For 1990-2009 cars

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. Verifies that the body control module (BCM) is providing ground to the horn relay.

2. Tests for voltage at the battery positive voltage terminal of the horn relay coil.

3. Tests for voltage at the battery positive voltage terminal of the horn relay switch. The HORN fuse supplies power to the battery positive voltage terminal ring and the horn switch.

4. The horns need maximum current flow to operate resistance, an open or a short to voltage when properly. A high resistance, greater than testing the relay control circuit. Refer to circuit 0.5 ohm, in the horn control circuit or the horn ground circuit could cause operating problems with the horns. Inspect the circuits for faults that would restrict current flow.

5. The horn relay control circuit includes the horn slip ring and the horn switch. The horn slip ring and the horn switch must be tested for a high resistance.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Check the vehicle with the scan tool. | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Is the vehicle equipped with keyless entry? | -- | Go to Step 3 | Go to Step 4
3 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. With a scan tool, command the Body Control Module Horn Output ON and OFF.

Does the horn operate?

--

Go to Step 4

Go to Step 5

4 | Depress the horn switch. Does the horn operate? | -- | Go to Testing for Intermittent Conditions and Poor Connections | Go to Step 6
5 | Test the body control module (BCM) portion of the horn relay control circuit for a high resistance or an open. Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 14
6 |

1. Turn OFF the ignition.
2. Disconnect the horn relay.
3. Connect a test lamp between the battery positive voltage circuit of the horn relay coil and a good ground.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 17

7 |

1. Connect a test lamp between the battery positive voltage circuit of the horn relay coil and the horn relay control circuit.
2. Depress and hold the horn switch.

Does the test lamp illuminate?

--

Go to Step 8

Go to Step 18

8 | Connect a test lamp between the battery positive voltage circuit of the horn relay switch and a good ground. Does the test lamp illuminate? | -- | Go to Step 9 | Go to Step 19
9 | Momentarily connect a 15-amp fused jumper wire between the battery positive voltage circuit of the horn relay switch and the horn control circuit. Does the horn operate? | -- | Go to Testing for Intermittent Conditions and Poor Connections | Go to Step 10
10 |

1. Reconnect the horn relay.
2. Disconnect the horn connector.
3. Connect a test lamp between the horn control circuit and a good ground.
4. Depress and hold the horn switch.

Does the test lamp illuminate?

--

Go to Step 11

Go to Step 20

11 |

1. Connect a 15-amp fused jumper wire between the positive battery terminal and the control terminal of the horn.
2. Connect another jumper wire between the negative battery terminal and the ground terminal of the horn.

Does the test lamp illuminate?

--

Go to Step 12

Go to Step 21

12 | Inspect for poor connections at the horn. Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 13
13 |

1. Connect a test lamp between the horn control circuit and the horn ground circuit.
2. Depress and hold the horn switch.

Does the horn operate?

--

Go to Step 16

Go to Step 24

14 | Check for poor connections at the harness connector of the BCM. Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 22
15 | Repair the high resistance in the horn control circuit or the horn ground circuit. A resistance as low as 0.5 ohms could cause operating problems with the horns. Did you complete the repair? | -- | Go to Step 25 | --
16 | Repair the open or high resistance in the battery positive voltage circuit of the horn relay coil. Did you complete the repair? | -- | Go to Step 25 | --
17 | Repair the open or high resistance of the horn relay control circuit. Did you complete the repair? | -- | Go to Step 25 | --
18 | Repair the open or high resistance in the battery positive voltage circuit of the horn relay switch. Did you complete the repair? | -- | Go to Step 25 | --
19 | Repair the open or high resistance in the horn control circuit. Did you complete the repair? | -- | Go to Step 25 | --
20 | Repair the open or high resistance in the horn ground circuit. Did you complete the repair? | -- | Go to Step 25 | --
21 | Important: Perform the BCM programming procedure when replacing the BCM. Replace the BCM. Refer to Control Module References Did you complete the repair? | -- | Go to Step 25 | --
22 | Replace the horn relay. Did you complete the replacement? | -- | Go to Step 25 | --
23 | Replace the horn. Did you complete the replacement? | -- | Go to Step 25 | --
24 | Operate the horn. Does the horn operate properly? | -- | -- | Go to Step 1