ABS Problems on American Made Automobiles
Many States and municipalities in the U.S. use domestic automobiles to fill their transportation requirements. Generally they prefer to buy American as opposed to using tax dollars to purchase foreign automobiles. Therefore mechanics working on government fleet vehicles often find themselves working on domestically built cars and trucks exclusively.
These organizations become an excellent source of identifying common problems and solutions as they are supported by car makers such as GM Fleet operations. In this article we will focus on a couple of ABS problems on American made automobiles plus provide tips for diagnosis and repair. On auto repair.answers.com there is an overview article about the ABS system and its warning lights. This is a great place to start if you want to learn more about the system, including normal operational characteristics.
ABS Active at Low Speeds
A common complaint from drivers that own GM and Chrysler vehicles built in the mid to late 90s is when they’re slowing down the ABS goes into active mode at around 5 mph. Defining the term active, is when the system starts to pulsate the brake pedal as if there were an emergency stop or lockup condition such as a slippery road surface.
In this situation it is a false activation as all of the wheels or rolling freely and none are locked up. Those experiencing the problem are vocal about it because having the brake pedal vibrating at every stop is annoying to say the least. From a safety stand point low speed braking distances can increase when cars suffer from this condition.
Low Speed Activation Solutions
People have reported problems with individual (WSS) wheel speed sensors malfunctioning at low speeds. If there’s a problem with the wiring or sensor itself the ABS light will turn on and set a code for that particular WSS. This can simplify diagnosis by directing your attention to one specific wheel. Since the sensors are external and adjacent to the road surface is not unusual to find broken, damaged parts or wiring.
When you have ABS going active at 5 mph without turning on the warning light or setting a code it’s time to get a close look at the air gap between the WSS and the reluctor ring which is a toothed gear that rotates with the wheel (pictured above). These problems can be caused by the teeth of the ring developing corrosion, rust or debris causing them to change shape or to have less defined gaps.
The slower the vehicle is traveling the harder it is for magnetic sensors to read the peaks and valleys of the gear. In some cases professional mechanics have found rust or metal debris bridging the air gap between the toothed ring and the sensor. At low speeds these types of situations can provide a false indication there is a wheel not moving or locked up when there’s not.
Problems with Chrysler Bendix ABS
Chrysler cars and minivans that include some of the most popular models ever built such as the Dodge Caravan had a problematic Bendix anti lock brake system installed in the early 90’s. These automobiles were experiencing large numbers of failures with the ABS pump motor and pressure sensors. These systems were designed as a one-piece unit that included the master cylinder and all valves, switches and solenoids.
As these systems aged primary seals in the master cylinder began to leak which would increase the pump running time as it would take longer to achieve full system pressure. Eventually the pump would run all the time until it burned out. Chrysler recalled the early 1990s vehicles having the Bendix 10 braking system installed. Although they will only repair problems related to the ABS pump motor. Jeep has a similar recall on Cherokee models with the Bendix 9 system that can be found on some late 80s Jeep vehicles.
Other Problematic ABS Systems
Bendix was not the only manufacturer to have problems with their anti lock brake systems. A GM partner and supplier also had problems with their ABS pump motors. Kelsey Hayes another manufacturer that teamed up with General Motors had their share of problems with the electronic brake control module. So many of these vehicles needed repair, businesses opened up and specialized in just fixing these control modules.
If you have a brake system problem on an American made car you’re probably not alone. Check your specific model for recalls and technical service bulletins. Use the owner’s manual to call customer service and ask them to run your vehicle identification number for any open recalls. If you are experiencing low speed activation it’s time for some diagnosis. Since speed sensing components are located in harsh environments visual inspection will be necessary to see if rust or corrosion needs to be removed from both the reluctor rings and the magnetic wheel speed sensor, also known for attracting metallic dust from brake pads.