GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2007 🢒 XLR 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 Horns Inoperative

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.

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

    3.

  3. Tests for voltage at the battery positive voltage terminal of the horn relay switch.

    4.

  4. The horns need maximum current flow to operate properly. A high resistance, greater than 0.5ohms, the horn control circuit or the horn ground circuit could cause operating problems with the horns. Check the circuits for faults that would restrict current flow.

    5.

  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, an open or a short to voltage when testing the relay control circuit. Refer to Circuit Testing and Wiring Repairs .

Step

Action

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Depress the horn switch.

Do the horns operate?

Go to Step 3

Go to Step 5

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 (BCM) Horn Output ON and OFF.

Do the horns operate?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 4

4

Test the BCM portion of the horn relay control circuit for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 24

Go to Step 13

5

  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 6

Go to Step 16

6
  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 7

Go to Step 17

7

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 8

Go to Step 18

8

Momentarily connect a 15-amp fused jumper wire between the battery positive voltage circuit of the horn relay switch and the horn control circuit.

Do the horns operate?

Go to Step 14

Go to Step 9

9
  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 10

Go to Step 19

10
  1. Connect a test lamp between the horn control circuit and the horn ground circuit.
  2. Depress and hold the horn switch.

Does the test lamp illuminate?

Go to Step 11

Go to Step 20

11

Inspect for poor connections at the harness connector of the horn. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 24

Go to Step 12

12
  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.

Do the horns operate?

Go to Step 15

Go to Step 23

13

Inspect for poor connections at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 24

Go to Step 21

14

Inspect for poor connections at the horn relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 24

Go to Step 22

15

Repair the high resistance in the horn control circuit or the horn ground circuit. The horn control circuit includes the HORN fuse and a resistance as low as 0.5ohms could cause operating problems with the horns. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 24

--

16

Repair the open or high resistance in the battery positive voltage circuit of the horn relay coil. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 24

--

17

  1. To disable the SIR system, refer to SIR Disabling and Enabling .
  2. Repair the open or short to voltage in the horn relay control circuit. The horn relay control circuit includes the horn slip ring and the horn switch. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 24

--

18

Repair the open or high resistance in the battery positive voltage circuit of the horn relay switch. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 24

--

19

Repair the open or high resistance in the horn control circuit. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 24

--

20

Repair the open or high resistance in the horn ground circuit. The horn control circuit includes the HORN fuse. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 24

--

21

Replace the BCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the repair?

Go to Step 24

--

22

Replace the horn relay. Refer to Circuit Protection - Fusible Links .

Did you complete the replacement?

Go to Step 24

--

23

Replace the horns. Refer to Horn Replacement .

Did you complete the replacement?

Go to Step 24

--

24

Operate the horns.

Do the horns operate properly?

System OK

Go to Step 1