Auto Service World
Feature   August 1, 2004   by Auto Service World

Light truck ABS malfunctions

It may just be perception, but a scan of technical bulletins and anecdotes would seem to indicate that light trucks suffer more than their fair share of ABS problems.

It may not be all in our imagination, however; light trucks, and the tendency for them to be equipped with 4WD, do complicate the control systems that ABS systems require.

One such problem relates specifically to the 4WD function. It even generated a recall campaign affecting 4X4 Blazers and S10 pickups from 1991 to 1996 and 1994 to 1996 respectively.

These vehicles suffered from the possibility that shifting into and out of 4WD could generate a condition that appeared to indicate a defective transfer-case logic switch.

This switch signals the anti-lock-brake system module, and changes the algorithm to compensate for the brake torque applied to the rear wheels through the drivetrain in 4-wheel drive.

If the switch malfunctions and the vehicle is operated in 2-wheel drive, an incorrect signal may be sent to the ABS module. The result is that insufficient brake pressure is applied, which will lengthen the stopping distance and maybe even cause a crash–which is why the recall occurred.

Of course, this is just one example of ABS malfunctions lengthening stopping distances.

In the 1990s, GM had an issue with its RWAL-equipped GMC and Chevrolet pickups equipped with leading/trailing type rear brake shoe systems.

The symptoms were a lack of stopping power, excessive pedal pressure or travel, and increased front pad wear. These signs seemed to point to a failure of the electro-hydraulic ABS valve, which had previously been noted as having a potential internal failure.

The fix was to first determine that the EH valve was not leaking internally into the accumulator, then to take a look at the shoes. The telltale sign of uneven wear, where the top 1/3 of the shoes is worn much more than the bottom 2/3, pointed to the fact that the shoes were not sliding on the lower tapered shoe mounting block. This lack of movement forced the brake pressure to be concentrated at the top of the shoes. The leading shoes are somewhat self-energizing due to the direction of rotation of the drum. If the tapered mounting block becomes a pivot point, rather than allowing the shoes to slide outward, the excessive wear will occur at the top.

When inspecting the brakes, a technician should tap downward on the top of each brake shoe with the palm of his hand. The brake shoe should easily move downward. Then he should tap upward, seating the shoe in its normal at-rest position. Again the shoe should move with normal resistance. The mounting block should be lubricated whether new shoes are installed or the existing shoes are being serviced.

This example serves as a good reminder that even with ABS, a brake problem could be the result of mechanical malfunction.

This is, of course, not always the case. More and more, weird braking anomalies lead technicians to suspect a system failure. Tops among the list of suspects is the wheel speed sensor.

Since these are located out in the open, they are subject to the elements more than any other ABS components. Salt, water, dirt, dust, and physical damage can cause a sensor to fail, or just generate intermittent signals, which is possibly worse. Checking the function of a wheel speed sensor has become more difficult over the past while, with different types of sensors being employed.

There are two types of wheel speed sensors that are available on 1999 and newer vehicles.

Variable Reluctance Wheel Speed Sensors produce their own voltage when rotating the wheel.

With the ignition key off, utilizing a digital volt ohmmeter, coil-winding resistance can be measured. If the coil resistance checks good, position the vehicle safely on jack stands to test sensor voltage generating capability. Typically, rotating the wheel by hand will produce an AC voltage output of about 60mv or more.

Magneto Resistive Wheel Speed Sensors rely on 12 volts DC from the ABS Computer.

This new type of sensor has two wires, and inside the sensor is a small power supply, returning approximately 0.90 volts back to the computer. This reading occurs when the valley of the tone wheel is aligned with the magnet of the sensor. As the tone wheel tooth approaches the magnet of the sensor, signal voltage should increase to approximately 1.65 volts. The computer measures the digital voltage and amperage signal for the interpretation of wheel speed. To check a magneto resistive sensor, the technician should position the vehicle safely on jack stands and very carefully back probe the wheel speed sensor. With ignition key on (engine off), one wire should supply voltage from the computer to the sensor. The other wire provides a signal voltage back to the computer. Technicians should be sure to have a good ground connection to the negative lead of their meter. Rotating the wheel very slowly should result in signal voltages changing from a little less than one volt to a little more than 1.5 volts.

A lab scope is recommended to test these sensors. Technicians should look for consistently sharp corners on the DC signal to the computer. For comparison purposes, all four-wheel speed sensors should display similar waveforms.

Of course, sometimes a system will set a number of trouble codes related to the ABS system. It is the kind of situation that can drive a technician crazy.

A trouble code tells him to look in one area, but then that component checks out fine. Then again and again. This is exactly the situation with some year 2000 GM vehicles, more specifically the S10, Blazer, Envoy, Jimmy, and Sonoma.

The electronic brake control module (EBCM) has been known to generate a flurry of trouble codes. As it turns out, this was the result of a build problem that placed the wiring harness too close to the exhaust manifold.

The harness runs down the driver’s side of the engine bay and the heat from the manifold can melt the wiring harness.

This can cause high resistance through the wiring, or short circuits to ground.

The EBCM has the capability to perform self-testing, and when it does so, it will identify the increased resistance and shorts and set codes pertaining to the faulty circuits.

The fix is to identify if the harness is located close to the manifold and if it is damaged. If this is the case, it needs to be repaired and relocated.

These are just a few examples of the types of challenges that ABS problems can pose, specifically regarding light trucks.

It is always useful for a professional counterperson to remember that regardless of the sophistication of ABS systems, it is often standard brake issues–such as noise–that can compromise good customer satisfaction.

Troublesome Trucks

1990-92 GM light trucks with 4WAL ABS systems can exhibit a code 61, 62, or 63. These codes signal a problem with reset switches, which are inside the Electro-Hydraulic Control Unit (EHCU).

These codes usually occur on vehicles that are three years old, or older.

The codes could be caused by contaminated brake fluid, which is allowing debris to flow through the system, or pushing the caliper piston in without opening the bleeder screws. The EHCU can be affected by contaminated fluid because of the very small orifices and passageways. One common problem is for one of the reset switches to become sticky and create a code.

If all three codes are present simultaneously, there is probably an electrical ground problem since these switches ground through the brake line. To determine if the contaminated fluid has caused this problem, the technician should remove the fluid from the reservoir and look for sediment in the bottom. If sediment is in the bottom of the reservoir, it is also likely to be in the whole system.

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