2008 Mercedes-Benz SL550 Horn Relay Replacement Cost

The horn relay allows the low amperage horn switch, located on the steering wheel, to power the horn with a higher amperage circuit. The relay is typically located in the underhood fuse block or power distribution center, and will be square or rectangular in shape. It should be noted that relays are used in very many circuits on a vehicle, so the relay should be identified prior to removal.

The horn relay powers the horn, which takes considerably higher amperage than can be safely passed through the horn switch, but must be powered when the horn switch is pressed. A relay is the perfect tool for the job, and has remained reliable for decades. When the horn switch is pressed, electrical power is sent from the horn switch to the relay. Inside the relay there is a large circuit and a small circuit. The small circuit is for a signal from the horn switch and causes the large circuit to close(connect) through electromagnetism. When the large circuit closes, power can now flow from the battery, through the closed circuit in the relay, and to the horn. To turn off the horn, the horn switch being released stops electric power to the small circuit, so the electromagnet in the relay no longer closes the circuit. That means that when the electromagnet in the relay looses power from the horn switch, the large circuit will open (disconnect). When the large circuit opens, the horn loses power and stops running.

Horn relay failure will manifest itself in one of three ways. It may cease functioning when the horn switch is pressed, always function, possibly with the vehicle turned off, or only function occasionally. These failures all point to the horn relay at some point during diagnostics, and the way the problem is presented will generally predict the type of failure. A horn relay that is stuck on all the time will constantly power the horn, sometimes with the vehicle off, as mentioned. Also, a weak horn relay will result in intermittent operation of the horn, while a failed relay will prevent the horn from functioning at all.