Brake pad ware warning light

[ah]

The last couple of weeks my warning light comes on for 2/3 miles and then goes off,and then doesn't come on at all,basically it's intermittent.i changed the front pads and the rear are low but not cut throw the warning light wire,any ideas why ?

[Brianmoooore]

There's the infamous cracked soldered joints on the 220 ohm resistor inside the cluster as one possibility.
Warning light comes on if the wire is cut or shorted to metalwork. A common cause of the latter is the wire to the sensor rubbing on the inside of the wheel rim, but it's unlikely to be that if you've had things apart and it's still the same after being put back together.

[ah]

Cheers Brian not to sure where or what the 220 ohm resistor is,could you give me more info please.

[Brianmoooore]

Remove the instrument cluster. On the back there is a small rectangular raised 'box', near the temperature gauge, that contains a 2W 220 ohm resistor that is part of the pad warning light circuit.
Open up the cluster, and locate the two soldered joints that hold this resistor to the main circuit board, right next to the temperature gauge. If they've never received attention before, the solder will either have severely deteriorated or be actually cracked.
Resolder, adding fresh solder, and put it all back together again.

[ChrisHC]

I have struggled with this, re-soldered the resistor although the joints looked good, checked connections and anything else that has been suggested. My local BMW specialist, a man who loves E30s said that on some cars it just seems impossible to solve and you may have to resort to removing the bulb. It is a shame, I would like to have it working, but in the end I admitted defeat and took the bulb out.

[Brianmoooore]

My local BMW specialist, a man who loves E30s said that on some cars it just seems impossible to solve and you may have to resort to removing the bulb.

Absolute rubbish! It's a very simple circuit, and ANY problem with it should be easily solved by anyone who knows how it works.
One end of the sensor loop, pin 21 of the blue socket is fed with 12 volts+ by the 220 ohm resistor, which will give about 11.5 volts actually on the loop (which is just a continuous loop of various wires), so the same voltage should appear on pin 26 of the white socket, where the loop returns to the cluster.
If the loop is broken, a 4700 ohm resistor pulls pin 26 down to zero volts, or the same thing happens if the loop is shorted to earth, with the full 12 volts then appearing across the 220 ohm resistor (which is why it's so large).
Pin 26 is connected (via a 390 ohm resistor) to the base of a PNP transistor, which has the warning lamp connected between its collector and earth, and its emitter connected to 12 volts via a forward biased silicon diode.
0.6 volts will be developed across the diode and another 0.6 volts across the emitter - base junction of the transistor, so any voltage less than 12 - 1.2 =10.8 volts on pin 26 will turn the lamp on.

[ah]

Cheers Brian that makes sense ?

[Kelson]

Brian, you clearly know a thing or two about this stuff. I have posed a question under a new topic regarding 3 flickering warning lights, the parking brake light, the pad wear warning light, and the low charge light. All of these have started to flicker erratically at less than full intensity. The reason for adding a note here is that the brake pad wear light sometimes illuminates at full intensity (no issues with pads etc). It can stay on for ages or go off and stay off for ages.

The thing that seems to link these is that when it is on at full intensity whilst driving, the 3 items mentioned do not flicker. If it is not on (as should be the case), then the 3 items mentioned above flicker. Any ideas? I had the board out of the instrument cluster but could not see any obvious solder issues (nor a large resistor as described above - mines a 1991 325i Touring)?

[Kelson]

Ah, just re-read the above. I'll go pull the cluster again and look for that raised 'box'. I was just looking at the PCB that slides out.

[Brianmoooore]

Already commented on your other thread.
The 220 ohm resistor can't actually be seen inside the cluster - it's sandwiched between the back cover and the main circuit board. Soldered joints are very near the temp. gauge.

[BenHar]

Details of the brake light resister repair are in the Wiki: http://www.e30zone.net/e30zonewiki/inde ... Flickering

Ben

[Kelson]

Yep, just sorted that and reflowed the solder. Will have to wait and see if its a cure as its intermittent, but at the moment, the light goes out as expected. Thanks