Brake Pad Wear Sensor, Failed sensor again and again and again.

[Pavesa]

Hi,

my wife got back with the XM yesterday and much panic over grating noise when braking. It turns out that the brake pad was worn down to the metal, hence the scraping. But really annoyingly, the sensor light never showed. Now, I've paid for the new pads and will have to replace at least one disc. This would be bad enough, but on my previous XM exactly the same thing happened and I wound up having to replace two brake discs! Can anyone tell me why the sensor might not be showing and what could be done about it? It's great having all the finesse of the XM but why on earth is there a problem with these sensors (seems senseless!)

Thanks all

Andrew

[noz]

Hi Andrew,

In most of the XM's I've worked on the cable in the wheel arch leading down to the hub for the connection to the break pads always seems to be broken. There are two cables in the wheel arch, one for the ABS sensor and one for brake pad wear. The brake pad wear cable is always broken. They look to be made of the same stuff and the same diameter but its always the same story.
It's very easy to check for correct operation. There's a little braided cable goes from each brake pad to a rubber junction mounted on the top of the hub. Temporarily pull the plugs out of the rubber junction. Take a spare piece of wire and short it between the connector inside the rubber junction and the brake disk. The light on the dash should come on. If not, there's a break in the cable between the rubber junction and the dash. However, if it's the usual reason then it will be very obvious when you stick your head under the wheel arch.

Hope that helps. Please let us know how you get on.

cheers

noz

ps
can I move this post to the electrical section?

This post has been edited by noz on November 02, 2006 06:56 pm

[onthecut]

Hello both.

Can I just add that as well as the inevitably broken wear indicator wires, the majority of my XMs have all suffered with uneven pad wear. As the wires are all but useless, I dispense with them and, crucially, with the tin clip that goes around the push through pin. This tin clip seems to me to be the prime cause of the uneven wear and removing it definitely improves things. If, say, at every oil change you have a quick gander to check the pads, the absence of the lamp really doesn't matter. So -- have I just been unlucky, or do more of you suffer the odd pad wear ?

Mike.

[Pavesa]

Hi,

thanks very much for the response on this. I'll follow this up.. Sure Noz, feel free to put this in electrical - I always get the wrong place!

[jorgy9]

Yes, my left-hand front pads have been found today to be too thin...this was in the process of another job (clutch)...because the relevant cable was cut, obviously...Lately the front brakes "faded" and went "noisy" very easily, just with a quick stop from high speed, and I though "look, the XM's braking material is so soft", but was pads probably as I don't remember having any such impression in the past.

regards
George

[jorgy9]

PS I'll try to rejoin the cables as I think that's a useful feature.

George

[techmanagain]

I too have had a problem with wear on one pad long before the other three and in the end identified it as being due to the little tin anti-rattle(?) item through which the pad locating pin has to pass. For some reason - which for the life of me I cannot fathom - this some times jams so that the caliper cannot slide as it should do, with the result that all the braking is done by one pad on the relevant side.
The answer to this was to remove the little slip of tin and all was well. What I cannot understand is why it is necessary to put the tin there in the first place?
Does anyone have know what it is for?
Incidentally, I have not had an unbroken pad-wear cable on ANY of the six XMs on which I have changed the pads! As soon as I -or my other XM pals - have a complaint of a scraping noise at the front, I order a new set of pads and so far have not had to replace a disc. I'm keeping my fingers crossed!!

[onthecut]

Hello Techmanagain.

As far as I can see, the tin clip is only there to support the useless wire that runs to the outer pad.

Mike.

[noz]

Hi Techman et al,

The purpose of the spring clip is anti-rattle and anti-squeal. When the piston and yoke are in their relaxed position there's a small gap between the backs of the pads and the respective part. The pad at this point is free to move and can produce a rattling sound as they very lightly touch and release the brake disk. In addition, when the pedal is pressed, depending on the materials used in the manufacture of the brake pad, you can get a squealing sound. I'm not entirely sure what causes it but it must be a repeated grab and release between the pad and disk at audio frequencies. The spring clip helps to alter that frequency such that it stops all together or moves out of the audible frequency range.

If you don't suffer from rattle or squeal then I can see how the removal of the spring clip can affect pad wear. The compromise and down side of the spring clip is that it keeps the pad in contact with the disk for longer, hence more wear. The assymetric wear must be to do with the single sided piston and sliding yoke design. In the CX there are two pistons behind each pad. You never get assymetric wear or lack of braking in a CX !

If you read my other post about dragging brakes you'll find that its the linkage to the master cylinder which becomes sticky and keeps the brakes applied. I had an issue with the wife's Vectra a couple of years ago. The tin clip on the rear disks had been fitted wrongly by the servicing garage. The clip was caught between the pads and therefore the pad force bore on the clip instead of the disk. The result was two badly corroded disks due to lack of use.

cheers

noz

This post has been edited by noz on November 03, 2006 08:02 am

[onthecut]

Hi Noz.

Surely the springy tin contraption in the main 'window' of the caliper is the anti squeal and rattle device ? Citroen only refer to the tin piece we've been talking about as a 'clip'.
Anyway, so we can all feel better, try the sliding calipers Vauxhall used in the early eighties. I reckon they lined the slide pin with friction material and then put Loctite in. Truly dreadful brakes ! It's interesting you mention CX twin pot calipers; my other regular ride has conventional twin pot Lockheed calipers -- sorry, it's not a Citroen -- and on more than one example, one pot each side goes sticky and the pads wear wedge shaped, so it's not a guaranteed recipe. I wonder if conventional brake fluid has less lubricity than LHM. I've certainly never had a Citroen caliper pot seize.

Mike.

[xmexclusive]

Hi All

The pad warning cables seem to have been broken on most of the XM's that have come my way. The ABS cables are the same material and follow virtually the same route without the same breakage problem. I tend to replace with new (at some cost) and have not yet had any repeat breakage. The broken ones I studied in detail looked so neat that it was as though they had been cut with a pair of wire cutters.
It did cross my mind that the pad light on is a mandatory MOT failure but a broken cable to the pad sensor is not. If a customer wants a MOT with repair work at the cheapest price either to sell the car or just keep it going for a few months and the pads/discs are worn but servicable .........................
I no longer take on trust anything about the mechanical condition of an XM until I have inspected it myself or overseen the work in hand.

Regards

XMexc