Does anybody understand the brake warning light circuit in a 1996 318ti? The manual shows a simple circuit of two sensors in series with a light bulb between 12v and ground. This doesn't make any sense because that would require both sensors to fail to get a warning light. And the lamp comes on briefly (normally) at start-up, so clearly the circuit is more complicated.
What is in the sensors? A crystal that gets shattered with any contact with a rotor? Or a simple ground contact? Is there a test for a sensor?
My problem is that the light is on for no apparent reason. Disconnecting the sensors does not extinguish the light. I have not replaced the sensors yet. Could anything else be causing this, such as (rear) brake light problems.
Check your brake pads; it's likely that either front pads and/or rear pads need to be replaced. Your car probably has 2 sensors (one on the front drivers side and the other on the rear passengers side). If either front pads OR rear pads are worn to the point where the sensor is tripped, you HAVE TO replace the sensor after replacing the pads. Just replacing the pads doesn't turn the warning light (on the dashboard) off.
FYI, the brake pad sensor basically a conductor when rubbed against the rotor, the wire breaks (i.e., continuity breaks). Hence, the dash light comes on.
The dash light comes on (and should come on) for a brief period upon starting the car; that's basically a self test of the circuit/system.
The light came on some time ago and I replaced the rear pads, although there was probably 1/8 inch left. The front pads had previously been replaced months before. There was no evidence of any scraping on the rear sensor. I did not replace it.
So a good sensor should show a short betweeen the two pins on its cord? I just today tested one new one (not yet installed) with an ohmmeter (Fluke DVM on the diode test--low voltage ohms test) and it read open. (???)