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Old 28th July 2015, 20:58   #11
Mike Noc
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Does it happen when braking from all speeds or only when going slowly, and does the frequency of the pedal judder slow down with the road speed?

Worth unplugging one sensor just to check that the ABS light does come on and hasn't been bodged in the past.














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Old 28th July 2015, 22:17   #12
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Does it happen when braking from all speeds or only when going slowly, and does the frequency of the pedal judder slow down with the road speed?
Seems to happen regardless of speed. I wouldn't say the frequency decreases with speed, but the severity of the judder increases. As does the noise.

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Worth unplugging one sensor just to check that the ABS light does come on and hasn't been bodged in the past.
I will try this first. Before anything.
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Old 28th July 2015, 23:02   #13
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When the car is fired up, the ABS self check is initiated and the light goes out, as per usual.
The car drives fine, no flickering of the ABS light. However, under braking (again no light) the pedal judders under foot as if the ABS is cutting in, accompanied by a grinding noise. Braking efficiency does not appear to be affected.
I had this on the wife's zt last year and turned out to be the rear sensors were covered with ferodo dust causing a poor connection.
A clean and alls been well since.

I had no warning light only pedal judder as you describe.
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Old 28th July 2015, 23:27   #14
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I have arranged a slot on the ramp at my friends garage tomorrow.

I will be visiting armed with knowledge from here, multimeter, oscilliscope cables and connectors. Hopefully I will get it nailed!
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Old 30th July 2015, 11:21   #15
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Managed to sort things out this morning. I started by unplugging each of the front sensors in turn to see if it triggered the ABS light to come on. Which it did.
When I re made the nsf connection I noticed the contacts were a little dirty. So I gave it a spray with elec contact cleaner and a scrub with an old toothbrush, allowed it to dry, connected it back up, took her for a test run and all is fine.

There must have been a good enough connection to pass the self test, but still cause a fault under braking. Fingers crossed it stays this way!
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Old 30th July 2015, 11:24   #16
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That's what we like to hear, a cheap, quick and easy repair.
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Old 30th July 2015, 16:03   #17
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Yep, always good to hear that a problem is sorted, well done that man !
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Old 30th July 2015, 17:52   #18
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just did a vensis today same problem as yours and same result ,was going post it but i see you have all ready sorted yours .
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Old 30th July 2015, 22:58   #19
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That's what we like to hear, a cheap, quick and easy repair.
Me too! Wish they were all like that!

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Yep, always good to hear that a problem is sorted, well done that man !
Thanks!

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Originally Posted by chrissyboy View Post
just did a vensis today same problem as yours and same result ,was going post it but i see you have all ready sorted yours .
No Probs! Thanks anyway!

All seems to be going well for now! 60 miles driven and no fault thus far. Furthermore, I was forced into an emergency braking situation on the way home as some idiot decided to cut me up. I braked hard and prayed, but all was good. Phheeew!

BIG THANKS to everyone for your help!
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Old 29th March 2020, 21:10   #20
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Originally Posted by chrissyboy View Post
Diagnosing Antilock Brake System Wheel Speed Sensors

Copyright AA1Car

When a wheel speed sensor (WSS) fails or there's a problem in the sensor's wiring circuit, it usually disables the ABS system and causes the ABS warning light to come on. Loss of a wheel speed signal is a serious problem because the ABS module needs accurate input from all its sensors to determine whether or not a wheel is locking up. Without this vital information, the ABS system can't do its thing.
Wheel speed sensors produce an alternating current (AC) output voltage that varies in frequency and amplitude with wheel speed. The faster the wheel turns, the greater the frequency and amplitude of the sensor's output signal. The strength of the signal can be affected by resistance in the sensor, resistance in the wiring and connectors, metallic debris on the end of the sensor, and the air gap between the sensor and tone ring mounted on the axle, hub, brake rotor, drum or CV joint.
A narrow air gap is usually necessary to induce a strong signal in the sensor's magnetic windings. Air gaps typically range from .016 in. to as much as .050 in. (0.40 to 1.3 mm) depending on the application. If the ABS warning light is on and you find a code for a wheel speed sensor (and the sensor is adjustable), the problem may be nothing more than too wide an air gap. Use a brass or nonmagnetic feeler gauge set the gap to the factory recommended specs.
Variations in the air gap can also cause fluctuations in the sensor's output signal. These may be caused by bad wheel bearings or missing, broken or chipped teeth on a tone ring. Even minor damage to the tone ring that's nearly impossible to see can sometimes cause a problem. One equipment supplier makes a test bench with a magnetic pickup and oscilloscope to check tone rings on remanufactured FWD axle shafts. The setup simulates the signal produced in a wheel speed sensor. Tests have shown that a variation in height of only about .010 inch on a single tooth can cause a noticeable fluctuation in the sensor's output signal!
WHEEL SPEED SENSOR CHECKS
One way to check a suspicious wheel speed sensor is to measure its output voltage. One way to do this is to plug a breakout box into the ABS module's wiring harness and attach the test leads from a digital volt ohm meter (DVOM) to the appropriate pins for the WSS circuit.
A good wheel speed sensor will generally produce an alternating current (AC) voltage reading of 50 to 700 MV when the wheel is spun by hand at about one revolution per second. Refer to a shop manual for the sensor's exact voltage specifications.
A low voltage reading or no reading calls for a direct measurement of the resistance in the WSS circuit (with the key off). This should be done through the breakout box to check the entire circuit. A good wheel speed sensor and circuit will typically have a resistance of 800 to 1400 ohms (specs vary, so refer to a manual for the exact numbers).
If the sensor circuit has too much resistance, reads open or is shorted (little or no resistance), measure the resistance across the sensor itself. If the sensor itself reads within specs, the problem is in the wiring or connectors. If not, then you have identified a bad sensor that needs to be replaced.
SCOPE CHECKS
This isn't something a do-it-yourselfer can do, but a dynamic check of a sensor's output with an oscilloscope is one of the best ways to analyze the performance of a wheel speed sensor. The waveform on the scope can reveal problems that might not be detectable by other means. A damaged tooth on a tone ring, for example, may not produce a noticeable change in the sensor's output voltage if you are reading the output with a DVOM or an analog voltmeter. But it may distort the waveform enough to upset the operation of the ABS system and set a fault code.
The scope connection can either be made through the breakout box or hooked directly to the wheel speed sensor. A "good" scope pattern should show a sine wave alternating current pattern that changes both in frequency and amplitude with wheel speed. Spinning the wheel faster should cause both frequency and amplitude to increase.
If the scope pattern is flattened (diminished amplitude) or is erratic, it usually indicates a weak signal caused by an excessively wide air gap between the tip of the sensor and its ring, or a buildup of metallic debris on the end of the sensor. A weak signal can also be caused by internal resistance in the sensor or its wiring circuit, or loose or corroded wiring connectors.
Damaged or missing teeth on the sensor ring will show up as flat spots or gaps in the sine wave pattern. A bent axle or hub will produce an undulating pattern that changes as the strength of the sensor signal changes with every revolution.
Something else that can be detected with a scope is mismatched parts. If the brake rotor, CV joint or axle has been recently replaced, and the new part did not have the correct number of teeth on the sensor ring, it will cause the sensor to read fast or slow compared to the others and set a fault code. Comparing the sensor patterns side-to-side will reveal this kind of problem.
Hi, I just noticed that your post was in 2015 so I hope you still connect with the forum. I have a grinding crunching noise from the O/S front wheel when braking, worse when moving slowly. It happened first when braking hard in slow moving traffic. When I remove the fuse to the ABS, the car brakes smoothly and silently. Obviously, the ABS light comes on and the handbrake light also. The airbag light has come on too but I’m not sure if this is connected with the brake issue or just a loose plug under this seat. Anyway, as the noise comes from the front O/S would it be worth replacing that particular sensor cable or does the noise not indicate the location of the fault?
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