Why We Love ABS Wheel Speed Sensor

Why We Love ABS Wheel Speed Sensor (And You Should, Too!)
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Anti-lock braking system (ABS) is an automobile safety system that allows the wheels on a motor vehicle to maintain tractive contact with the road surface according to driver inputs while braking, preventing the wheels from locking up (ceasing rotation) and avoiding uncontrolled skidding. It is an automated system that uses the principles of threshold braking and cadence braking which were practiced by skillful drivers with previous generation braking systems. It does this at a much faster rate and with better control than a driver could manage.
ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces for many drivers; however, on loose surfaces like gravel or snow-covered pavement, ABS can significantly increase braking distance, although still improving vehicle control.

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ABS Brake types:
Anti-lock braking systems use different schemes depending on the type of brakes in use. They can be differentiated by the number of channels: that is, how many valves that are individually controlled and the number of speed sensors.

• Four-channel, Four-Sensor ABS

This is the best scheme. There is a speed sensor on all four wheels and a separate valve for all four wheels. With this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force.

• Three-channel, four-sensor ABS

There is a speed sensor on all four wheels and a separate valve for each of the front wheels, but only one valve for both of the rear wheels. Older vehicles with four-wheel ABS usually use this type.

• Three-channel, three-sensor ABS

This scheme, commonly found on pickup trucks with four-wheel ABS, has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. The speed sensor for the rear wheels is located in the rear axle. This system provides individual control of the front wheels, so they can both achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness. This system is easy to identify, as there are no individual speed sensors for the rear wheels.

• Two-channel, four sensor ABS

This system, commonly found on passenger cars from the late '80s through early 2000s (before government mandated stability control), uses a speed sensor at each wheel, with one control valve each for the front and rear wheels as a pair. If the speed sensor detect lock up at any individual wheel, the control module pulses the valve for both wheels on that end of the car.

• One-channel, one-sensor ABS

This system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle. This system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS kicks in. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness. This system is also easy to identify, as there are no individual speed sensors for any of the wheels.

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How Does it Works?

There are many different variations and control algorithms for use in ABS. One of the simpler systems works as follows.

• The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 km/h) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

• The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.
  
  
  
• This replaces the need to manually pump the brakes while driving on a slippery or a low traction surface, allowing to steer even in the most emergency braking conditions.
  
  
  
• When the ABS is in operation the driver will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. This pulsing also tells the driver that the ABS has been triggered. Some ABS systems can cycle up to 16 times per second.

Symptoms of Wear or Failure

• A failing ABS wheel speed sensor can turn on the ABS light and disable the traction and stability control systems (if equipped).
  
  
  
• In some cases, the speedometer will stop working and the Check Engine Light will come on
  
  
  
• Improper ABS activation

Related Repair Advice

• In some vehicles, the sensor is incorporated with the hub assembly and the entire hub assembly must be replaced
• Wheel speed sensors are reliable, but may be damaged if they come into contact with road debris
• Many conditions—including corrosion, worn bearings, or other problems in ABS system—can set a wheel speed sensor trouble code. Any problem with the ABS system needs to be diagnosed before wheel speed sensors are replaced, as a new sensor may not solve the problem.
• ABS wheel speed sensors can be easily damaged during other repairs that call for the wheels to be removed (especially brake work). If you have had recent repair work performed, return the vehicle to the shop to inspect for damage

 

On the other hand...
http://www.tacomaworld.com/forum/2nd-gen-tacomas/68167-abs-truck-dangerous.html

 

Hmmmm, you may be correct.

 

This.

 

While I do agree that the ABS system is really screwed up in the Tacoma, it is NOT simply for those "who can't steer and brake at the same time". When you hit the brakes and a wheel locks up, you have NO steering control because you have no traction. While the Tacoma system IS screwed, a properly operating 4W ABS system can pulsate and control each wheel individually, which you can't. Period. Being able to pulsate the brakes allows you to have some traction, which will allow you to steer. I have driven countless vehicles with ABS and I can guarantee you that in low traction situations (rain, snow, etc) that a properly functioning ABS system can save your a$$. Now let me get my rain coat on because I am sure that the is about to commence. (And yes, I do know that the Tacoma ABS system sucks, but I am referring to ABS systems in general)

 

The ABS system in the Tacoma is the most sensitive I've ever driven and sucks balls in snow

 

I would rather have no ABS on snow , like how we used to drive and survive on snow

 

Umm...

 

I completely agree on the Tacoma. The system is way too sensitive for sure. Hey OZ, since you see a lot of snow, have you driven any other vehicles that have ABS, and had it activate on you in a low-traction spot? I'd like to get your insight as well.

 

My point with ABS here is that you, as a human being, cannot do the following:
1. Pulsate the brakes to an INDIVIDUAL wheel, while allowing normal brake pressure to go to the other 3.
2. Pulsate the brakes as rapidly as a computer can.
3. Maintain the SPECIFIC pressure required to get the wheels to JUST before lockup, and modulate the pressure as needed.

So in your instance, if a single wheel locks up, sure, you can take your foot off the brake and reapply, but by then, if you were doing only 30 mph, you would have traveled approximately 20 feet, even if it took you 1/2 of a second to remove your foot from the brake, and reapply. In that amount of time you could hit another car, a pedestrian, etc. I am not saying that you are not capable of driving or handling a car with ABS. What I AM saying, is that no human can operate and manage the brakes, thus maintaining traction (thus steering) as fast as a computer can. Again, this in general reference to ABS systems, not the Tacoma itself, which we ALL know sucks.

 

So I should disable my ABS or what? lol, I got better tires now, slid down my street last year, luckily to an open intersection..had to change pants though haha

 

Matt, I would simply suggest that knowing the Tacoma system is not the best, when traveling on uneven traction areas, give yourself additional time to stop, and additional distance between you and the vehicle in front. The problem with disabling the factory ABS system is that IF, and let us pray it never happens, IF you have an accident, and someone is injured or killed, you could be criminally and civilly liable.

 

I would like to add that my Dad's 07 Tundra does great with ABS on ice and snow. I still prefer no ABS like my current 95 Taco and my 99 Jetta.

On another note, my Mom's previous SUV (Hyundai Tucson) had all tons of problems with ESC (Electronic Stability Control) and ABS. Everyone in the family hit someone because of the ABS disengaging for extended amounts of time (~2 seconds, a VERY long time when coming to a stop). Same car also started rolling down a hill backwards even when in 4WD with ESC on. Turned that crap off and I got a little bit of wheel spin and climbed right up the icy mountain road so I could get some xc skiing in!

 

Yes , I've driven other ABS equipped vehicles here on snow , and can honestly say I can't remember ever having it kick on unless I hammered on the brakes unlike the Tacoma that tends to engage almost every time you touch the brakes

 

In snow there can be an advantage to ploughing

 

x2

 

It is those instances where I am referring to. Sure YOU have been lucky enough to not really need it, but I have personally avoided wrecks because the ABS system in the car I was in gave me much needed traction to allow me to steer away from a car. It's like that old adage: Better to have it and not need it, than need it and not have it.

As for your comment about ABS warning lights passing inspection still: While ABS systems are not required to pass state inspections, vehicles that are required by federal law to have ABS from the factory can cause a sticky slope if someone is injured, and the investigation shows that someone disabled their ABS. Even if one is not found criminally liable, a civil suit could cost a person everything. This is the reason I always recommend against disabling something from the factory.

Thanks for your input. I know that snow is a whole nother world from just wet roads.

I will take your word on that. I guess plowing can generate heat and melt what is under the wheels, giving more traction than ice perhaps?

 

Sometimes ploughing can help , in fact most times , that's why it is a fact that stopping distances on snow are INCREASED with ABS .

On dry surfaces , ABS works by pulsing the brakes so you alternately have a quick braking phase and a quick steering phase . On snow , the problem is your braking phase generally consists of a quick slide generating very little braking because of the lack of resistance between the snow and the tire , in ploughing , although you may initially meet limited resistance and therefore limited braking , the resistance increases as the snow accumulates and the tire gains resistance . You can then release the brake , allow the wheel to rotate and steer and then re-apply . Essentially the same concept as ABS but on snow it is beneficial for the braking period to be longer to allow for resistance to be built up . Rapid ABS pulsing feels like you are skipping across the surface .

 

Amen!


I don't know about anyone else, but I've avoided more accidents by using my steering wheel rather than my brake pedal.

 

Close. The ABS system will allow maximum braking force until one or more wheels slip, then the system will modulate the wheel(s) that are locked up, and reduce the pressure JUST enough to allow the wheel to continue to have rotation, but the best braking WITHOUT locking up. The idea is that you obviously have the most traction (thus steerability) if the wheels are at the point just before lockup. It does not "switch" between braking and steering.