Horns Malfunction
Horns Malfunction
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
The body control module (BCM) monitors the horn switch signal circuit, when the horn pad is pressed, the BCM detects the drop in the horn switch signal circuit and provides a ground for the Horn PCB relay within the underhood fuse block. The underhood fuse block will then provide B+ voltage to the horn control circuit, sounding the horns
Diagnostic Aids
A short to ground in the horn relay control circuit or a short to voltage in the horn control circuit will cause the horns to sound continuously until the horns overheat and become inoperative.
Rotate the steering wheel while pressing the horn pad to identify intermittent and poor connections within the steering column.
If diagnosing a Horn- Poor Tone condition, inspect the following:
* Debris or water in the horn assembly
* Proper horn mounting hardware torque-Refer to Fastener Tightening Specifications (Specifications).
* Debris in the joint where the horns attach to the vehicle
* Debris in direct contact with the horn
* Vehicle components vibrating while the horn is sounding
Reference Information
Schematic Reference
Horn Schematics (Electrical Diagrams)
Connector End View Reference
Component Connector End Views (Connector Views)
Description and Operation
Horns System Description and Operation (Description and Operation)
Electrical Information Reference
* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)
Scan Tool Reference
Control Module References (Programming and Relearning) for scan tool information
Circuit/System Verification
1. Ignition ON, observe the scan tool Horn Switch parameter while pressing and releasing the horn pad. The parameter should change from Inactive to Active.
• If the value does not change, refer to Horn Switch Malfunction.
2. Command the Horn Relay ON and OFF with the scan tool, the P12 horn should turn ON and OFF when changing between the commanded states.
• If the P12 horn is always ON or always OFF, refer to Horn Malfunction.
• If the sound emitted from the P12 horn is not clear and even, refer to Horn - Poor Tone.
Circuit/System Testing
Horn Switch Malfunction
1. Ignition OFF, disconnect the harness connector at the S33 horn switch.
2. Test for less than 30 ohm between the ground circuit terminal B and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, verify the scan tool Horn Switch parameter is Inactive.
• If not the specified value, test the signal circuit terminal A for a short to ground. If the circuit tests normal, replace the K9 BCM.
4. Install a 3 A fused jumper wire between the signal circuit terminal A and ground, verify the scan tool Horn Switch parameter is Active.
• If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 BCM.
5. If all circuits test normal, test or replace the S33 horn switch.
Horn Malfunction
1. Ignition OFF, disconnect the X4 harness connector at the X50A fuse block-underhood.
2. Connect a test lamp between the control circuit terminal K5 and B+.
3. Ignition ON, command the Horn Relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
• If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the K9 BCM.
• If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 BCM.
4. Ignition OFF, connect the X4 harness connector at the X50A fuse block-underhood. Disconnect the harness connector at the appropriate P12 horn.
5. Test for less than 5 ohm between the ground circuit terminal 1 and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
6. Connect a test lamp between the control circuit terminal 2 and ground.
7. Ignition ON, command the Horn Relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
• If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the X50A fuse block-underhood.
• If the test lamp is always OFF and the circuit fuse is good, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the X50A fuse block-underhood.
• If the test lamp is always OFF and the circuit fuse is open, test the control circuit for a short to ground. If the circuit tests normal, test or replace the X50A fuse block-underhood.
8. If all circuits test normal, test or replace the P12 horn.
Horn - Poor Tone
1. Ignition OFF, disconnect the harness connector at the appropriate P12 horn.
2. Test for less than 1 ohm between the ground circuit terminal 1 and ground.
• If greater than the specified range, test the ground circuit for high resistance.
3. Disconnect the X3 harness connector at the X50A fuse block-underhood.
4. Test for less than 1 ohm between the control circuit terminals listed below:
* Terminal C2 at the X50A fuse block-underhood and terminal 2 at the P12B horn-low note.
* Terminal C1 at the X50A fuse block-underhood and terminal 2 at the P12A horn-high note.
• If greater than the specified range, test the control circuit for high resistance.
5. If all circuits test normal, test or replace the P12 horn.
Component Testing
Horn Test
1. Ignition OFF, disconnect the harness connector at the appropriate P12 horn.
2. Install a 15 A fused jumper wire between the control terminal 2 and 12 V. Install a jumper wire between the ground terminal 1 and ground. Verify the P12 horn emits a clear and even tone.
• If the sound emitted is not clear and even, replace the P12 horn.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Steering Wheel Horn Contact Replacement (Service and Repair)
* Horn Replacement (Service and Repair)
* Accessory Wiring Junction Block Replacement (Accessory Wiring Junction Block Replacement)
* Control Module References (Programming and Relearning) for BCM replacement, setup and programming