Brake pad warning light-not sensors

Mon Apr 23, 2012 1:00 pm

Guys
Changed pads all round on the touring about 3 months ago, complete with new sensors. No problems at all-light (Which was previously on) went out.

Driving down the motorway today and noticed that the light was on. I noticed when at a standstill at lights that it flickered quite a lot, and seemed to flicker more when the engine was revved....

When I got home, I jacked it up and checked both sensors. Neither is anywhere near the disc and both are completely undamaged and fitted correctly. Just in case, I seperated the connectors and cleaned with switch cleaner and reconnected. Checked the wiring loom as far as I could and there was no apparent damage.

Looked through the archives and noticed that mention has been made of an instrument cluster fault that would cause this. It might be a coincidence, but I swapped the service indicator board two weeks ago as the old one had dead batteries. This solved all the cluster problems but now this has happened.

Could someone clarify what I need to check within the cluster and how I access it? Is this likely to be the cause of my problem?

Many thanks.

Mon Apr 23, 2012 1:22 pm

One 'possible' is a large resistor mounted on the main cluster circuit board,just behind the temp gauge.These have been known to develop a 'dry joint'. Re-soldering this fella may just cure your problems.

As you have already stripped a cluster to swap the SI board,you should be able to locate the offender!

Mon Apr 23, 2012 1:58 pm

Thanks. Will it be easy to identify? It must be worth a try.

Mon Apr 23, 2012 2:10 pm

http://www.e30zone.net/e30zonewiki/inde ... nsor_Light

Ben

Mon Apr 23, 2012 2:12 pm

As I remember it,it's the only biggie in the area.

I doubt that you will do any damage if you re-solder several!

The brake pad sensor circuit is a bit complex,Brian did explain it in detail about a year back,but the common faults are this resistor and/or the cabling shorting to earth...and you seem to have covered that possible fault.

Mon Apr 23, 2012 2:12 pm

BenHar wrote:http://www.e30zone.net/e30zonewiki/inde ... nsor_Light

Ben

Brilliant! Exactly what I needed! Thanks.

Wed Apr 25, 2012 2:25 pm

Just thinking. If I'm right, the warning light comes on when the wire breaks-is that right? So just disconnecting a sensor at the plug would also put the light on ?
In that case, is it possible to make sure the sensor isn't faulty by just connecting the two terminals in the loom plug together? Am I right in thinking that this would put the light out?
Cheers.

Wed Apr 25, 2012 2:32 pm

Actually, it's the other way around; when the pad wears down, the two contacts of the sensor are joined together by the metal of the brake disc. So disconnecting the plug should make the light go off; stick a wire across the plug will make the warning light come on.

Wed Apr 25, 2012 2:37 pm

Brake pad sensor light circuit is a bit complicated!

The light comes on with ign,along with most other cluster lights,as a bulb check function.
The light also comes on when the sensor pad wears through and shorts to earth,via the disc.So if the wire is broken or rubbing on the wheel rim,perhaps,this will also cause the light to come on.

If you dis-connect the sensor,the circuit thinks that it is earthed,so the light will come on.So yes,if you dis-connect the sensor and add a loop of wire to the loom plug,this should eliminate sensor faults.

Wed Apr 25, 2012 3:55 pm

Great-I'm going to try that first before stripping the clocks out again...

Thanks.

Wed Apr 25, 2012 4:01 pm

daimlerman, you star. I might just wikify that comment :winkeye:

Wed Apr 25, 2012 4:27 pm

This is the simplifed version of the circuit!

Wed Apr 25, 2012 6:15 pm

The light comes on if:
1/ The sensor wire is broken/worn through.
2/ The wire is shorted too earth, via the disc partly wearing through the wire, or anywhere else.
3/ The solder joints of the 220 ohm resistor are cracked.
The light stays off if:
1/ The ignition is off
2/ The wire is intact, including through the sensors.