Wear indicators alert motorists when brake pads are due for replacement. Typically, a dashboard warning light will be activated when the friction material is worn to 2-3mm thickness, prompting the driver to book into a garage for brake service. There are 3 different types of wear indicators:
Acoustic wear indicator
This is the simplest type of indicator, and is based around a spring steel. When it comes into contact with the brake disc, the driver will hear a loud screeching.
Electric 1-way wear indicator
This type of indicator is fixed on the backing plate, above the friction material. A single cable runs between the brake pad and a dashboard warning lamp. When the friction wears away to the extent that the cable touches the brake disc, the electric circuit will be closed and the warning lamp will be activated.
Electric 2-way wear indicator
This type of indicator is also fixed on the backing plate, above the friction material. The difference to the 1-way electric wear indicator lies in the type of plug and sensor. The plug has two integrated contact pips, connected with one cable. The cable is glued into the friction material, and a short circuit occurs when the when the friction material is worn, causing the brake warning lamp to light up.
The diagram shows the wear indicator fixed above the backing plate. It is then locked above the spring, which is in the sensor.
In a different type of construction, the wear indicator is fixed on the backing plate in combination with a retaining spring.