Shared Knowledge

"BMW - Brake Pad Wear Light"

13 August 2016

Summary: The brake pad wear light will come on either because the brake pad is thin and must be replaced OR, there is an open connection at one of the brake pad wear sensor connectors.

Background:

BMW Z3, 1997, 66,000 miles.

Brake pad wear light came on.

On the 1997 Z3, there is a brake pad wear sensor on the left front and right rear (from driver's seat).

The brake pad wear light is a normally closed circuit, meaning if brake pad wear "sensor" is in tact, the brake pad wear light does not come on.

Once there is an open in the brake pad wear sensor circuit, the brake pad wear light will come on.

In Virginia, a safety inspection will flag a thin brake pad a long time before the brake pad wear sensor gets chewed and turns "on" light.

Connectors:

When brake pad wear light came "on", checked pad thickness by using flashlight and also feeling with fingers.

Front pads had plenty of pad left.

On rear, harder to see pad depth so jacked car and removed rear wheel.

Pads on rear were also fine!

Looked specifically at brake pad wear sensor that is pressed into small recess in brake pad and both front and rear sensors were not worn and looked fine.

With wheel off right rear, traced break pad wear sensor wire to junction box mounted on chassis and discovered the plastic cover that was once on the box had come off for some reason, leaving connector for wheel speed sensor (automatic braking system, ABS) and brake pad wear sensor exposed to elements.

The speed sensor and brake pad wear sensor, connectors are terrible. These connectors use very thin pins (size of sewing needle) and wires going into connector are not sealed in any way.

Below is picture of rear connection junction box with no cover. In image, brake pad wear connector has been removed and taken apart. This connector normally sits just below the speed sensor connector shown in the box.

Although not pictured, there is a similar junction box on the left front wheel well.

If brake pads fine and wear sensor appearing to be ok, had to be connection.

Unhooked brake pad wear sensor from wiring harness connector and using key, started car. Brake light wear light was "on".

So problem was with one of the 2 brake pad wear sensor connectors.

Solution:

With brake pad wear sensor connector pulled out of wiring harness connector, used electrical contact cleaner on both wiring harness and brake pad wear sensor pins.

Using silicon sealant, sealed where wires entered connectors. Both connectors. Liquid electrical tape was also used after silicon dried.

Placed a small amount of electrical anticorrosion compound inside connectors on pens and female sockets.

Used small dab of plumbers grease on "rubber" gasket of one connectors such that when connectors rejoined, plumbers grease made a good seal.

With wiring harness connector inserted into brake pad wear sensor connector, turned "on" car to ensure brake pad wear light was "out"

Solution of Plastic Box Cover:

Junction boxes are plastic and since "cover" portion of rear speed and brake pad wear sensor connector was missing, bought a piece of aluminum flashing from local Home Depot.

Measure width of box and cut section out of flashing long enough to cover box opening and then cut flashing to leave 1/4inch of 90 degree bend in place.

Placed cut flashing over plastic junction box with 90 degree bend piece sticking over top of plastic box.

Strapped in place with cable ties.

Use silicon sealant around flashing and also around where speed and brake pad wear sensor cables entered and exited the plastic box.