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Inspection Procedure 11: ASC-ECU power supply circuit system.
 
 
caution
  • If the battery terminal is not tightened properly, a dump surge may occur and the power supply voltage may become abnormally high for a short time.
  • Whenever ECU is replaced, ensure that the CAN bus lines are normal.
  • When the hydraulic unit (integrated with ASC-ECU) is replaced, always carry out the calibration of the steering wheel sensor and the G and yaw rate sensor (Refer to and ).
 
 
OPERATION
  • The ASC-ECU contains the power supply circuit (+BV terminal) for the solenoid valve. The solenoid valve is energized by the valve relay, which is incorporated in the ASC-ECU.
  • The ASC-ECU contains the power supply circuit (+BM terminal) for the pump motor. The pump motor is energized by the motor relay, which is incorporated in the ASC-ECU.
  • The ASC-ECU contains the power supply circuit (IG1 terminal) for itself. When the ignition switch (IG1) is turned ON, the voltage is applied to the relay incorporated in the ETACS-ECU to turn ON the relay, and the power is supplied from the fusible link through the multi-purpose fuse.
  • When malfunction occurs in ASC-ECU power supply, the communication with scan tool MB991958 becomes unavailable.
 
 
PROBABLE CAUSES
  • The CAN bus line is defective.
  • Battery failure
  • Improper tightening of battery terminal
  • Charging system failed
  • Damaged harness wires and connectors
  • Fuse and fusible link malfunction
  • Improper tightening of ground bolt
  • Malfunction of ASC-ECU
 
 
DIAGNOSIS
STEP 1. Check the battery.
Refer to GROUP 54A - Battery Test .
Is the battery in good condition?
Go to Step 3.
Charge or replace the battery. Then go to Step 2.
 
STEP 2. Check the charging system.
Refer to GROUP 16 - Charging System .
Is the charging system in good condition?
Go to Step 3.
Repair or replace the charging system component(s).
 
STEP 3. Measure the resistance at the ASC-ECU connector.
(1) Connector check: the ASC-ECU connector, the fuses (relay box) and the fusible links
(2) Disconnect the ASC-ECU connector.
(3) Measure the resistance between +BV terminal and body ground.
OK: No continuity
Is the check result normal?
Go to Step 4.
The power supply circuit may be short. Repair the wiring harness wires or connectors between the ACS-ECU connector +BV terminal and the fuse (relay box), the fuses (relay box) or the fusible links. Then go to Step 11.
 
STEP 4. Measure the resistance at the ASC-ECU connector.
(1) Connector check: the ASC-ECU connector and the fusible links
(2) Disconnect the ASC-ECU connector and remove the fusible links.
(3) Measure the resistance between +BM terminal and body ground.
OK: No continuity
Is the check result normal?
Go to Step 5.
The power supply circuit may be short. Repair the wiring harness wires or connectors between the ACS-ECU connector +BM terminal and the fusible links, or the fusible links. Then go to Step 11.
 
STEP 5. Measure the resistance at the ASC-ECU connector.
(1) Connector check: the ASC-ECU connector and the junction block
(2) Disconnect the ASC-ECU connector.
(3) Measure the resistance between the IG1 terminal and body ground.
OK: No continuity
Is the check result normal?
Go to Step 6.
The power supply circuit may be short. Repair the wiring harness wires or connectors between the ACS-ECU connector IG1 terminal and the junction block. Then go to Step 11.
 
STEP 6. Measure the voltage at the ASC-ECU connector.
(1) Disconnect the ASC-ECU connector.
(2) Measure the voltage between +BV terminal and body ground.
OK: Battery voltage
Is the check result normal?
Go to Step 7.
The power supply circuit may be open. Repair the wiring harness wires or connectors between the ACS-ECU connector +BV terminal and the fuse (relay box) or the fuses (relay box). Then go to Step 11.
 
STEP 7. Measure the voltage at the ASC-ECU connector.
(1) Disconnect the ASC-ECU connector.
(2) Measure the voltage between +BM terminal and body ground.
OK: Battery voltage
Is the check result normal?
Go to Step 8.
The power supply circuit may be open. Repair the wiring harness wires or connectors between the ACS-ECU connector +BM terminal and the fusible links, or the fusible links. Then go to Step 11.
 
STEP 8. Measure the voltage at the ASC-ECU connector.
(1) Disconnect the ASC-ECU connector.
(2) Ignition switch: "ON"
(3) Measure the voltage between the IG1 terminal and body ground.
OK: Battery voltage
Is the check result normal?
Go to Step 9.
The power supply circuit may be open. Repair the wiring harness wires or connectors between the ACS-ECU connector IG1 terminal and the junction block. Then go to Step 11.
 
STEP 9. Measure the resistance at the ASC-ECU connector.
(1) Disconnect the ASC-ECU connector.
(2) Measure the resistance between the GND1 terminal and body ground as well as between the GND2 terminal and body ground.
OK: Continuity exists (2 Ω or less).
Is the check result normal?
Go to Step 10.
The ground circuit may be open. Repair the wiring harness wires or connectors between the ACS-ECU connector GND1 terminal and the fusible links as well as between the GND2 terminal and the fusible links. Then go to Step 11.
 
STEP 10. Retest the system
Make sure that the scan tool MB991958 cable is properly connected and the V.C.I. switch is ON.
Is the communication with scan tool MB991958 possible?
The trouble can be an intermittent malfunction (Refer to GROUP 00 - How to Cope with Intermittent Malfunction ).
Replace the hydraulic unit (integrated with ASC-ECU) (Refer to ). Then go to Step 11.
 
STEP 11. Retest the system
Is the communication with scan tool MB991958 possible?
Return to Step 1.
This diagnosis is complete.