Hydraulic ABS^Article

March 1, 1999
SPRINGTIME always brings new changes, and this year is no exception. This spring, hydraulic antilock brakes systems (HABS) on straight truck chassis are

SPRINGTIME always brings new changes, and this year is no exception. This spring, hydraulic antilock brakes systems (HABS) on straight truck chassis are going to be in your shop very soon.

In the past a great majority of medium duty users have opted to not use hydraulic antilock. Effective March 1, ABS is mandatory on all commercial vehicles over 10,000 pounds GVW that are equipped with hydraulic brakes.

This legislation will affect light- and medium-duty commercial vehicles, including straight trucks, commercial and specialty fabricated busses, cab and chassis over 10,000 GVWR, cut-a-ways, walk-in vans, and vehicles fabricated for recreational usage. In effect, most commercial vehicles that pass through truck equipment shops for upfit and body installation will now carry ABS componentry. In most cases, these light-and medium-duty ABS equipped vehicles will be hydraulic or air over hydraulic.

At this year's National Truck Equipment Association (NTEA) Convention in Indianapolis, vehicle manufacturers discussed how upfitters should work on HABS equipped chassis. Trailer/Body Builders will carry a complete report on the Convention in the April issue. Understanding a few simple nuances of the HAB system might help to avoid possible upfit problems.

Important System Facts

The HABS electronic control module (ECM) is powered by the vehicle's electrical system. It is important to remember this fact when doing any welding or chassis modification that might run current through the electronic componentry. A good rule of practice is to remove both positive and negative leads from the battery when welding. At the outset of any welding, make sure the ground is as close to the work as possible. This helps eliminate loops and power surges through the ECMs.

Do not splice or tap into the HABS electrical harnesses. HABS is designed to be a dedicated electrical system. As a body installer, it is important to educate technicians not to splice into or otherwise use any ABS electrical harness for power or grounding purposes.

When shortening or lengthening a chassis, refer to the body builder's book. Shortening a chassis would normally allow the upfitter to simply "loop" the ABS harness in frame. Use gentle loops, and do not pinch the wire in tight tie-downs. Make a visual check to confirm that the newly installed body does not rub or pinch the harness.

When lengthening a wheelbase, contact the chassis OEM or dealer to see if a "wheel jumper" harness is available. These harnesses attach to the existing ABS electrical harness and allow the body builder to add additional length to the wheelbase. Chassis manufacturers at the recent NTEA convention stressed that upfitters should not attempt to fabricate any part of this harness.

Some chassis manufacturers are producing wheel jumper kits for specific classes of vehicles in their line. As an example, Isuzu is planning to produce the kits for FRR, FSR, FTR, and FVR in the 2000 model year. Other manufacturers are styling their jumper kits to only be applicable for Class 5-7 wheelbase changes.

Ford's model year 2000 introduction of the F-650 and F-750 (at 30,000 GVW) will have hydraulic antilock brakes as standard equipment. Ford is reviewing the comments of body builders to determine possible needs for wheel jumper kits.

Distributors should check the body builders book for details and additionally verify that the chassis manufacturer authorizes wheelbase alterations.

Wheelbase alterations may require shortening or lengthening the hydraulic lines. Some OEMs recognize this and are supplying hydraulic brake line for this purpose through local truck dealers. These supplemental parts are made of standard approved brake line. However, OEMs strongly recommend that connections be double flared to insure line integrity. Remember that this line will have some additional flexing and quicker internal pressure changes.

Brake line clearance and fluid temperature also are important. Truck manufacturers typically are requiring at least 2" clearance between the brake line and any new body installations. This will help insure that lines are not pinched and there can be airflow between the line and the body installation.

The effectiveness of hydraulic fluid is reduced when the temperature reaches the point of boiling water. Installers of asphalt spreaders and mid-ship feed/salt spreaders with an underbelly motor, or ambulance builders with generator installs, might check design plans. Some unusual applications create a great amount of heat buildup under the fabricated body. Awareness of this could be important for brake operation and trouble-free body installations.

In cases where the wheelbase is drastically changed, the HAB system may need to be recalibrated. There aren't any hard rules on this subject, but General Motors has published a service bulletin on this subject. It would be advisable to contact your body builder's book and call the OEMs help line, or check with your local dealer.

The Light Comes On

Cab and chassis manufacturers have installed a dash-mounted ABS indicator light. This light will activate for a few seconds during initial key-on of the vehicle. If the light remains on, there may be a problem with the ABS componentry. The vehicle might be driveable. However, a driveable vehicle does not signal a successful body installation if the ABS indicator light remains illuminated.

HABS works in a similar fashion to air ABS. On straight trucks that use hydraulic brakes, a typical HABS will have four sensors and four modulators (4S/4M). Navistar, Ford, General Motors, and Freightliner generally use this type of 4S/4M system.

Manufacturers also offer 4S/2M systems. Akibno, wh ich builds HABS for Isuzu and Mitsubishi Fuso, uses both three-and four-channel systems.

How It Works

Antilock brakes are designed to provide the end user with the optimum balance between vehicle stability/steering and stopping distance. This is accomplished by a complex electro-mechanical-hydraulic combination of sensing if the wheel is turning or is locked-up during braking

On the steer axle the wheel's rotational velocity provides steerability to the truck while maintaining traction with the road. On the drive axle the wheels maintain traction while brake pressure is applied. The object of any ABS system is to provide a positive traction to a rotating wheel while still having braking forces applied to that wheel.

This complex balance is reached is by an electronic control module (ECM) that measures wheel speed, and a modulator that mechanically changes applied brake pressure through hydraulic modulation. In other words, there are two major components to an average ABS system. One measures wheel speed and one controls braking forces. Additionally there may be other controller or relay parts involved in the HABS that the chassis manufacturer has designed into the system.

Tone Is Important

The tone sensor and ring are two parts that can possibly be damaged during body installation. In some instances shop personnel remove wheels in order to expedite the body installation process. In limited cases where shop personnel get into the hub assembly, tone ring damage can happen.

"Tone sensors should be removed and protected any time a body installer must remove a wheel for an extended period of time", says Nagesh Ananth, brake systems engineer for LucasVarity. Ananth describes the tone sensor as a precision variable reluctance reader that produces and reads an alternating current off the tone ring. Protect the facing of the tone reader and the tone ring. This device sends wheel revolution information to the ECM.

For example, on Ford front axles the tone sensor is mounted into a flange with a setscrew placement. When the setscrew is placed into the correct mounting position, the tone sensor will be set to the correct distance from the tone ring.

The tone sensor is housed in a push-through housing on GM, Ford, and Navistar rear axles. The tone sensor, after removal from the housing, should be gently pushed back into place to zero tolerance against the tone ring. During vehicle operation, the tone sensor should wash-back automatically to an acceptable clearance.

As with the examples above, it is important to contact the local dealer or OEM to ensure that the proper removal and installation practices will be observed for that vehicle model and HABS model. The tone sensor should have a .000" to less than .035" clearance to remain in effective operation. There is not a great amount of clearance for sensor play in this operation.

Tone sensor rings can be mounted on axles or hub assemblies. There are several different styles of mounting this assembly. Contact the equipment manufacturer for further information.

When the electronic control module senses that the wheel assembly is in lock-up, it modulates the hydraulic pressure to the brake mechanism to allow the wheel to have some rotational capability.

Past Success

LucasVarity currently holds in excess of 80% of the HABS market in Class 5-7 with its EBC 410M HABS system. "The EBC 410M is a system that is being delivered to GM, Ford, and Navistar for production installation as the mandate takes effect in March", says Bob Shutz, Senior Business Manager. Shutz says the EBC 410M has been a premandate option for many chassis manufacturers for several years. For example, GM has offered HABS since 1996 and has reported little difficulty in usage for body upfitters.

LucasVarity also produces HABS for light-duty applications. Shutz confirmed that this HABS is engineered in a similar fashion to the EBC 410M. There are changes in componentry, but the average body installation for these light duty chassis would not require extensive HABS work.

Bill Thomas, marketing programs manager for Freightliner Corporation, reports that Freightliner has used a Meritor WABCO hydraulic antilock system since 1996 for its medium-duty product. This premandate option was used by many federal, state, and local municipalities, and also by some private fleet operators.

For most body installation purposes the OEM's body builder's book should help answer any installation questions. Also ask your local dealership for any service bulletins that might be helpful. LucasVarity is a resource if you have a problem directly related to a LucusVarity antilock product. Shutz' direct line is 734-266-4986.

Meritor WABCO has hydraulic ABS information kits available and a number for information on Meritor WABCO products at 800-535-5560.

The Future Is Clear

Hydraulic antilock brake systems are here to stay. There will be future product enhancements such as automatic traction control (ATC) and electronic stability control (ESC). As Shutz explained, "LucasVarity is moving ahead with new product development into traction control for vehicles." HABS may become an intrinsic part of vehicle control programming as opposed to just a system of the vehicle. The future won't stop with antilock.