The what, when and ‘wear’ of brake pad wear sensors

Now fitted as standard on many modern vehicles, brake pad wear sensors, also known as brake pad wear indicators, will become an increasingly popular part of any brake job. So, it’s essential you know both how they work. And when to replace them….

What is a brake pad wear sensor?

Brake pads wear over time. However, the amount of time it takes to do this can vary greatly. Road conditions, driving style, speed, load, and the type of pads and discs can all impact the speed at which this happens, making it difficult for drivers, and technicians, to determine when a new set may be needed. The introduction of the brake pad wear sensor removes this uncertainty by letting drivers know when the pad has worn, and in some cars, even predicting when this will be. As well as helping plan future brake jobs, it also ensures the brake pads are in good working order for a safe and controlled ride.

How do brake pad wear sensors work?

Today’s brake pad sensors are an evolution of the traditional metal tabs found in many older braking systems. By rubbing against the discs, these make a loud screeching sound, providing an audio clue to the driver that the pads have reached their maximum wear limits.

Electronic brake pad wear sensors eliminate the tab by using a small sensor - a loop of wire with a low current passing through it. As the pad wears out, the loop is exposed and make contacts with the rotor or disc, creating an open circuit. This illuminates the dashboard warning light, indicating it’s time to replace the brake pads.

The latest sensors go one step further. As well as alerting the driver to a worn pad, they can also gauge how much mileage is left. Known as two-stage sensors, they feature two resistor circuits that run parallel with each other at different heights in the sensor housing. When the first circuit is breached, resistance in the sensor increases and the system calculates how much life is left in the brake pad using inputs such as wheel speed, mileage, brake pressure, brake temperature and brake operating time. This information is displayed on the dashboard, either as a physical number on vehicle start up, or a warning light which changes colour as the pad wears. Once the second circuit is broken, the circuit becomes open. This triggers the warning light that advises it’s time for a brake service.

Whilst many brake pad wear systems feature a sensor on the inboard pad at each corner of the vehicle, the number can vary - between one and four – depending on the design of the braking system. The position of the senor can also vary. The most common type of electronic wear sensors are embedded directly in the brake pad material, and therefore are not removable. Others are mounted on the brake pad itself.

When do brake pad wear sensors need to be replaced?

As brake pad wear sensors are designed to break, they should be replaced as a matter of course every time new brake pads are fitted. It’s also advisable to regularly inspect the sensors, in between pad changes, and replace where necessary. Over time, heat from the brakes can damage both the wiring and clips. They are also prone to damage during other work on the vehicle such as installing new brake discs.

To meet the growing demand for replacement parts, Delphi provides a wide range of OE quality brake pad wear sensors, available either with the pads, as per the OE, or as individual items.