Hi folks,

This one just cropped up on me (2 weeks after MOT pass thankfully), driving yesterday and after about 20 mins of driving brake warning light comes on. Brakes are still working fine (not soft, spongy or more travel on the pedal) and handbrake isn't on.

Got home, turned engine off and checked fluid level, looks full. Turned engine on, drove backwards down the drive and back, light doesn't come back on.

This morning though, after about 5 minutes of driving the light comes back on again and stays on the whole journey.

I know normally this indicates either handbrake on, or low fluid, is there another condition this light will come on and stay on during driving that it's warning me about?
Discs and pads were replaced end of last year so can't see it those being worn down so anything else to check?

Would key-dance give me anything or do I need plug-in proper analyser to see anything brake related?

Thanks for the tips guys, am after where to start looking!

 

Is it not the ABS/Traction Control warning light Andy?

 

Hi Alan,

Nah, had that on my last one, this is purely the brake warning light (the dual purpose, handbrake indicator).

I've had TRAC OFF illuminated for a while (6 months plus) and this just happened.

I just sat in works car park to try a few things:
Checked, light goes on/off with handbrake as it should when no engine running. Brake lights all working and coming on/off as they should (so discounts the switches/electrical basics)
Then started up and started pressing the brake on and off, trying to get the light to illuminate, and it wouldn't! Pressed the brake hard and watched for any travel, no movement at all (not dropping) so don't think it's air, leak, etc.

So conclusion is that it's not happening when car is stationary, must be only when it's moving which makes me think speed sensor?

 

ABS read sensor usually effects audible pump vibration / rumble, that in turn illuminates track off Andy. Traction control light and ABS will be inop forever till the puter counts 4 wheels. Check the offending sensor and ring.

 

Thanks QinteQ, helps my understanding of them as I knew they did an initial 'startup' test but assumed if a fail they'd cause trac off and abs light to be on.

I'm going to test out the sensors, is it the usual, do an ohm resistance reading on each one and compare, then switch to AC voltage and spin looking for a couple of volts from the hall effect sensor return?
I have my suspicions on front left as I remember the wire being a bit dodgy when I last did discs and brake pads and I know it doesn't take much for a hall sensor not to keep giving back data.

thanks as always, will post back after testing and seeing what values I get.

 

Well there we go, should have just done this at first!

Tested both front speed sensors, front right get a reading, go to test front left, no continuity at all, not a thing. That's also the one I had my suspicions because of the dodgy wire so I've ordered a front left from a Latvian chap on ebay for £20.95 so will fit that when it arrives and go from there.

 

I've bought from Latvia- safe as houses. Wiring is always as we know an awkward one. If you take the wheel off to fit get a hammer, little chisel for the valleys in the reluctor and 'bastard' brush and give the rust a knock before fitting to make sure it does not knack the new sensor. Best of luck.

 

Thanks QinteQ, yep good bit of brush and cleaning required!

I had a go at doing this whilst my other problems went on, part arrived and I swapped it out. With the sensor completely disconnected the brake warning light is illuminated (Interestingly it doesn't trigger the ABS warning light at all, ABS light comes on with ignition then goes out), so not sure how a missing speed sensor doesn't trigger that light, but good to know for future.

Changed the speed sensor and cleaned up the ring too, rusty and not in good nick but should get me by.
The new speed sensor doesn't quite fit perfectly. It's a fine art, they sit very close to the spinning sensor ring but don't touch it, the replacement must be a mm or two thicker and was touching the sensor ring. Moving it and you can stop it doing that but bolting it into that position was difficult.
Then I snapped the bolt that holds the sensor in place, half in half out, so at least I could remove the sensor but I now have to drill half the bolt out. So beware! The bolts aren't too good and don't take much force to snap!