2001 Mercedes-Benz ML430 Blower Motor Relay Replacement Cost

The blower motor relay allows the low amperage blower motor switch, located in the cabin of the vehicle, to power the blower motor with a higher amperage circuit. The relay is typically located in the 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 blower motor relay powers the blower motor which takes considerably higher amperage than can be safely passed through the blower motor switch, but must be powered when the blower motor switch is pressed. A relay is the perfect tool for the job, and has remained reliable for decades. When the blower motor switch is active, electrical power is sent from the blower motor 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 blower motor 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 blower motor. To turn off the blower motor, the blower motor switch being deactivated 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 blower motor switch, the large circuit will open (disconnect). When the large circuit opens, the blower motor loses power and stops running.

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