How to Troubleshoot a Car Hornby Robert Moore
Diagnosing your inoperative car horn doesn’t require a Ph.D., but you won't find it the easiest task to complete either. It may be as simple as replacing a blown fuse, or you may have to dive right in and remove your steering wheel to replace the clock spring that is hidden inside the steering column. The following diagnosis procedure will work for most vehicles on the road today, but you’ll want to have a service or repair manual handy to aid you identify wires and access some components of the horn system.
Diagnosing your car’s faulty horn requires you to use a multimeter capable of reading voltage and ohms. You can rent or purchase a multimeter at most auto parts stores. If you don’t know how to use a multimeter, check out St. Lawrence University’s guide to using a digital multimeter.
A constant ground is connected to one side of your horn switch. When you press the horn switch, it completes the ground circuit -- through the clock spring -- to the control side of the horn relay. This energizes the relay and closes the power circuit to the horn or horns, depending on your make and model. The horn has a constant connection to ground. The same fuse protects both the control and active sides of the horn relay. Your vehicle may be slightly different by design, so review the wiring diagram in the service or repair manual for your specific vehicle before continuing.
Checking the Basics
The horn may operate on the same circuit that powers the accessory outlets, interior lights and possibly the radio. If you notice any of these devices aren’t working, you have likely blown the accessory fuse in your car's fuse box. The fuse may be labeled “Horn” or “Accessory,” and is commonly located in the under hood fuse box or in the interior fuse box on the driver's side of the dash. Review your car’s owner’s manual for fuse box locations and diagrams of the fuse boxes, if needed. Locate the fuse and remove it from the fuse box. Look at the piece of thin metal inside the fuse. If it looks burnt or is broken, replace the fuse and try to honk the horn. If your car's horn still doesn’t work, it is time to check the horn relay.
The horn relay is most likely located in the under hood fuse box and will be labeled “Horn.” For testing purposes, try switching the horn relay with another identical relay in the fuse box and attempt to honk the horn. If the horn works, replace the faulty relay. If the horn still doesn’t work, proceed to the next section for some more in-depth testing.
Remove the bumper cover.
For this procedure, you’ll need to locate the horn on your vehicle and work with live voltage. Horns are commonly located behind the bumper cover and mounted to the steel bumper support.
Your car's bumper is secured to the car with a few bolts and a lot of plastic retainers. Generally, you’ll find a screw or bolt in each wheel well that secures the bumper to the fenders. Some models have additional supports that are bolted to the steel bumper support. The plastic retainers will run along the bottom of the bumper -- normally securing the bottom of the bumper and undercarriage of the vehicle. You’ll also find plastic retainers that secure the top of the bumper cover to the radiator core support. Once all of the retainers are removed, gently pull the bumper away from each wheel well, and then pull it straight off the vehicle from the center. Place the bumper cover on a soft blanket or towel to prevent it from being damaged.
When you’ve removed all of the bumper cover retainers, the bumper cover should come off with ease. Do not attempt to pry the bumper cover or use excessive force, as it will damage the bumper cover.
Check the horn's ground supply.
Disconnect the wiring harness from the horns. Review your horn’s wiring diagram in your car’s repair manual to determine which wire color is the ground wire for the horn. Set your multimeter to DC current on the 20-volt scale. Push the negative lead of the volt meter into the corresponding pin in the wiring harness for ground. Touch the positive lead of the multimeter to the positive battery terminal. If the horn has a good ground, you should get a reading equal to your car’s battery voltage -- roughly 12.6 volts. If you do not get a voltage reading, trace the wire to its connection on the body. Repair the ground connection or wire as needed.
Check the horn.
Look at the two male terminals inside the connector of each horn. Clip one end of a test lead to the pin that receives power when the horn is activated. Connect the opposite end to the positive battery terminal. Clip one end of a second test lead to the negative battery terminal. Touch the end of the second lead to the ground pin inside the horns connector. If your cars horn doesn’t sound, replace the horn.
Disconnect the negative battery cable.
Loosen the negative battery cable with a battery wrench, 8mm wrench or 8mm socket and ratchet. Remove the cable from the battery terminal and isolate it so that it cannot accidentally come into contact with the negative battery terminal.
- In the following steps you’ll be working close to the driver-side airbag. If you do not disconnect the negative battery cable, one or more airbags may inadvertently deploy causing you serious injury or even death.
- Do not use any type of memory saver or retained power device.
- Your radio may have a built-in security code. Make sure you have this code for your radio before disconnecting the battery. You may find the code in your car’s owner’s manual or on a sticker inside the glove box.
Check the wire between the horn relay and the horn.
Set your multimeter to the Ohms setting, or audible Ohms setting, if equipped. Remove the horn relay and look at the diagram on the side or bottom of it. Plug the positive lead of your multimeter into the terminal on the fuse box that pin No. 30 of the relay plugs in to. Touch the negative lead of your meter to the pin for the power supply wire in the horn’s wiring harness. You should get a low resistance reading -- less than an ohm -- or hear an audible beep from your meter. If you don’t hear the beep or get an infinite reading on the meter, look for a break or damage to that wire between the harness connector and the fuse box. Repair the wire as needed.
- If you cannot locate the security code for your radio, contact your local car dealership to obtain the code. Alternatively, obtain your radio code by using a website like Radio-Code.com for a small fee.
- If you use a website to obtain your security code, you’ll need to supply your vehicle's VIN and you may need to supply the radio’s serial number.
Locate and Identify the horn switch wires.
Remove the upper and lower steering column cover from the steering column. This usually involves removing a couple of screws from the bottom cover, but check your vehicles repair manual to avoid inadvertently breaking either cover. Inspect the wires in the steering column and locate the wires for the horn switch. One wire will be a ground wire, and the other will carry ground to the horn relay when the horn switch is pressed.
Test the horn switch.
Remove the horn switch from the steering wheel. Sometimes the switch is secured by screws hidden in the back of the steering wheel, or you may be able to remove it by gently prying it out of the steering wheel. Refer to your car’s repair manual if you need assistance. Disconnect the wiring harness from the horn switch. With your multimeter on the ohms setting, touch the leads to the two terminals on the switch. Press the switch and observe the meter. You should see a small resistance value. If you receive an infinite reading, replace the horn switch. If you do get a good reading, the clock spring is most likely at fault. Follow the procedure in your car’s repair manual to replace the clock spring.
Put everything back together.
Once you’ve completed necessary repairs, install any components you removed during the testing procedure. If you need to replace any of the plastic retainers for the bumper cover, you can get them from your local car dealership or at most parts stores. After everything is installed and tightened, install the negative battery cable to the battery. The first time you drive the vehicle, it may idle and shift oddly for the first few minutes. During this time, the on-board computer system is going through its adaptive learning process in which it learns how to idle and shift correctly. Drive the vehicle as you normally would during this time. Everything will return to normal after you drive for a few minutes.
Things You'll Need
- Standard or Phillips screwdriver
- Two jumper wires, 1 to 2 feet long
Robert Moore started writing professionally in 2002. His career started has head writer and Web designer for VFW post 1224 in Hamburg, Michigan. He has prepared business plans, proposals and grant requests. Moore is a state of Michigan-certified mechanic and is pursuing an Associate of Arts in automotive technology from Lansing Community College.