bypassing brake pad light.

[King Luis]

so i changed my brakes and a. didn't buy to sensors and b. couldn't replace the old ones. now instead of taking out my cluster and ripping out the bulb, can i just run a wire from one side of the sensor to the other and seal it up???

[PeaveyBassist]

You can tie the 2 wires together, wrap them in electrical tape (or heatshrink) and the light wont come on again. Really though, I would get the sensors and just zip tie them out of the way so they never get touched. They are pretty cheap.

Will

[Mike325]

Yep, eaither one will work.

[King Luis]

so the sensor doesn't need to be on the pad?
cool...i'll sort it out this weekend.

[Jscotty]

In order to reset the light you might have to put the key into the ON position for about 2 to 3 minutes. Once it goes out, the light is reset.

[King Luis]

In order to reset the light you might have to put the key into the ON position for about 2 to 3 minutes. Once it goes out, the light is reset.

i'm not talking about the service indicator light...i'm talking about hte brake lining like...the in between the P and the !

[WillisE30]

Yeah, once you replace the sensors you do have to "reset" the light in the way Jscotty described. I don't see why you wouldn't just leave the system intact...

[Mike325]

Fuck your cluster is sexy.

[ldsbeaker]

not his cluster, just a photo. Guaranteed.

[King Luis]

not his cluster, just a photo. Guaranteed.

and if i was a billionaire i would send you $500.
i'll just rig something up on the weekend to by pass it.
my old sensors didn't even go off yet they were basically one with the pads. couldn't even get them off.

[DJB]

First, the advice to reset the sensor by turning on the ignition key for 2-3 minutes is bogus. The e36 and later have computer controlled brake lining warning lights. The one in the e30 is simple electronics with no memory.

The system is easy to diagnose and fix if you have the right information. The usual failures are worn through wiring caused by incorrectly routed cables or missing clips, or a bad resistor on the instrument cluster.

Here is my write-up


Brake lining indicator light

Summary: The light is triggered either by the wire grounding (usually
against the brake rotor as the sensor is partially worn away) or the
wire breaking (usually as the sensor is further worn away). If the
sensors are fine, re-flow the solder on the instrument cluster resistor.

Operation

Most e30s with four wheel disk brakes have a brake wear monitoring
system. The system uses two replaceable sensors to monitor the front
and rear brake pad wear. The sensors are mounted in a notch on the
outer edge of the inboard brake pad on the left front and right rear.
As the pad wears, the sensor faces are ground away with the brake pad
friction material.

The sensors are simply a wire embedded in an insulating material. The
brake wear circuit is similar to alarm system wiring: the indicator
light illuminates when the sensor wire is either grounded or is broken.
When a sensor face first starts to wear against the rotor, the wire is
initially exposed and grounds against the rotor only when braking.
Later the wire is worn through, breaking the circuit. Thus the lamp is
initially illuminated only when braking, and later stays illuminated.

The sensor circuit is fed +12V through a 220 ohm resistor on the
instrument cluster circuit board, through the right rear sensor, then
through the left front sensor, then back to the circuit on the cluster.

If the wear indicator circuit doesn't work and the sensors are fine, the
most common source of problems is the 220 ohm resistor. This can be
easily tested without taking apart the instrument cluster. Turn on the
ignition to power the circuit and illuminate the wear indicator.
Measure the voltage and available current on the right rear sensor
connector. One of the contacts should read +12V, and provide 55-65 mA
to ground. The contact is the blue/white wire, not the green/yellow,
but it's trivial to probe both rather than look for the wire color. Use
the brake rotor as a clean ground.

If you aren't getting voltage or the correct current, the problem is
very likely on the cluster circuit board. Remove the board and re-flow
the solder around the 220 ohm resistor. This resistor is mounted next
to the C1 blue connector on the PC board. It sticks off of the board on
long leads. There are on-line pictures to locate the resistor, and you
can confirm the correct part by looking for the red-red-brown color bands.

http://e30.bmwdiy.info/brake-lining-light/brake-lining-light-03.jpg

This might sound daunting if you haven't soldered before, but this is the
easiest soldering task ever. Let the soldering iron warm up for a few
minutes, then touch the soldering iron tip to each resistor lead for two
to five seconds. You should see the solder surface change texture as it
melts, filling in the cracks that have grown over time.

Add a dab of silicone to mechanically support the resistor so that the
solder doesn't crack again. Don't cover the resistor, as it has to
dissipate almost 1 watt of heat if the sensor wire is touching ground.

That heat is likely why the resistor is mounted so far off the board to
start with -- the designers didn't want to use a expensive 1.25W
resistor, when a common 1/4W resistor was adequate most of the time.
But they didn't consider that worn-through sensors and holding the
brakes on for an extended period (e.g. stop-and-go traffic) are
correlated. The resistor is at 4X its power rating, and the only place
for the heat to dissipate is through the resistor leads. Now you have
the mechanical leverage from mounting the resistor far from the board
"working" the solder combined with thermal expansion cycles from the
overheating. No wonder there is cracking!

Worst-case power analysis for the 220ohm sensor supply resistor.
220ohm, 10% tolerance = 198-ohms, 5% tolerance = 209-ohms
Nominal 12V power is typical 13.8V operating, 15V hot load.
Power = V*I = V*V/R
Worst-case 15V*15V/198ohms = 1.14W
Worst-case, 5% 14.4V*14.4V/209ohms = 1.00W
Typical/nominal 13.8*13.8/220 = 0.87W
Design safety worst case 18V*18V/198ohms = 1.64W
So the designer should have specified at least a 1.25W part.

Note: I previously assumed that the lamp check was done by providing the
sensor power through the unloader relay. That would test the lamp drive
circuitry as well as the lamp. Instead the lamp check is done with a
diode isolated feed from the start circuit.

While you have the cluster out, with soldering iron at hand, re-flow the
solder around the temperature meter mounting hole. One of the contacts
is through the sleeve around the brass mounting stud. The stud pulls
the sleeve into the soft solder on circuit board to make contact. Over
time the solder deforms, resulting in bad contact. Tightening the
mounting nut will help for a bit, but building up the solder is a longer
term fix. Better, solder on a thin brass washer.

[rcko]

I been refurbishing an '88 325is I just acquired and this wknd while working the brakes I discovered that the right rear pad lining sensor connector on the chassis side was ripped off and just hanging on the strut. I have not been able to find a replacement plug to rewire so here is the question:

How do I go about bypassing this. Does connecting the wires together fix the issue as mentioned earlier? I could make my own set or connectors but that would mean rewring a new sensor everytime.

Anyways, what do you all suggest?

[jlevie]

If you want to retain the pad wear functionality splicing in the connector from a wrecked car would be the solution. Otherwise shorting the wire will turn the light off.

If you'll pay the postage I'll mail you the connector that I cut off my track rat night before last. PM me if you are interested.

[rcko]

rede30 offered his connector so I mailed him the $. THanks for the offer.