We have a very elderly (25 years ish I think) Mercedes and the disc brake warning light has started to come on intermittently, sometimes when I brake, sometimes just when it feels like it. Sometimes it just flickers, sometimes it stays on.
I took it to our garage and he checked the disks and told me I still have 5,000km of driving.
So not the brakes then. Anyone any idea why the light would be coming on?
Thanks for any thoughts/suggestions.
Itās usually the brake pad wear indicator that illuminates that particular light on the dashboard. The discs may be fine for another 5,000km but the pads may not be. Alternatively an electrical issue causing a false indication.
Thanks for your reply. Sorry Peter, I have no understanding whatsoever of cars - I just drive them. What is the difference between a āpadā and a ādiscā please?
The lad at the garage got a light thingy and got down on his knees and inserted the light into the wheel and then gave me the 5,000km info.
Hi @SuePJ . The ādiscā is a metal disc (often called a rotor) that is attached to (and rotates at the same speed as) the carās wheel. The āpadsā are small (typically say 8cm x 4cm) pads of a high friction material. They are held in a sort of clamp (called a caliper) so that they are opposite each other on either side of the disc - see the diagram below.
When you apply the brake, the clamp forces the pads onto the disc and the resulting friction slows the disc/wheel assembly - and thus the car.
This process generates a lot of heat, and slowly wears away both the discs and the pads. The discs usually last much longer than the pads. I suspect that the mechanicās diagnosis was that the pads have 5000 miles left in them yet. The cause of the flickering warning light could be many things, but itās most often a corroded terminal on the connection to the sensor that triggers the light.
Ask the mecano to have a look and clean the terminals.
(Edited to correct phone typos)
Yes, in my days of self maintenance the āsensorā was just a wire through the pad that touched the disk when the pad got worn down to a certain level. So it did flicker until there was a solid ground after a little more wear. This was a big improvement on just waiting until the worn pads scored your disks 
The āsensorsā are probably no more sophisticated today.
Un plugging the pad connection by the mechanic would have been a good yes/no test of the pads. Although it was observed there could be 5000kms of pad left, if one side of the caliper is tight it could mean uneven wear of the pad causing the light to flicker or illuminate. Without a proper look its all just guesswork.
what did the last CT say about the brakesā¦ ??
Did the figures (bottom right hand side) show any differences between L/Right Front/Back etc etc
That is definitely the brake pad wear warning light on a mercedes, if it was a problem like fluid or a leak in one of the dual circuits it would be red and a stop warning would come up.
The CT would not show any imbalance between the brakes due to pad wear, if they are like the UK you would only get a caution/advisory if they were close to the legal limit for pads which is 3mm.
Iām now intriguedā¦
what does the quite detailed bumpf/figures actually relate to ā¦
If itās not telling you/me/usā¦ how good/bad/indifferent the brakes areā¦ whatās the point ???
(I appreciate this is off track re the Brake Warning Lightā¦ but Iād like to know the answer, please)
When I had an ABS light come onā¦ it was a faulty switch (I think)ā¦ but the garage took my car, checked it and returned it asapā¦
It shows any imbalance or inefficiency with the brakes front to back or side to side, but not the cause which could be a multitude of reasons, sticking caliper, worn brake pads/shoes, rusted disk, thin disk, warped disk, fluid needing changed etc, the test is for highlighting the efficiency of the brakes not the cause which is a garages/mechanics job.
On the Nissan Qashqai or any other renault 1500cc engined vehicle its a blocked engine breather pipe. Its shows as an ABS light but nothing to do with the braking system.
Lack of vacuum, which technically is still part of the braking system, hence the warning light 
It usually brings up a C1130 fault code though a blocked dpf can do the same.
Thatās what I thoughtā¦ and why I asked what the CT for the Merc had saidā¦ just asking the question, thatās allā¦ I thought it would be interesting to know if it showed imbalances or inefficienciesā¦ or if it showedā¦ nuffink at allā¦
Yes but not the ABS system. If thatās how it works (and it probably is) itās a kludge. Iām very cynical about just how much the systems in recent (last ten years) cars are integrated. A lot of new features have been added during that period but they seem to be more āislands of automationā than a system. That, coupled with the half trained halfwit (being invoiced in a main dealer at ā¬120+ an hour) staring at the diagnostic screen doesnāt fill me with confidence on problem determination. I experienced that incompetence first hand with our VW Tiguanās electrical problem.
I hope electric cars are better, but ID4s have a bad rep too. Hence my switch to an i4. Weāll see if thatās any better 
On the Nissanās and many others the warning light covers the braking system and abs systems.
No its more part of the EGR system.
Itās linked if either the engine breather or egr block you start to loose vacuum to the brake booster/servo, the codes are thrown for both faults.
Ahh makes some sense now.
A lot of codes are just generic codes that cover a whole system, hence why garages just keep on changing parts till the fault goes away 
Yes because they cannot properly diagnose a fault. Should be able to being the āprofessionalā likewise replacing the not at fault part and refunding the customer for it should also stop a lot of malpractice.