Fix a Horn that doesn’t work
If you’ve tried to beep your horn to alert somebody to get out of the way and your horn doesn’t work, read this post. It describes how many horn systems work and provides Horn doesn’t work along with troubleshooting steps.
First, understand that there are many ways to wire a horn in a modern vehicle. Click on the image below to download a PDF of three common methods. In an older vehicle, the horn button provides a path to ground for the control coil portion of the horn relay. Once the control coil is grounded, the relay contacts close and provide power to the horn.
In a more modern vehicle, the horn button still switches power to ground. However, instead of actually tripping the horn relay, the button acts as an input to the body control module. It’s the BCM that provides ground to the control coil on the horn relay. Car makers moved to this design to provide horn sounds for the vehicle alarm, keyless remote entry system, and options such as OnStar.
Late model vehicles are even more complicated. They use many of the components described in the above paragraph, but instead of using the BCM to provide ground, the BCM transmits a digital horn request signal along the data bus. The digital communication is received by a totally integrated power module. The TIPM is a combination fuse box/relay center with smart electronics inside. It receives the digital request and provides ground to the horn relay.
So, if your horn doesn’t work, the fault could lie anywhere in the system. Here’s the troubleshooting method.
Step 1: Check the horn fuse. Horns are located at the front of the vehicle and subject to water, road salt and grit. Over time, those elements can corrode the horn diaphragm or degrade the coil windings that cause the diaphragm to vibrate. That can short the power to ground an blow the fuse. If the fuse checks out, move to step 2.
Step 2: Locate the horn and disconnect the electrical connector. Connect one probe of a digital voltmeter to the electrical connector and the other probe to ground. Then actuate the horn. You should see battery voltage. If so, the horn button, BCM, TIPM, and relay are working and you have a bad horn. Confirm by attaching a fused jumper from battery positive to the horn. If the fuse blow, the horn is toast. If you don’t see battery voltage at the horn connector, the problem could lie in the horn relay, TIPM, BCM, Horn switch, clock spring, or wiring harness. Of all those components, the horn relay has the highest failure rate.
Step 3: Test the horn relay. There are steps in this process. The easiest first step is to locate another relay in the fuse/relay center with the same part number and swap it in place of the horn relay. Then operate the horn. If it works, the horn relay was bad. Buy a replacement at an auto parts store or the dealer. If a replacement relay doesn’t work, move on to the next test.
Using a digital voltmeter, remove the horn relay from it’s socket. Check the socket terminals for battery power at the relay contacts and control coil (key on). If the control coil and contacts are getting power, next check for control coil ground when actuating the horn button. If you see ground, then you most likely have a wiring harness issue between the relay socket and the horn.
Step 4: If you don’t get ground on the horn relay control coil when pressing the horn button, the problem could be a data bus, BCM, clockspring, horn switch, or wiring harness issue. You’ll need a high end scan tool to diagnose a data bus problem. And, you’ll need a professional grade wiring diagram to diagnose a wiring harness issue. If you don’t have those, refer this repair to a pro.
©, 2015 Rick MuscoplatPosted on by Rick Muscoplat