TRUCKS: Fleet Managers Required To Brake for ABS

Bob Deierlein

Until recently, price-conscious waste industry fleet managers looking to reduce costs often omitted optional anti-lock braking systems (ABS) when spec'ing trucks. They couldn't justify the costs to purchase, maintain and train employees on the system. However, ABS now are required on all hydraulic braked trucks of more than 10,000 pounds gross vehicle weight (GVW) produced after March 1, 1999, to reduce accidents and improve highway safety.

Fortunately, the systems have improved and prices have been reduced recently, so ABS generally are considered to be cost-effective. Although the price now is moot, typically, the $1,500 to $2,000 add-on cost is buried in the vehicle's price.

Fleet managers say ABS can reduce skidding and jackknifing, and assist drivers during a panic stop by modulating the brakes but allowing the driver to steer.

"During panic braking without ABS, a driver rapidly applies force to the brake pedal and either backs off or applies too much pressure for the tire road condition," says Tom Musselman of Ford Motor Co., Dearborn, Mich., who addressed the ABS topic at a Truck Maintenance Council presentation in Kansas City, Mo. "Too little pressure means longer stopping distances; too much pressure means wheel lock-up. If the front wheels lock-up, the driver will not be able to steer. If the rear wheels lock-up, the vehicle spins out of the lane."

An ABS includes a wheel end exciter ring and sensors, an electronic control unit (ECU), a hydraulic control unit (HCU) and an ABS wiring harness. When the driver brakes firmly, sensors monitor wheel speeds. If the situation demands it, the ABS is invoked. Using the wheel speed information, the ECU and HCU modulate brake pressure at all wheels. Hydraulic pressure is adjusted at each wheel individually to prevent wheel lock-up.

While ABS prevents wheels from locking; provides steering ability during braking and directional stability; ensures optimal braking efficiency; and increases vehicle and occupant safety, "don't assume shorter ABS stopping distances under all conditions. Twice the speed implies approximately four times the stopping distance," Musselman cautions.

"Also, inform drivers that there is a slight varying pedal pulsation during the entire stop," he adds. The ABS motor and pump may chatter, and wheels may chirp as they approach lock-up on some surfaces.

To operate ABS effectively, drivers should:

* Be aware of conditions that affect braking. Slow down prior to braking and apply the brake firmly and hold it firmly during the entire stop.

* Don't pump the brakes and don't drive aggressively.

Maintenance is minimal, Musselman says, and control unit designs have a long life. Also, check and adjust sensors when servicing wheel equipment.

Additionally, ABS has self-diagnostic capabilities at startup. An ABS bulb will signal at ignition while the system self-checks the relay, valves, coils, motor, pump, sensors, etc. Fail-safe features continuously monitor the system for potential faults, taking action when detected. After ignition, if the ABS lamp turns on, it means the system is disabled. The general response is to relight the lamp and return the vehicle to conventional braking.

Overall, "ABS have no major effect on vehicle maintenance," Musselman says. "Bleed the brakes at the calipers as you do currently - solenoids remain open and do not trap air. When you remove and re-install the rotors or adjust the wheel bearing end play, reset the wheel speed sensor. Use the special tool to press the sensor into light contact with the exciter ring (similar to current ABS for air braked vehicles). Do not tap or splice into the ABS electrical circuit."

Brake fluid also should be compatible with the system to prevent metal parts corrosion, lubricate cylinder and pistons, control elastomer swell and prevent vapor lock.