Mercedes-Benz ML450 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.

A failed starter relay must be replaced before the vehicle is operated. If the engine will start and run, but the starter intermittently engages or always engages, this will lead to a failed starter motor and damaged engine flywheel. Even if the vehicle operates properly, if the starter relay has a known issue it should be solved before the vehicle is used.

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 starter 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 Ignition switch may provide a switched ground to the starter 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 ignition switch is turned to the 'start' position.

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.