[dds02400]

Just had my brake inspection 2 weeks ago, 9mm front and 6mm rear on the report. Then I realize my wheel lock key is not in the trunk... I called dealer... The technician claims that the key wasn\'t there so he measured the pad without taking wheels off. Is that possible ? Or they have a special tool?

[hurricane56]

Maybe they have a set of measurement calipers with extra long jaws?

[dds02400]

obviously they are saying they didn't lose my wheel lock key. I am just upset they didn't document that on my service report that "Tech wants to remove wheels and couldn't find the wheel lock key in the trunk".

[dds02400]

yes....the tech told me that....he opened the trunk and the wheel lock key wasn't there in the time of service.....well, i guess he "intended" to remove my wheels anyway.

[David1]

I have seen the tool. it looks like a tire depth gauge. There is a whole in the shield on the calipers where it goes through. so no need to remove the wheels. Even the service advisors can do it.

[Blue Streak]

Micrometer. It measures the rotor thickness.

[David1]

Originally Posted by Blue Streak

Micrometer. It measures the rotor thickness.

We are talking about pad thicknes measurment not the rotor.

[JoosyJoos]

They probably just glanced at your pads and said "ya those look alright".

Same strategy I use at the track after every session... but to answer your question they just use a basic caliper to measure the thickness of the pad and rotor normally.

[David1]

They don't just glance at them. They have tool that measures the pad thickness without pulling the wheels. They can do all four corners in about 2 minutes.

[vasillalov]

Ummm, the wear sensors report actual percentage of brake compound left on the pads. All they need to do is plug into the car computer.

[Shawangadonga]

Here at BMW we do not just glance at the pads we use special tool # 34 1 260. It looks similar to a pen. We use an access port through the pad retainers or pad shields, we are then able to measure the actual thickness of the pad itself without pulling off all 4 wheels. The cluster measurements are only an estimate. It changes depending on your driving style.

[e90pilot]

Originally Posted by vasillalov

Ummm, the wear sensors report actual percentage of brake compound left on the pads. All they need to do is plug into the car computer.

Nope. The ware sensor is an on/off switch. Just 2 wires encased in plastic. Once you ware away the plastic, the wires will touch the rotor and complete the circuit. It is only installed on one wheel per axel. The ware indicator you see on your CBS is just an estimate based on pedal pressure and deceleration. I've had my ware sensor trigger well before the mileage reached.

[vasillalov]

Thanks for the clarification. I learn something new every day...

[dbjb]

Originally Posted by e90pilot

Nope. The ware sensor is an on/off switch. Just 2 wires encased in plastic. Once you ware away the plastic, the wires will touch the rotor and complete the circuit. It is only installed on one wheel per axel. The ware indicator you see on your CBS is just an estimate based on pedal pressure and deceleration. I've had my ware sensor trigger well before the mileage reached.

Close. What actually happens is that the 2 wires connect to a hairpin. When that hairpin is worn away, the wires no longer connect and then trigger the replace signal. That is why some just tape the wires together vs buying a new sensor. If you know what to look for, the sensor is unnecessary.

You can check depth without taking the wheel off. There is a little tool that looks like a tire depth gauge tool. They probably used that between your spokes. Or, perhaps they had an extra wheel lock key in the shop.

[speedball73]

Originally Posted by Shawangadonga

Here at BMW we do not just glance at the pads we use special tool # 34 1 260. It looks similar to a pen. We use an access port through the pad retainers or pad shields, we are then able to measure the actual thickness of the pad itself without pulling off all 4 wheels. The cluster measurements are only an estimate. It changes depending on your driving style.

I'm not sure this (and the previous reply stating "they can do all four wheels in a few minutes") are accurate. I'm pretty sure that the access port used to measure the brake pad is the hole where the wear sensor is installed (wear sensors are installed in left front and right rear inside pads). So you can only use that measurement method for right front and left rear brake pads (at least on brake-sensor equipped models like e90).

The bentley manual procedure for the 3-series states that you measure brake pad depth on right front and left rear only.

If I'm wrong I'd love to know, find it quite annoying to measure brake pad depth without removing the caliper!

--speedball

[SackOHammers]

I think people still don't know how the brake sensors work.
There are two "wear wires" in them.
When you put in a new wear sensor you should reset the brake wear mileage meter.
When the pad wears halfway through, it breaks the first wire. Then the car knows how many miles it took to wear through half your brakes. From there it tries to predict how many miles you have left on your brakes. Of course, it's entirely dependent on how fast or how slow you wore through the first half. But that's how you get your estimate for the second half of your pad.
And when the second wire finally breaks (regardless of estimate), you get the warning light in your dash that it's time to replace the pads.

[mantis]

Originally Posted by David1

I have seen the tool. it looks like a tire depth gauge. There is a whole in the shield on the calipers where it goes through. so no need to remove the wheels. Even the service advisors can do it.

depth mic its called

[rich_mane]

Originally Posted by speedball73

I'm not sure this (and the previous reply stating "they can do all four wheels in a few minutes") are accurate. I'm pretty sure that the access port used to measure the brake pad is the hole where the wear sensor is installed (wear sensors are installed in left front and right rear inside pads). So you can only use that measurement method for right front and left rear brake pads (at least on brake-sensor equipped models like e90).

The bentley manual procedure for the 3-series states that you measure brake pad depth on right front and left rear only.

If I'm wrong I'd love to know, find it quite annoying to measure brake pad depth without removing the caliper!

--speedball

http://www.samstagsales.com/bmw/bmw90886341260.jpg

There's a picture of the tool.

All four corners of the brake setup have the inspection holes in the metal retainer/clip and the pads themselves. It's very easy to measure the outside pad thickness with this tool, but the wear sensor is located on the inner pad.

If you trip the wear sensor you may still need to change the brakes, even if the outer pad is good you may be close to metal-on-metal contact with the inner pads.

Originally Posted by SackOHammers

I think people still don't know how the brake sensors work.
There are two "wear wires" in them.
When you put in a new wear sensor you should reset the brake wear mileage meter.
When the pad wears halfway through, it breaks the first wire. Then the car knows how many miles it took to wear through half your brakes. From there it tries to predict how many miles you have left on your brakes. Of course, it's entirely dependent on how fast or how slow you wore through the first half. But that's how you get your estimate for the second half of your pad.
And when the second wire finally breaks (regardless of estimate), you get the warning light in your dash that it's time to replace the pads.

And sorry to burst your bubble, but this is wrong. The wear sensor never makes contact with the rotor until the pad is worn down enough to get there. Once the sensor starts rubbing with the rotor, the plastic casing is worn down and eventually breaks the metal connection inside, creating an open circuit in the wire, which is the indication that it's time to change the brakes. The mileage left prior to this happening is simply predicted up until that point.

[SackOHammers]

Originally Posted by rich_mane

And sorry to burst your bubble, but this is wrong. The wear sensor never makes contact with the rotor until the pad is worn down enough to get there. Once the sensor starts rubbing with the rotor, the plastic casing is worn down and eventually breaks the metal connection inside, creating an open circuit in the wire, which is the indication that it's time to change the brakes. The mileage left prior to this happening is simply predicted up until that point.

Oh, you didn't burst my bubble. The principal behind what I said is all true. These are two-stage wear sensors. When the pads wear enough, the first stage is triggered. And your car knows how many miles went between the time you reset the meter and the time the first stage was triggered. It then estimates you have that many miles again until the second stage is triggered. The second stage is when the wire wears completely through and you get the warning light in your dash.

So, yes, there is no wearing or contact until the first stage is reached, but that's a minor point. Stage one is when the sensor starts to wear. Just know that if you ever reach first stage with your wear indicator, you put in new pads, and then you try to reset your meter, you'll get an SES light. So if you ever go through 50% wear (trigger the first stage), you need to get new wear sensors. Because you'll run into a foul if your stage 1 is triggered, but your mileage meter is reset.

None of this information is new. There are plenty of posts on how this works.

[rich_mane]

Originally Posted by SackOHammers

Oh, you didn't burst my bubble. The principal behind what I said is all true. These are two-stage wear sensors. When the pads wear enough, the first stage is triggered. And your car knows how many miles went between the time you reset the meter and the time the first stage was triggered. It then estimates you have that many miles again until the second stage is triggered. The second stage is when the wire wears completely through and you get the warning light in your dash.

So, yes, there is no wearing or contact until the first stage is reached, but that's a minor point. Stage one is when the sensor starts to wear. Just know that if you ever reach first stage with your wear indicator, you put in new pads, and then you try to reset your meter, you'll get an SES light. So if you ever go through 50% wear (trigger the first stage), you need to get new wear sensors. Because you'll run into a foul if your stage 1 is triggered, but your mileage meter is reset.

None of this information is new. There are plenty of posts on how this works.

Yeah you're definitely right about all of this.

To be honest I'm not even sure what part I was disagreeing with in your original post lol. I must have misread something because rereading it now, I'm really not even sure what point I was trying to make hahah so sorry about that.

[SackOHammers]

Well, to be fair to you, I was going on about there be two wires and there aren't.
As you said, there is no contact between the sensor and the rotor until the brake pad wears about half way down. Then the sensor starts making contact with the rotor. This is stage one. How I understand it, your car now expects contact with the rotor from here on out. This is why, if you reset the meter and there is sensor contact with the rotor, you get an SES error. Because it knows a sensor is at stage 1 (half pad wear).
So then at half pad wear, you have contact with the rotor until you wear the sensor and pad down enough that the sensor wire is completely worn through and broken. Then no electrical contact is made through the wire circuit again. This should trip stage two. Which means your pads should be replaced. It also causes it to flash the warning light on your dash.
I'm 90% all that's accurate, but I may be missing a fine point. I may be missing some detail about how the wire and contact works, but the principal is all there. At the time you posted, you may have seen that I had some of that wrong the first time around.