Brake pad warning light circuit. Help needed.

Thu Apr 27, 2006 9:46 am

Hi,

325i, 88/89, ABS

Having failed an MOT on several things including this dash light being illuminated I though I would try and troubleshoot it rather than just pulling the bulb. I have disconnected both sensors front n/s and rear o/s both checked positive for continuity yet the light remains on. I have had a look through the wiring diagrams in the Bentley manual but couldn't find the circuit. I traced the sensor wires as far as I could, the front one comes up thought the inside wing and into the fuse/relay box and the rear one disappears up through the chassis and presumably ends up at the dash console or fuse box? All exposed sensor wires look in good condition with no nicks or abrasions.

Where do these sensor wires go/end up? And what checks can I make to solve this one?

Thanks in advance

Neil

Thu Apr 27, 2006 12:38 pm

In my experience this is a console fault... where rthe speedo, lights etc are.

Over time a resister soldered on the board vibrates the solder loose, causing
an open circuit that keeps the light on.

Take out the console and resolder every solder connection you can find, the
currently faulty solder blobs will have dark rings and/or lifeless dull solder.

When the console is out you can also buzz through the wires in the sensor
circuit from the console connectors - I think one comes in on each connector
but I'm not sure which pins any more.

Thu Apr 27, 2006 1:54 pm

Something not quite right about OP. ABS light on is a MOT failure, pad warning light isn't.
Sounds like you're trouble shooting the pad wear circuit!

Thu Apr 27, 2006 1:57 pm

No good pulling the bulb either. Your failure points are logged on the VOSA computer, so it will be obvious to the tester that ABS is fitted, and the warning light must have a test function. (On with ignition).
Always amazes me that people submit cars for MOT that are obviously going to fail!

Thu Apr 27, 2006 5:22 pm

It is the pad wear sensor that I am trouble shooting. The ABS system is working fine. Another failure point was the HC emissions which I dont have the means to test but have since rectified. I didnt think the pad wear sensor light was a failure but it was failed on that point by the garage. The other failure was rust on one of the front wings "seriously affecting its strength" apparently.

And before anyone points out, the pads have seen less than 1K miles.

Neil

Thu Apr 27, 2006 10:24 pm

PAD WEAR INDICATOR IS NOT A MOT FAILURE!
The one about the wing sounds dubious as well.
If you are certain the pad sensor loops and their connectors are OK, then the problem is a simple one inside the cluster.
On the back of the cluster, behind the temp gauge, there is a small rectangular box moulded into the plastic. This box houses a large 220 ohm resistor.
Unscrew all the self tappers that hold the cluster together, and carefully part the two halves. On the printed circuit board, just above the temp gauge, you will see the two solder joints for this resistor, both of which will probably be in need of re soldering.

Fri Apr 28, 2006 9:38 am

Thanks Brian!

I had the cluster apart last night checking for dry joints but I just laid the component you mentioned to one side. Ill open it up and check it tonight.

As for the MOT I was quite surprised to see the PAD SENSOR and wing as a failure :/ Unfortunately I had to leave the car with a friend who dropped it off at the garage and picked it up for me so I didn't have a chance to look over the failure sheet and raise any issues. I will be taking it back to the same garage now the problems have been sorted and I will be having words.

Thanks for the advice guys! Hopefully I will be back on the road in the very near future.

Neil

Fri Apr 28, 2006 10:21 am

On a similiar note...

My pad wear light doesn't illuminate at all, even when the ignition is on prior to starting the engine. This is correct right?

Cheers,

R

Fri Apr 28, 2006 4:40 pm

Brianmoooore wrote:PAD WEAR INDICATOR IS NOT A MOT FAILURE!
The one about the wing sounds dubious as well.
If you are certain the pad sensor loops and their connectors are OK, then the problem is a simple one inside the cluster.
On the back of the cluster, behind the temp gauge, there is a small rectangular box moulded into the plastic. This box houses a large 220 ohm resistor.
Unscrew all the self tappers that hold the cluster together, and carefully part the two halves. On the printed circuit board, just above the temp gauge, you will see the two solder joints for this resistor, both of which will probably be in need of re soldering.

Ditto re pad wear indicator, mine's not had one for 2 MOT's

I ordered a replacement indicator, the bit that connects from the pad to the car, but the connectors were wrong? Oh this was after I replaced front pads.

Sat Apr 29, 2006 10:57 am

Just a follow up to say that located the offending resistor right where Brain said it would be, behind the temp gauge. It seems as though the larger/heavier components on the board are the ones that are more susceptible to vibration induced fatigue. But all is sorted now and I have a nice un-illuminated lower instrument cluster.

Thanks again.

Neil

Sat Apr 29, 2006 7:33 pm

i have the same problem and have soldered what i thought was the dry joint but it is still the same can anyone post a picture to show the dry joint so i know i did the right one

Sun Apr 30, 2006 1:50 pm

Sorry gazza its all back in the car now. I guess I should have taken some pics when I had it out.

It was the large resistor behind the temp gauge that had the dry joint on mine.

Sun Apr 30, 2006 2:19 pm

gazza wrote:i have the same problem and have soldered what i thought was the dry joint but it is still the same can anyone post a picture to show the dry joint so i know i did the right one

The two joints are directly in line with the ends of the rectangular lump on the back of the cluster, and nearly hidden by the temp gauge inside.
If soldering them doesn't put the light out, then the exrernal pad sensor loop is either resistive/open circuit, or shorted to earth somewhere.

Sun Apr 30, 2006 6:03 pm

cheers lads ill have another go

Sun Apr 30, 2006 7:14 pm

Just cut the wires going to the pads and twist them together. I had to do this as the new pads i bought did not have the connector for the pad sensor.

The light should go off when you do this.

Bid dodgey i know but never mind!

If a jobs worth doing its worth bodging!!! Lol

Sun Apr 30, 2006 9:52 pm

gazza wrote:i have the same problem and have soldered what i thought was the dry joint but it is still the same can anyone post a picture to show the dry joint so i know i did the right one

As I mentioned above - you should re-solder every joint you can find. This means getting some new solder and quickly - with a decent hot iron - do each joint so they all look shiny.

The resistor is the first component to go but not necessarily the last, if you're going to go to the trouble of pulling the panel you need to spend the extra 10 minutes to make sure everything is as good as new and all the bulbs are good. maybe fit new BMW illumination bulbs too.

I did mine that way and it should be good for the next 150,000 miles

Sun Apr 30, 2006 10:04 pm

Finny wrote:Just cut the wires going to the pads and twist them together.

Everything on an E30 should work exactly as it did when it left the factory. Everything, with the possible exception of headlamp levelers, otherwise go buy a Ford or Saxo, or whatever.