Diagnose this fault (braking)

Peter Diffey

Peter Diffey

Peter Diffey

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Joined
28 Feb 2007
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Hello gents

Car: 1999 Renault Megane Scenic 1.6

Problem: This doesnt happen everytime, but when braking from any speed down to a stop, the brake pedal will judder significantly BUT only just before the car reaches a stop.
This is with constant braking force and not under heavy braking.
It feels slighly like when the ABS kicks in, but the judder is more violent and you can actually hear the pedal making the juddering noise.

As previously stated this only happens when coming to a stop and actually occurs from say less than 10mph down to 0mph.

Any suggestions what the problem could be?
:)
 
This is caused by the ABS activating, and is quite a common fault. The problem will either be a duff wheel speed sensor, or a reluctor ring on one of hubs or CV joints will be broken or badly corroded,

Because the sensors are inductive, their output voltage is low at low speeds, and rises as speed increases. The ABS computer only measures the frequency of the pulses rather than amplitude(i.e. peak voltage), but requires a minimum amplitude to operate. If the sensor or reluctor ring is faulty the output can fall so low at low speeds that it appears to the ABS computer as though the wheel has stopped turning. This is why it happens just as you come to a stop.
 
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Dogbreath said:
This is caused by the ABS activating, and is quite a common fault. The problem will either be a duff wheel speed sensor, or a reluctor ring on one of hubs or CV joints will be broken or badly corroded,

Because the sensors are inductive, their output voltage is low at low speeds, and rises as speed increases. The ABS computer is only measures the frequency of the pulses rather than amplitude(i.e. peak voltage), but requires a minimum amplitude to operate. If the sensor or reluctor ring is faulty the output can fall so low at low speeds that it appears to the ABS computer as though the wheel has stopped turning. This is why it happens just as you come to a stop.
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That is very interesting :) learn something new everyday :p

So would you say this is something that can be forgotten about? i.e. not dangerous?
 
This is typical or warped discs. It feels worse when really slow as the disc roatation is slower and the warp becomes more pronounced.
 
Dandle said:
This is typical or warped discs. It feels worse when really slow as the disc roatation is slower and the warp becomes more pronounced.
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I dont think it is this, because the juddering of the brake pedal just sort of kicks in like ABS does, its not progressive.

Also it doesnt happen everytime that you use the brake, only randomly probably about 5% of the time at most.
 
I would have thought if you had a sensor misreading you would have had the ABS light on to show something isn't right, as it will be sending wrong signals even when the car isn't braking. Thats the way Ive found ABS systems to work in the past.
 
I wouldn't want ABS activating unnecessarily if it were my car. You'd be a fool not to have them checked at the very least, you don't take chances with your brakes.
 
Dandle said:
I would have thought if you had a sensor misreading you would have had the ABS light on to show something isn't right
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The ABS computer can not tell the sensor is 'misreading' under this condition because it's seeing a signal reducing in frequency as the car slows, which then disappears. This is exactly the same as it would see when a wheel locks. If a sensor is faulty enough to give no output at all, or no output until a considerable speed has been reached then the ABS can detect this.

This is really more annoying than dangerous, since it usually only happens just as you pull up to a stop, but it will put more wear on brake and suspension components so should be fixed ASAP.
 
Dogbreath said:
The ABS computer can not tell the sensor is 'misreading' under this condition because it's seeing a signal reducing in frequency as the car slows, which then disappears. This is exactly the same as it would see when a wheel locks. If a sensor is faulty enough to give no output at all, or no output until a considerable speed has been reached then the ABS can detect this.

This is really more annoying than dangerous, since it usually only happens just as you pull up to a stop, but it will put more wear on brake and suspension components so should be fixed ASAP.
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While driving normally though that wheel will be giving an odd signal to the other three would that not trigger an abs fault light up?
 
Dandle said:
This is typical or warped discs. It feels worse when really slow as the disc roatation is slower and the warp becomes more pronounced.
Click to expand...

Just a quick question mate, i have a judder through the pedal and also the steering wheel. Could this be warped discs?
 
Most common fault on renaults of this year is the abs ring on the cv joint corroding and eventually breaking which gives a weak signal up until it breaks.
 
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