1997 Mercedes-Benz S420 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.

If the horn is not functioning properly, many states will not permit the vehicle to operate on public roads or pass safety inspections. Also, this will pase a safety risk since the horn may not work in an emergency, which may allow a preventable collision to occur. When the horn, or any other safety feature on a vehicle is not functioning to standard, it should be addressed right away.

Relays fail all the time, in new and old vehicles of all makes and models. Relays are generally reliable, but most vehicles will have at least one relay failure through the course of its service life. No matter how well made and how well maintained a vehicle is, the slightest bit of corrosion and years of wear on contact surfaces will eventually break down a relay. Also, if the relays are quickly powered on and off, such as with a dying battery, the contacts can arch and erode quickly.

Diagnosing a relay is simple with testing. On the exterior of most relays there is a diagram of the switches inside the relay. In the case of the horn relay, there normally be only one switch. Applying power and ground to the small circuit with proper polarity should make the relay click. This, however, is only the first half of the test. The secondary test is to measure the resistance across the relay, and compare that to the data on the relay or from the service manual. If the resistance is too high across a relay, the powered component will not receive the high amperage current it needs, and therefore, the relay must be replaced. If the relay does not click, the same applies, but the second part of the test will yield infinite resistance.

Replacing a relay is only a matter of identifying the relay according to diagrams in the owner's manual, under the lid of the power distribution or fuse block, or in the service manual for the vehicle. Once testing of the relay is complete, the failed relay is thrown away only after ensuring the new relay is exactly identical in function and size, and the new relay is firmly pressed in, pulled out, and once again pressed into the power distribution or fuse block. This will ensure that complete contact is made between the relay and the terminals on the vehicle.

Never attempt to repair a relay. There is generally nothing serviceable in a relay, and erroneous reconstruction will only lead to humidity entering the relay, a relay that does not function properly, or even melting of the entire relay. Relays are inexpensive sealed units that should be replaced, not tampered with.

If a relay is proven good, but still does not function, the terminals can be tested in the junction block with a test light or multimeter. A multimeter is prefered because of the accuracy of readings, but a test light will inform the technician if power is present or not. Also, the horn switch may provide a switched ground to the horn relay, or it may provide a switched hot (battery voltage). This should be known when testing the relay and the power distribution block. In the case of a switched ground, there will only be continuity between the battery negative cable and the relay when the horn switch is pressed.

Relay testing and replacement is simple, straightforward, and could be a fun project to share with a family member. The issues start to emerge when the relay is diagnosed as operable, but is not activated when the ignition switch is turned on. If testing of electrical circuits is understood, and a multimeter with long leads is present, this repair can be accomplished at home, but if the circuit is not understood and the relay is good, leave the job to a professional. This will likely prevent further damage or electric shock.