REGENERATE DTC 

• Clear Continuous Memory
• Attempt to reset DTC 579 by performing the following:
  1. Start engine
  2. A/C on, wait 20 seconds after A/C clutch engagement is heard ("click").
  3. A/C off, wait 5 seconds
  4. Repeat steps 2 and 3 four more times (a total of five A/C clutch engagements, with each engagement lasting about 20 seconds, must be seen by the VCRM during the same power-up to set DTC 579 for insufficient A/C pressure change).
• A/C off
• Engine off
• Rerun Key On Engine Off (KOEO) Self-Test

Is Continuous Memory DTC 579 present?

Yes	No
DTC 579 is a hard fault. GO to XB53	The ACP sensor is now able to detect an A/C system pressure change. Either an intermittent ACP sensor circuit fault exists, or an A/C system intermittent or marginal mechanical condition exists. GO to XB66

XB53 CHECK A/C SYSTEM PRESSURE AND PRESSURE CHANGE 

• Key off
• Install A/C System Manifold Gauge Set. Refer to the Climate Control Group of the Service Information for installation instructions
• A/C off
• Start engine
• Note the A/C high pressure gauge reading
• While monitoring the A/C high pressure gauge reading, turn the A/C on. Ten seconds after clutch engagement, note the pressure (the pressure should increase).
• Key off
• A/C off

Did the A/C high pressure gauge reading change more than 20 psi within 10 seconds of clutch engagement?

Yes	No
GO to XB54 (the A/C pressure changed as expected. ACP sensor and circuit will now be checked).	A/C system pressure did not change as expected. GO to the Climate Control Group of the Service Manual to check for A/C system mechanical concerns. After any service, CLEAR Continuous Memory. ATTEMPT to reset DTC 579 (refer to XB52 for procedure only). RERUN Quick Test

XB54 CHECK ACP SENSOR OUTPUT 

• Key off
• Disconnect ACP sensor
• A/C system Manifold Gauge Set installed
• Jumper the ACP SIG RTN circuit between the ACP sensor vehicle harness connector and the ACP sensor connector
• Jumper the ACP VREF circuit between the ACP sensor vehicle harness connector and the ACP sensor connector
• Jumper the ACP signal circuit at the ACP sensor vehicle harness connector to the battery negative post
• Connect a DVOM between the ACP signal circuit at the ACP sensor connector and the ACP SIG RTN circuit jumper wire
• A/C off
• Start engine
• Note the A/C high pressure gauge reading and the ACP sensor voltage
• While monitoring the ACP sensor voltage, turn the A/C on. Ten seconds after clutch engagement, note the voltage
• A/C off
• Key off

Did the ACP sensor voltage change more than 0.2 volts within 10 seconds of turning the A/C on?