2012 Mercedes-Benz S350 Starter Relay Replacement Cost

The starter relay is used to send very high amperage to the starter motor while operating the starter motor with a low amperage circuit. This relay is normally located under the hood, inside the power distribution center or fuse block. It is square or rectangular in shape, and will look like many other relays that will likely be adjacent to the starter relay.

The starter relay powers the starter, which takes considerably higher amperage than can be safely passed through the ignition switch, but must be powered when the ignition switch is turned to 'start'. A relay is the perfect tool for the job, and has remained reliable for decades. When the ignition switch is turned to the start position, electrical power is sent from the ignition 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 ignition 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 starter. To turn off the starter, the ignition 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 ignition switch, the large circuit will open (disconnect). When the large circuit opens, the starter loses power and stops running.

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