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Inspection Procedure 4: Horns do not sound while the theft-alarm system is triggered.
 
 
cautionBefore replacing the ECU, ensure that the power supply circuit, the ground circuit and the communication circuit are normal
 
 
 
 
TECHNICAL DESCRIPTION (COMMENT)
If horns do not sound, the horn input signal circuit or the ETACS-ECU may be defective.
 
 
TROUBLESHOOTING HINTS
  • Horns may be defective
  • Horn relay may be defective
  • The ETACS-ECU may be defective
  • The wiring harness or connectors may have loose, corroded, or damaged terminals, or terminals pushed back in the connector
 
 
DIAGNOSIS
Required Special Tools:
  • MB991223: Harness Set
  • MB992006: Extra Fine Probe
STEP 1. Using scan tool MB991958, read the diagnostic trouble code.
Check if DTC is set in the ETACS-ECU.
(1)
cautionTo prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.
Connect scan tool MB991958. Refer to "How to connect scan too (M.U.T.-III) ."
(2) Turn the ignition switch to the "ON" position.
(3) Check whether the ETACS-ECU related DTC is set.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is the DTC set?
Diagnose the ETACS-ECU. Refer to .
Go to Step 2.
 
STEP 2. Check which horn is defective.
Check which horn does not sound when the theft-alarm system is triggered.
Which horn does not sound?
Horn (HI) and horn (LO) : Go to Step 3.
Horn (HI) only : Go to Step 9.
Horn (LO) only : Go to Step 12.
 
STEP 3. Check horn relay connector A-19X for loose, corroded or damaged terminals, or terminals pushed back in the connector.
Is horn relay connector A-19X in good condition?
Go to Step 4.
Repair the damaged parts.
 
STEP 4. Check the horn relay.
Refer to .
Is the horn relay in good condition
Go to Step 5.
Replace the horn relay.
 
STEP 5. Measure the voltage at horn relay A-19X.
(1) Remove the relay, and measure at the relay box side.
(2) Measure the voltage between horn relay connector A-19X (terminal No. 2, 4) and the body ground.
  • The voltage should measure approximately 12 volts (battery positive voltage).
Is the measured voltage approximately 12 volts (battery positive voltage)?
Go to Step 7.
Go to Step 6.
 
STEP 6. Check the Wiring harness between horn relay connector A-19X (terminal No. 2,4) and fusible link (36).
Check the power supply lines for open circuit and short circuit.
Is the check result normal?
The trouble can be an intermittent malfunction (Refer to GROUP 00, How to use Troubleshooting/inspection Service Points - How to Cope with Intermittent Malfunction ).
Repair the wiring harness between theft-alarm horn relay connector A-19X and fusible link (36).
 
STEP 7. Check ETACS-ECU connector C-410 for loose, corroded or damaged terminals, or terminals pushed back in the connector.
Is ETACS-ECU connector C-410 in good condition?
Go to Step 8.
Repair the damaged parts.
 
STEP 8. Check the Wiring harness between horn relay connector A-19X (terminal No. 1) and ETACS-ECU connector C-410 (terminal No. 11).
Check the output line for open circuit and short circuit.
Is the check result normal?
Go to Step 9.
Repair the wiring harness between horn relay connector A-19X and ETACS-ECU connector C-410.
 
STEP 9. Check horn (HI) connector A-55 for loose, corroded or damaged terminals, or terminals pushed back in the connector.
Is horn (HI) connector A-55 in good condition?
Go to Step 10.
Repair the damaged parts.
 
STEP 10. Check the Wiring harness between horn (HI) connector A-55 (terminal No. 1) and horn relay connector A-19X (terminal No. 3).
Check the output line for open circuit and short circuit.
Is the check result normal?
Go to Step 11.
Repair the wiring harness between horn (HI) connector A-55 and horn relay connector A-19X.
 
STEP 11. Check the horn (HI) work normally.
Connect the battery as shown, and verify that the horn sounds.
Does horn (HI) work normally?
Go to Step 12.
Replace the horn (HI). Then, go to Step 12.
 
STEP 12. Check which horn is defective.
Check which horn does not sound when the theft-alarm system is triggered.
Which horn does not sound?
Horn (HI) and horn (LO) : Go to Step 13.
Horn (HI) only : Replace the ETACS-ECU.
Horn (LO) only : Go to Step 13.
 
STEP 13. Check horn (LO) connector A-51 for loose, corroded or damaged terminals, or terminals pushed back in the connector.
Is horn (LO) connector A-51 in good condition?
Go to Step 14.
Repair the damaged parts.
 
STEP 14. Check the Wiring harness between horn (LO) connector A-51 (terminal No. 1) and horn relay connector A-19X (terminal No. 3).
Check the output line for open circuit and short circuit.
Is the check result normal?
Go to Step 15.
Repair the wiring harness between horn (LO) connector A-51 and horn relay connector A-19X.
 
STEP 15. Check the horn (LO) work normally.
Connect the battery as shown, and verify that the horn sounds.
Does the horn (LO) work normally?
Go to Step 16.
Replace the horn (LO).go to Step 16.
 
STEP 16. Retest the theft-alarm system.
Does the theft-alarm system work normally?
The trouble can be an intermittent malfunction (Refer to GROUP 00, How to use Troubleshooting/inspection Service Points - How to Cope with Intermittent Malfunction ).
Replace the ETACS-ECU.