Exactly. The good thing is they aren't very expensive. Under $9 for the pack of four, but I could buy lunch for the cost of the three I don't need.

 

Hi Lugnut.
Looks like BikerDrew has answered the number of sensors in a packet issue.

In the UK the sensors are dispensed from a "big box" so you can have as many as you wish without wasting any. Unfortunately to save time and be sure that I had everything I needed (without a dry run) I purchased four sensors for the rear axle so have three left over which I will use when the fronts need changing....if I can still find them!!

These type of cut wire sensors are very sensitive. They detect pad wear LONG before any physical damage occurs to the brake rotors or caliper pistons. (As a guess I would define LONG as 10% of pad life since pad life is user dependent)
Although brake performance is linked to pad thickness the light comes on when performance remains more than adequate. This is to enable the vehicle owner to make the arrangements to get the vehicle into a repair shop at a convenient time.

The sensors can also determine when the rotors require changing since the small lip that begins to form on the edge of the disk rotor CAN BE the first thing that cuts into the wire sensor. In fact on my vehicle >100k the pads (original) were still OK for thickness (sensor not exposed thru friction material) but the discs were at minimum thickness (lip cut sensor).

 

You misundertood and are trying to make an issue again where there isn't one. I didn't say the brake system would disintegrate into a puff of smoke when the warning light flickers. Go get a life, will ya?

One of my front pads was cracked. All four front pads were worn into the wear sensors. That alone put about a 1/2" long slit about 1/16" wide into each pad. They were in bad shape no matter what conditoin your pads were in. Sheesh. As if the condition of your pads have anything to do with the condition of mine.

I don't see how a rotor can cut a sensor wire. The sensor wires are well free and clear of all the rotors on my car. You must have installed the sensors incorrectly on your car. On my car, the only part of the sensor that ever touches a rotor is the rubber-insulated end that inserts into the pad. Since you're going to count the number of periods I use in my sentences, I guess I have to mention that as the pad wears down, the rotor wears the rubber insulation off of the sensor and completes a circuit that turns on the warning light.

 

Lugnut,
May I suggest that I didn't misunderstand. You stated "If you wait for the light, you need to MOVE QUICKLY as soon as it starts to flicker." I merely stated that there was NO IMMEDIATE need to take action other than to get the vehicle booked in at the earliest convenient date.
Please try to be a little more tolerant of someone who is only trying to impart some useful information to you and others. Not trying to pick a quarrel.

I do however challenge your statement "I don't see how a rotor can cut a sensor wire. The sensor wires are well free and clear of all the rotors on my car. You MUST HAVE INSTALLED THE SENSOR WIRES INCORRECTLY on your car."
PLEASE NOTE: The sensor is just a loop of insulated wire.

The sensor wire loop pushes into the friction material but exposes a gap between the friction material and where the sensor attaches to the pad backing plate via a plastic moulding.
The pads do not sweep the full radius of the rotor. There is about an 1/16th inch outboard that is not sweeped and this causes a rust / wear ring to build up around the periphery of the rotor.
This ring becomes more pronounced as the rotor wears. There are clear guidelines in WIS as to how much wear is allowable. If it exceeds these amounts damage to the caliper pistons can result.
As the pads wears the rust ring can cut into the sensor immediately adjacent to the plastic fixing BEFORE the pads have worn sufficiently to expose the sensor in the pad material and cut the sensor at that point. See photo. The sensor in the photo switched the w/l ON even though the hole in the pad has not even been exposed.
In this way it can highlight worn discs where the pads are still serviceable.
Of course if you have to change the rotors you would opt to fit new pads.

As a final point the light comes on when the FIRST sensor wire in the circuit is broken. Not as you indicated when it makes contact with the rotor..."the rotor wears the rubber insulation off of the sensor and completes a circuit that turns on the warning light." The circuit works in this way so that should one of the connections to the sensor become loose / displaced / corroded the warning lamp will illuminate and subsequent investigation will reveal the problem. Since pads are no longer changed at set service intervals, due to differing customer usage, it is important that the warning system can be relied upon.
You must have been very unlucky for all sensors especially front and rear to be cut/worn at exactly the same time.

Problem solving requires great attention to the detail.

(I've tried to attach a .jpg <250k but I keep getting an upload error. Any ideas? I've turned off interlace.)

 

Thanks for over four large paragraphs of useless info. I'm not saying it's technically incorrect, just useless.

I'll bet you have trouble making friends.

 

means it's using single wire contact type and not loop-break type.

 

Hi Sleepwalker.
I respect your helpful answers. In fact the design is very subtle.

The brake pad w/l illuminates when voltage ACROSS A RESISTOR (in series with the sensors) drops to 0v.

This situation arises when the looped sensor wire is severed by the rotor or forwhatever reason the circuit becomes OPEN.

It is correct however that IF the sensor wire successfully manages to ground thru the rotor it has a similar effect...a flickering light when applying the brakes / cornering (vs steady). However electrical continuity between the sensor, rotor and the chassis is not guaranteed and therefore does not meet product liability risk assessment requirements.

 

yes, that's right.

i think his car was before 1994 version which still uses the first type.
i remember that the steel plate backing for the pad on some models also got extended edge which touches the rotor to make audible noise.

lately, we even got two stage warning for the brake pad sensor (MB and BMW uses both). it was those types that cannot be re-used since the circuit was already open.

 

Thanks Sleepwalker. I agree.

 

I didn't realize it was a looped wire.