The first relay was invented in 1895 by Joseph Henry. A relay is basically an electrical switch that opens and closes under control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It can be able to control an output circuit of higher power than the input circuit that's why it is also considered to be a form of an electrical amplifier.

In your Mercedes Benz car, the Mercedes relay allows you to switch devices such as headlights, parking lights, horns, engine computer, etc. with low amperage outputs.

Relays are used in different types of applications. These are to control high-voltage with a low voltage signal, to control a high-current circuit with a low-current signal, to conserve energy, to perform logic functions, to perform time delay functions and to isolate the controlling circuit from the controlled circuit when the two are at different potentials.

One example of a relay in your vehicle is the fuel pump relay. The common indicators of a fuel pump relay failure are a no-start engine and usually a burnt smell. As a general rule, a hot relay is a bad relay. It should not be hot even though the fuel is drawing too much current. Bad relay contacts can also be another cause of fuel pump relay failures.

Connector failures to your Mercedes relay are usually one of the causes of damage to most automotive electrical and electronic components. The current flow through the relay heats up the relay which in turn heats up the connector. The connector looses its grip with the relay. It also begins to overheat from its new resistance to current flow. As a result, you will most probably smell something hot or burning and later on, you can even see the plastic around the connector melting. Eventually, the relay itself will fail in no time.

Anything that has to do with the electrical flow in your vehicle that goes wrong can pose a great deal of danger. Generally, a Mercedes relay failure can either be fixable or non-fixable. To avoid further damage, be sure to know when your relays need replacements or repairs. You might hear this all the time but it's actually true --- It's best to be ready than to be sorry.