# Can Bad Wheel Speed Sensors Cause Loss of Pedal?



## filmnoire (Sep 11, 2009)

Hey, all. I'm wondering if anyone knows whether it's possible for a faulty wheel speed sensor on an ABS-equipped car to cause loss of braking in an emergency stop situation? My ABS light on my 2000 Jetta GLS has been coming on and going off intermittently for approximately one month. Twice during that time, I've had the experience of jamming hard on the brakes to make an emergency stop and had the pedal go straight to the floor, with an accompanying "vibration." When the pedal is re-applied, everything returns to normal braking. The car will have no stopping problems for weeks at a time, though the the ABS light continues to come on and off most every day. There are no leaks anywhere in the system, the power booster checks out ok, and the pads are all in good shape. The fluid level in the MC has remained consistent throughout. So my initial thought was that the inner seals in the MC were going bad and it should be replaced. HOWEVER, because I don't have a VAG COM, I wouldn't be able to cycle the air out of the ABS after installing a new MC. Since there's no way to check an MC other than replacing it, I'm loathe to do it for fear of air getting into the ABS. While reading up on how ABS works, I learned that the solenoid valves release pressure on the calipers when they sense an imminent lock-up. So I'm wondering if a bad signal from one of the speed sensors on the front wheels (which do about 70 % of the braking) could possibly trick the system into remaining in the anti-lockup position when the brake is hit hard? I bring this up only because the pedal has only failed during emergency stops so far. I know there's a gearhead out there that knows the answer to this, so please take pity on this DIY'er and lemme know. Thanks! http://****************.com/smile/emthup.gif


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## GTijoejoe (Oct 6, 2001)

*Re: Can Bad Wheel Speed Sensors Cause Loss of Pedal? (filmnoire)*

In short the answer to your question is YES.
If your wheel speed sensor (WSS) is tripping the ABS system into fault intermittently than it is just barely on the edge of the system spec...meaning that it can also cause faulty wheel slip signal to your modulator when its not needed, which will release/reduce brake line pressure when its unexpected....seemingly when your are hard at braking......
You'll need to find which WSS it is, you can measure the resistance of all four sensors from the sensor harness coupler, usually all that are ok will roughly be the same resistance and that one that isn't will surely be different.
Hope that helps http://****************.com/smile/emthup.gif


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## filmnoire (Sep 11, 2009)

*Re: Can Bad Wheel Speed Sensors Cause Loss of Pedal? (GTijoejoe)*

Hey, thanks Joe. You were the only person to weigh in on this one. http://****************.com/smile/emthup.gif I'll have to see if I can find a meter somewhere.
Much appreciated! http://****************.com/smile/emthup.gif


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## filmnoire (Sep 11, 2009)

*Re: Can Bad Wheel Speed Sensors Cause Loss of Pedal? (GTijoejoe)*

That definitely helps. Just picked up a meter and am ready to test the WSS's. Shouldn't I be testing across the contacts on the individual sensors themselves though, instead of the harness? Also, I assume the key needs to be in the "ON" position for current to flow through? Sorry, I'm just finally learning to use a meter after all these years. Can't believe I waited so long!








Thanks, bud.


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## filmnoire (Sep 11, 2009)

*Re: Can Bad Wheel Speed Sensors Cause Loss of Pedal? (filmnoire)*

Sorry, JoeJoe. I must have been out to lunch when I read this the first time. Somehow I thought you were telling me to measure the resistance up at the ABS module.







You could say I've had a lot on my mind lately.







Anyway, thanks again for chiming in on this one. I needed to know that I wasn't crazy in thinking it could be a WSS issue.







I'll check 'em. http://****************.com/smile/emthup.gif


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## GTijoejoe (Oct 6, 2001)

*Re: Can Bad Wheel Speed Sensors Cause Loss of Pedal? (filmnoire)*


_Quote, originally posted by *filmnoire* »_Sorry, JoeJoe. I must have been out to lunch when I read this the first time. Somehow I thought you were telling me to measure the resistance up at the ABS module.







You could say I've had a lot on my mind lately.







Anyway, thanks again for chiming in on this one. I needed to know that I wasn't crazy in thinking it could be a WSS issue.







I'll check 'em. http://****************.com/smile/emthup.gif 

correct the actual WSS coupler for the sensor http://****************.com/smile/emthup.gif


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## filmnoire (Sep 11, 2009)

*Re: Can Bad Wheel Speed Sensors Cause Loss of Pedal? (GTijoejoe)*

Thanks again, 'Joe. http://****************.com/smile/emthup.gif


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## waabaah (Jun 24, 2006)

*Re: Can Bad Wheel Speed Sensors Cause Loss of Pedal? (filmnoire)*

a wheel speed sensor can play tricks on the brake pedal and miake it sink or rise but i fail to see how the brake pedal can go to the floor because of a wheel speed sensor.
i would think the pedal would move around 1mm to 5mm if abs related and not 2-4 inches to the floor.
your going to want to fix the wheel speed sensor that is causing troubles with the light and the abs operation but dont get your hopes up thinking it will solve your brake pedal going to the floor.


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## filmnoire (Sep 11, 2009)

*Re: Can Bad Wheel Speed Sensors Cause Loss of Pedal? (waabaah)*

Well, I knew it was a shot in the dark, but my logic was that if an ABS unit cuts fluid pressure to designated wheels during a potential lockup situation, then a bad malfunction could theoretically cause massive braking failure. Theoretical at best, I know. And it shouldn't be able to work that way. But it's very suspicious that the pedal has failed only twice in the last 6 weeks, both times under emergency stopping situations. I think that in all likelihood, I may have 2 separate issues going on simultaneously, but I've never had an MC take this long to fail completely, so it's a bit odd. My next move is to measure each WSS individually with a meter. I'm pretty sure one is defective, particularly since hitting a bump in the road can make the ABS light turn ON or OFF. Seems to me that a true ABS issue would throw a continuous warning. After that, I'll monitor the braking very carefully for a while and see if the MC fails the test of time...


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