Several days ago, I asked for help with a friend's E30 brake pad indicator
lite. I want to thank those of you who replied. After checking for open
circuit or grounded circuit in the pad sensor wiring, my friend pulled the
instument cluster. As I understand it, He jumpered the two electrical pins
in the cluster connector where the two wires from the sensors come into the
connector. If the problem was in the sensor circuit (and not the circuitry/
board in the cluster) then the lite should have gone out with the jumper
installed. It did not. Anyway, my friend checked the circuit board and
found a green resistor that was loose (cracked solder). He resoldered the
resistor & resoldered some other points that looked questionable. When he
reconnected everything, the lite was out. Don't know if he checked the
circuit to see if the lite would still come on under the right conditions,
but I bet he did.
The following are the two helpful replies to my original post. (Also thanks
to jeffrey@pe.net, Pat Egan, Harvey Chao, and Benton Adair.
[From: Jim Shank ]
This is the topic of the "Calabresions" article in the May 1996 Roundel--page 81.
In brief, this is often a problem in the instrument cluster circuit board --like
a bad solder joint. If your friend really has verified it's not
at the sensors in the wheel, he'll have to remove the inst. cluster to track
it down further (even if it's not on the instr. cluster board). See the
article for details.
[From: prreitz@amp.com (Paul R. Reitz)]
The following info is for a '91 E30 (325 iX), but is probably the same on
all other BMWs.
The circuit is as follows: from battery (+12v) there is a series resistor (4.7 kohm?)
to a long piece of wire that goes first to one wheel sensor,loops through the sensor
(yes, the sensor is just a loop of wire!), runs to the other sensor, loops through
that sensor, then terminates at the input of a voltage-sensing circuit at which point
there is a (3.3 kohm?) resistor to ground. The electronic circuit lights the
warning bulb if the voltage at the end of the loop drops to near zero.
Several things can cause this:
- 1) an open circuit *anywhere* within the loop will cause the voltage to
drop to zero. This can be cause by the (intended) wearing through of the
sensor wires, or can be caused by a faulty connector or any other break in
the sensor wiring.
- 2) the wire loop gets grounded at any point, such as when either wear
sensor has worn to the point of exposing the wires to the metal rotor.
Consider this to be a pre-warning, as it *must* occur prior to the wires
wearing through. Although this is the intended action, it is occuring on
our car as I speak not because of pad wear alone, but because rust forming
on the outer edge of the rotor "blooms" the thickness of the rotor at the
edges and prematurely wears the sensor, exposing the wires. When it rains,
the warning light will sometimes flicker when the brake pedal is depressed.
If the wiring to the sensors rubs on a caliper or inside of a wheel, this
might also occasionally short to ground.
- 3) the voltage-sensitive circuit could die.
Do the following checks: test each sensor for continuity. If either is
open, well, there's at least one problem. If both show nearly zero ohms,
as they should, unplug both sensors and see if you can detect a voltage on
*one* of the pins in either sensor plug on the chassis wiring. There should
be one point with something like 3-8 volts (with ignition ON), the actual
voltage is not critical. Unfortunately, I don't know whether it's the front
or rear sensor that is fed first, maybe someone else knows. This would at
least help you find determine whether the problem is in the chassis wiring
or a sensor. If there is voltage at the circuit in the dashboard, then the
problem is there.
--Paul Reitz