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ABS Not ABS'ing?

Q. I recently brought a 2000 Daewoo Leganza SX. It has a 4 cylinder and has auto transmission and fuel injection. It has PS, A/C and cruise control. I had only 2000 miles on the car when I had the accident. The car has ABS. I was driving on a wet surface (I was on the highway and it was raining). The car started to slide and I stepped on the brakes. The car simply went out of control at that point and skid until I was able to crash into a side barrier. I thought that anti lock brakes did not "lock up" when you apply pressure.

Should I have pumped the brakes instead?


A. The most common misconception of ABS is that they will stop on a dime on any surface. As with normal brakes you should adjust your driving to the road conditions at the time. Under no circumstances should you pump an ABS system, that defeats the whole purpose of ABS brakes. You should apply a firm constant pressure to the pedal and let the ABS do the pumping. It can do it a whole lot faster than you can.

Let me describe how the Anti-lock Braking System works and you'll get a better idea of what might have happened to you. This applies to vehicles with four wheel ABS. There are two wheel ABS systems, and the principles are the same, only either the front or rear wheels have the ABS.

The Main components are the ABS hydraulic unit, Vehicle Speed Sensors (VSS), wheel sensors and the ABS Control Module. Inside the ABS hydraulic unit are motors that do the actual brake "pumping" and controls the valving of fluid to each wheel. There is a sensor mounted near each wheel to measure the speed the wheel is turning and when it stops turning. When you step on the brakes the wheel sensors report to the ABS control module that all wheels are turning evenly and there is no wheel lock up, the module lets the brakes perform as normal brakes. When you apply the brakes on a slick surface, the control module measures the four wheel speeds and activates the motors in the pump to apply pressure to each wheel in such a way that the wheels will get the proper pressure to stop the vehicle evenly.

Say, for example, the left two wheels are on ice and the right two are on dry pavement. You step on the brakes and the left wheels lock up. The control module sees this and activates the pump to release the pressure to those wheels so they will turn and the vehicle will travel in a straight line. As soon as the wheels start turning again, the pump will again apply pressure to those wheels until they lock up again and start the cycle over. The idea is to keep the wheels turning so you have steering control yet have the car stop as quickly as it can.

ABS performs best in a panic stop on dry pavement. It stops the car quicker than regular brakes and allows you to keep steering control of the car. However, when the road is ice or rain covered the wheels will lock up sooner and the ABS will cycle quicker. It still improves your braking and steering ability, but you will not stop "on a dime" as it were. You still need to consider road conditions when you drive with or without ABS brakes.

There are two possibilities in this situation, the ABS either failed to operate or road conditions were such that the ABS did the best it could and could not cope with them. You mentioned that the car started sliding before you applied the brakes which indicates to me that road conditions were quite slick and nothing would help control the car. There is a very good possibility that your car was hydroplaning which is just like driving on a sheet of ice.

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Additional Information provided courtesy of AllDATA and Warranty Direct
© 2000-2008 Vincent T. Ciulla

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