[satish_141]

Hi all, I got my 320d serviced at sytner high Wycombe, I was expecting the car to be better after that, unfortunately my mpg is gone down to 35 from 40 mpg! I rang the garage and they say what they have done wouldn't affect mpg! One more thing is on the day car was serviced it says 7000 miles warning in my next service indicator for front brakes, but it's more than a month now I did more than 1000 miles but it still shows 7000 as warning for front brakes! Can't be right! I am sure they have messedup the OBC, any ideas on what I should do now? The worry is if OBC is stuck then I won't know when exactly to change my brakes, so any suggestions? Please help, thank you all!

[briers]

I wouldn't worry about it, the OBC is just a guestimate. My changes all the time.

As for MPG, well maybe they reset your trip in which case it adjusts more frequently. Or if they are a bit clueless like some they might have left your car idling for a while on the ramps.

[satish_141]

Hi Ben,

Thanks for the response, but I am slightly confused..should I ignore the OBC warning for 7000 miles all the time for front brakes, then how will know when I actually need changing?

RE MPG, what would happen if they had left the car on ramp for long time, why would my car only show it is doing only 35MPG as opposed to 40 mpg as before, I am just trying to understand how could this be the cause?

[xenon]

The life left on the brakes is a estimate as Ben said. Mine has been on 6000 for months. It gets more accurate over time but you will know if the pads or discs need replacing as the pad-wear sensor will ground out and you'll get the warning so don't worry about. Re. MPG - again I suggest Ben is correct. Given the weather they probably left the car idling for a couple of hours, driving down the mpg. Don't worry about it!

[AlwynMike]

Your brake pad monitor looks at each press of the pedal, the length of application and the speed. It can then "work out" how far gone the pads are. As has been said, this is only a guesstimate. If you leave well alone, it will probably take a dive and reduce rapidly at some stage, particularly if you indulge in a particularly spirited high speed braking run.
Mine went down from 2700 miles to 200 miles in the space of 25 miles after I had an uncharacteristic blast down some lanes. I looked at the pads and they were OK, but getting low, so I changed them the following week.
As a fallback, there is an old fashioned wear sensor that when it touches the disc when the pads have about 1mm remaining, will immediately take the remaining miles down to zero and then count down in negative numbers.
You should ALWAYS replace the sensor when changing the pads (one per axle, not one each side).

Mike

[Jon D]

Originally Posted by xenon

The life left on the brakes is a estimate as Ben said. Mine has been on 6000 for months. It gets more accurate over time but you will know if the pads or discs need replacing as the pad-wear sensor will ground out and you'll get the warning so don't worry about. Re. MPG - again I suggest Ben is correct. Given the weather they probably left the car idling for a couple of hours, driving down the mpg. Don't worry about it!

Agreed.
As to the fuel consumption: reset the journey computer in conjunction with a brim to brim check. That will give you an accurate MPG figure and indicate the degree of computer inaccuracy.

[Jon D]

Originally Posted by AlwynMike

As a fallback, there is an old fashioned wear sensor that when it touches the disc when the pads have about 1mm remaining, will immediately take the remaining miles down to zero and then count down in negative numbers.
You should ALWAYS replace the sensor when changing the pads (one per axle, not one each side).

That's interesting - one sensor per axle and not per pad, do you know how it works?
I assumed it to be a soft metal conductive pin embedded in the pad, which completes the warning circuit when it contacts the disc.
And before anyone chimes in, I do know that to 'assume' often makes an 'ass' of 'u' and 'me' - hence the question .

[xenon]

You're correct, Jon. There is just one "sensor" per axle, on the off-side, connected to the inner-most pad. It works exactly as you describe - calling it a "sensor" is over-egging the pudding somewhat. It's just a metal pin surrounded by an epoxy coating. When it wears sufficiently it grounds on the disc and trips the warning. Unless the sensor has actually worn and tripped it's not strictly necessary to change them.

[Jon D]

Originally Posted by xenon

You're correct, Jon. There is just one "sensor" per axle, on the off-side, connected to the inner-most pad. It works exactly as you describe - calling it a "sensor" is over-egging the pudding somewhat. It's just a metal pin surrounded by an epoxy coating. When it wears sufficiently it grounds on the disc and trips the warning. Unless the sensor has actually worn and tripped it's not strictly necessary to change them.

Thanks Xenon, I was getting carried away on a flight of fantasy about devices including lasers, ultrasonics and proximity sensors.
Now back down to earth with good old-fashioned low tech .

[xenon]

I must admit I was intrigued when I saw the pad life remaining mileage and I too speculated as to how this was derived and could only think it was a capacitive or inductive circuit, the characteristics of which would change as the amount of pad material deminished but I understand it works as described earlier, i.e. how frequently brakes are applied and at what speed.

[satish_141]

Thank you all for the responses..I am so happy that I joined this form and getting all my doubts clarified by many BM lovers and also techy guys SUPER..

[satish_141]

sorry meanrt FORUM, not form...typo ;(

[AlwynMike]

Guys,

At risk of repeating myself.

You MUST replace the sensor when replacing the pads. I know that a lot of you will feel that this is unnecessary when the sensor hasn't worn. However, with the experience of the 5 series forum, the majority of people who have changed their pads without the sensor have had tremendous difficulty resetting the iDrive (some on that forum say to only replace with a genuine BMW item, but I wouldn't go that far!). Having an electronics background, I too thought that this would not be possible, but it does seem to be the case. By all means try it without the sensor change, but be prepared to remove a wheel again to replace the part.

I am a dyed-in-the-wool cheapskate, but I replaced the sensor with my pads.

BTW, the sensor is a two wire item, and when it touches the disc and wears away, a circuit is broken, which puts the light on - still low tech, but not as low tech as some!

Mike

[xenon]

Fair point Mike but I didn't replace the sensor when I did mine (hadn't worn through) and there was no problem resetting the OBC.

I agree that it's best practice though for the sake of £15.

[m1bjr]

If the sensor is still made (and no resistance across the pins) there is no need to replace it.
Period.

[Jon D]

Originally Posted by m1bjr

If the sensor is still made (and no resistance across the pins) there is no need to replace it.
Period.

I'm not familiar with the BMW system, but should it not be the other way around?
1. pad not worn through to pins - open circuit - maximum resistance - no warning - sensor ok.
2. pad worn - pins contact disc - closed circuit - no resistance - warning on - sensor needs to be replaced.

[MEGA]

My understanding: The "sensor" is just 2 wires, and is a constant circuit.

When it grinds down and earths against the disc: Then the resistance is massively increased - and the circuit is broken.

broken circuit = brake pad indicator comes on