E320 rear brake disc hot and esp/run flat warning on

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jshodgson

Active Member
Joined
Aug 25, 2017
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138
Location
Torquay UK
Car
E class 320 cdi sport 2007
Changed the rear pads and noticed poor mpg on a long run so checked temp of discs and rear o/s was 120 degrees whilst the other 3 were about 60. Then the ESP light came on and a warning that the run flat indicator was inactive.

On idle the warning light won’t come on. When you press the brake it doesn’t come on. When you release the brake pedal it comes on.

I replaced both the guide pins on the hot wheel as one of them was deformed and noticed the pad had come away from the piston as the clip that holds it in had popped off. I put it back on refitted everything but still have the problem.

Any other ideas of what I could check? Before I changed the pads there was no issues at all and the ESP light came on about 250 miles after I fitted them so not sure if it is just coincidental that it happened when I changed the pads or if it has come on because of the pd change.
 
Changed the rear pads and noticed poor mpg on a long run so checked temp of discs and rear o/s was 120 degrees whilst the other 3 were about 60. Then the ESP light came on and a warning that the run flat indicator was inactive.

On idle the warning light won’t come on. When you press the brake it doesn’t come on. When you release the brake pedal it comes on.

I replaced both the guide pins on the hot wheel as one of them was deformed and noticed the pad had come away from the piston as the clip that holds it in had popped off. I put it back on refitted everything but still have the problem.

Any other ideas of what I could check? Before I changed the pads there was no issues at all and the ESP light came on about 250 miles after I fitted them so not sure if it is just coincidental that it happened when I changed the pads or if it has come on because of the pd change.
The rear pistons on the W211 are apparently plastic and deform when hot so are probably stuck . @Jobsworth was I think who told me ??
 
Jack the offending wheel off the ground and spin it by hand. How does it compare to the rear n/s? Do this before and after applying the brakes.

As said above, it's probably a sticking piston but the ESP light is forming a theory at the back of my mind that line pressure isn't being released for that wheel.
 
Did some more testing today and it looks like both rears are much hotter than the fronts. Tested them a few times after long runs and the rears are 120-130 degrees whereas the fronts are 50-60.

MPG is down by about 10-15%.

ESP and run flat warnings remain on and it comes on exactly after the second press of the brake pedal of every journey (comes on when the pedal is released). No error code is recorded on my generic OBD readers.
 
When fitting the pads, did you check that the pads were 'free' where they abut the caliper?
 
If a caliper is sticking and generating more heat, that heat will find its way into the tyre and cause a pressure increase. It could be that the pressure warning is because it's too high.
 
When fitting the pads, did you check that the pads were 'free' where they abut the caliper?
The wheels could spin as the piston was pushed all the back when they were fitted. The pistons went back nice and easily too. The guide pins were cleaned, greased and refitted. Everything looked ok when they went back together. The poor fuel economy happened immediately and the warning lights came on after about 250 miles or so.
 
If a caliper is sticking and generating more heat, that heat will find its way into the tyre and cause a pressure increase. It could be that the pressure warning is because it's too high.
The warning light for the run flat indicator comes in 20 seconds after a cold start once the brake pedal has been pressed twice. Even without the car moving. And always with the ESP warning. If I don’t touch the brakes the warning light doesn’t come on at all even if I drive it for ages.
 
The wheels could spin as the piston was pushed all the back when they were fitted. The pistons went back nice and easily too. The guide pins were cleaned, greased and refitted. Everything looked ok when they went back together. The poor fuel economy happened immediately and the warning lights came on after about 250 miles or so.
That doesn't answer my question.
The piston and pins can be as free as Woodstock but if the (moving) pad hangs up in the caliper the brake will bind.
 
That doesn't answer my question.
The piston and pins can be as free as Woodstock but if the (moving) pad hangs up in the caliper the brake will bind.
Help me out here. The pad that sits on the piston side clips firmly into the piston so not sure what you mean by ‘hangs up in the caliper’. As far as I can see there is no upward/downward movement of the pad on the piston side. Just in/out movement. How would I tell that the pad is hung up in the caliper?
 
Help me out here. The pad that sits on the piston side clips firmly into the piston so not sure what you mean by ‘hangs up in the caliper’. As far as I can see there is no upward/downward movement of the pad on the piston side. Just in/out movement. How would I tell that the pad is hung up in the caliper?
In this instance 'hanging up' means jamming. With a sliding caliper the area where the ends of the pad sit collect grime and can corrode a bit - both of which can prevent the pad from moving and cause it to hang up.
 
Jack the offending wheel off the ground and spin it by hand. How does it compare to the rear n/s? Do this before and after applying the brakes.

As said above, it's probably a sticking piston but the ESP light is forming a theory at the back of my mind that line pressure isn't being released for that wheel.
I read somewhere that the brake pedal/light switch can cause these symptoms so have changed it tonight and will test it tomorrow. Doubt it will affect the hot brakes but may eliminate the warning lights and leave something mechanical like the line pressure?
 
The pistons in the rear calipers are made of phenolic resin. With age and use they mushroom very slightly towards the outer end. When pushed all the way back to fit new pads the fat bit ends up back inside the bore of the caliper body and it can bind when the brake gets hot. I’ve seen it hundreds of times.
 
Help me out here. The pad that sits on the piston side clips firmly into the piston so not sure what you mean by ‘hangs up in the caliper’. As far as I can see there is no upward/downward movement of the pad on the piston side. Just in/out movement. How would I tell that the pad is hung up in the caliper?
The pad on the piston side is restrained during braking by its edge bearing against the caliper body. The pad has to have clearance to freely slide laterally across said surfaces.
When I fit new pads, once the piston is pushed back in I offer up the new pad and ensure that it can slide (a build up of dirt on the caliper body can prevent this) and once I'm happy the caliper is clean enough, if the pad doesn't move freely, I take a file to its edge(s). Excess flash or a blob of paint from production often requires removal. This checking is sometimes easier with the pad reversed, ie, if the clip that locates into the piston is in the way.
The above is routine for any pad fitting and if there's doubt you should check but post # 13 may cure your problem.
 
In this instance 'hanging up' means jamming. With a sliding caliper the area where the ends of the pad sit collect grime and can corrode a bit - both of which can prevent the pad from moving and cause it to hang up.
Thanks for clarifying.
I took off the carrier off and cleared out all of the crud. Cleaned them off, cleaned the caliper before I fitted the new pads.
 
Replacing the brake light switch seems to have solved the ESP warning light and the run flat warning. Did 200 miles today with no warning light. So must have been a coincidence that failed at the same as the pads were changed.
 
Replacing the brake light switch seems to have solved the ESP warning light and the run flat warning. Did 200 miles today with no warning light. So must have been a coincidence that failed at the same as the pads were changed.
Coincidences are a bugger for messing with your head!
 
Checked the wheels today. Passenger side moved easily. Drivers was stuck.

Pads move freely on both wheels.

Drivers side the of that fits into the piston was loose compared to the passenger side. Almost like the metal clip that holds it against the piston wasn’t working as it should. I bent the prongs out a bit more and popped it back in.

Not sure if that could be a reason for the sticking pads or to just get on with it and replace that caliper.
 
Checked the wheels today. Passenger side moved easily. Drivers was stuck.

Pads move freely on both wheels.

Drivers side the of that fits into the piston was loose compared to the passenger side. Almost like the metal clip that holds it against the piston wasn’t working as it should. I bent the prongs out a bit more and popped it back in.

Not sure if that could be a reason for the sticking pads or to just get on with it and replace that caliper.
If the pad wasn't sitting square then it could have jammed 🤷. Fingers crossed this is the solution.
 

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