Kelsey-Hayes rear wheel antilock (RWAL) systems have been gotten a bum rap in recent years. The RWAL system on 1991-96 Chevy S-10 and S-15 pickups, Chevy Blazer, GMC Jimmy, 1992-95 Chevy/GMC Suburban and 1990-95 Chevy Astros have been the subject of numerous "investigative reports" by various news media, the target of a class action lawsuit (dismissed in August '87) and an NHTSA investigation which found nothing (in spite of having received more than 7,000 complaints including 1,600 accidents and more than 500 injuries!).
The Kelsey-Hayes rear wheel antilock brake system was first used by Ford in 1987 on F series trucks, and was later added to the Ford Ranger, Bronco, Bronco II, Explorer, Aerostar and Econoline vans. Ford calls their version of Kelsey-Hayes EBC2 system "RABS" for Rear-wheel Antilock Brake System."
The GM RWAL version is found on '88 and later "C" and "K" full-size pickups, "S" and "T" series pickups, some "S" series Blazers, '89 and up Astro minivans, '90 to '92 "R" and "V" series light trucks and "G" series vans.
Dodge has also used the Kelsey-Hayes RWAL system since '89 on its "D" and "W" 150/350, Dakota and Ram pickups. Geo, Isuzu, Mazda, Nissan and Subaru have used the system since 1991.
Considering how many Ford, Chevy, GMC and Dodge
trucks have the Kelsey-Hayes rear wheel antilock brake system,
it's not surprising that a certain number of these vehicles would
experience some kind of problem during their lifetime. NHTSA says
there's no inherent defect in the system, so any failures that
occur are the result of normal service conditions.
If a small piece of dirt or rust gets into the unit, it may prevent the dump valve inside the EH valve from closing. The dump valve will then leak fluid into the accumulator when the brakes are applied. The misrouted fluid allows the pedal to drop, and the pedal may go all the way to the floor without applying the brakes. No ABS warning light or fault code will be found either because the limited diagnostics on this system can't tell if the dump valve is fully closed or not.
Wagner Brakes recommends the following procedure to find out if the problem is a bad master cylinder or a leaky EH valve in the RWAL system. To rule out a bad master cylinder, disconnect the rear brake line at the master cylinder and install a plug to block off the port. Have an assistant depress the brake pedal about an inch to purge any air from the outlet, then tighten the plug while the assistant holds the pedal in this position. After tightening the plug, continue to apply pressure to the pedal to prevent damage to the primary cup in the master cylinder as the cup moved across the vent port.
If the pedal holds and no longer drops, it isn't the master cylinder that's causing the problem.
Reconnect the brake line to the master cylinder and block off the outlet port on the EH valve. Then apply pressure to the brake pedal again to see if the pedal drops. If the pedal goes down, the EH valve is leaking fluid into the accumulator. The EH valve needs to be replaced.
It's important to note that one of the underlying causes of EH valve failure is moisture-contaminated brake fluid. If the fluid is more than a couple years old, it may contain a lot of moisture. GM trucks do not have a recommended service interval for brake fluid, but most experts say that flushing the brake lines every couple of years for preventative maintenance can greatly reduce the risk of internal corrosion inside the brake lines and EH valve. But most people totally ignore their brakes until something goes wrong. So the best you can do is flush the system and replace the old fluid with new when the EH valve is replaced -- then recommend a fluid change every two years to prevent a repeat occurrence.
Air trapped in the EH valve can be another source of trouble with the RWAL system. Some RWAL EH valves (the smaller ones) do not have a bleeder screw, which makes the unit hard to bleed if air gets into the lines. The trick here is to loosen the brake line connections at the valve to vent air when bleeding the brakes.
Brakes can be bled in the usual way manually or with pressure equipment. The wheel bleeding sequence is RR, LR, RF, LF. If a pressure bleeder is used, the combination valve must be held open.
The conventional master brake cylinder and power booster supplies brake pressure to the EH valve, which contains two solenoid valves: a normally open isolation valve to block pressure from the master cylinder to the rear brakes during antilock braking, and a normally closed dump valve for relieving pressure in the rear brake circuits. The EH valve also contains a pressure accumulator for storing fluid pressure during the dump or release phase of operation, and a reset switch which allows the system to maintain proper brake pressure.
When the ABS control module detects a difference in the average speed of the rear wheels compared to the vehicle's overall speed, it initiates antilock braking. The ABS isolation solenoid is energized to prevent any further increase in brake pressure at the rear wheels, and then the ABS dump solenoid valve is opened to release pressure from the rear brake circuits so the wheels can regain speed and traction. Pressure is reapplied when both solenoids are de-energized and return to their normal positions. The cycle is repeated continuously for as long as ABS braking is needed or until the vehicle stops.
The control module that regulates the operation of the ABS solenoid valves is separate from the control valve, and is located next to the master cylinder on some applications. On Dodge trucks, it is on the passenger side cowl panel under the dash. On Ford Bronco IIs, it is located under an access panel by the driver's door pillar.
The control module receives a speed signal from a single vehicle speed sensor. On Ford and Dodge applications, the speed sensor is in the differential and the sensor ring is on the ring gear. On GM applications, the speed sensor is located in the transmission tailshaft, and the sensor ring is on the transmission output shaft.
On GM trucks, the speed sensor signal first passes through an intermediate module on its way to the antilock controller. This module is called the "Digital Ratio Adapter Controller" or "DRAC." The DRAC module translates the analog speed sensor signal into a digital signal that can be processed by the ABS control module. The DRAC module also divides the basic speed sensor signal into three separate frequencies of signals that are used by other vehicle systems:
The DRAC module is calibrated to the final drive ratio and original equipment tire size of the vehicle. Replacing the original tires with ones of a different size or aspect ratio will change the speed sensor signal, which in turn can adversely affect the operation of the ABS system as well as torque converter lockup and the accuracy of the speedometer and odometer.
If non-stock tires are installed on a GM vehicle with a DRAC module, therefore, the DRAC module must either be replaced or recalibrated. On 1991 and earlier "C" and "K" pickups, the DRAC modules can be recalibrated for different tire sizes and axle ratios by changing the configuration of an 8 pin connector that plugs into the instrument panel circuit board connector. A DRAC recalibration kit from GM is necessary for this procedure. By referring to a speedometer calibration chart in the factory service manual, the correct pin positions can be determined for any tire size and gear ratio combination. The specified pins are then broken off the connector. This alters the DRAC circuits when it is replaced in the instrument cluster to recalibrate the vehicle speed signal.
The "S" and "T" Blazers
and vans as well as '92 "C" and "K" pickup
truck applications have a DRAC that is a sealed plug-in module
referred to as a "Vehicle Speed Sensor Buffer." It is
matched to the original gear ratio and tire size of the vehicle,
and cannot be recalibrated. Any changes in tire size or axle gearing
requires replacing the DRAC or buffer with one that's correctly
calibrated for the tire size application.
In GM applications, the brake warning light will be brightest when grounded by the parking brake, a little dimmer when grounded by the hydraulic system and dimmest when grounded by the ABS controller.
To determine the cause of a red brake warning light in a GM application:
The flash code sequence for Ford, GM, Dodge and import trucks with the Kelsey-Hayes rear-wheel ABS system are the same, but the flash codes are slightly different. If you're reading flash codes, start counting from the first long flash. Then add the short flashes that follow. The total of the long flash plus any short flashes that follow equals the number of the code.
Because the system can only do one code at a time, the first code must be read out, diagnosed and repaired before the system can generate any additional codes.
Beginning in August 1990, a software change was made in the ABS controller on all General Motors RWAL equipped vehicles. The change transforms codes 6, 9 and 10 into "soft" (non-latching) codes that only illuminate the warming light as long as the fault is present. If the fault is intermittent and no longer exists, the light will go out with the next ignition cycle. The conditions under which the codes are set remains the same, as do the basic diagnostic procedures.
A scan tool must be used on the newer GM applications
to read any soft (non-latching) codes that might be present because
grounding the "H" terminal on the ALDL connector erases
To manually clear a stored fault code on GM
and some Dodge trucks with RWAL, remove the ABS fuse or disconnect
the battery for 5 seconds. Ford RABS codes will automatically
clear when the ignition is turned to the off position.
Kelsey-Hayes RWAL and RABS systems are vulnerable to a number of false codes. If terminal "H" on the ALDL connector is grounded when the warning light is not on, it will set a code 9. Nothing is really wrong with the system and the code can be cleared.
On 1989-90 GM "S" and "T" series trucks, and 1988-90 "C" and "K" series trucks with manual transmissions, a false code 7 can be set if the blower motor is set on high, and the ignition is turned off while the brakes are applied and the truck is still moving. This "glitch" was eliminated on 1991 models when a change was made in the ABS module.
Another false code problem has been noted on some 1988 and '89 "C" and "K" series GM trucks. A code 10 may be set when the vehicle is traveling over 37 miles-per-hour and the ABS controller sees an open circuit or no voltage in the stop lamp switch circuit. Later models have a revised brake pedal switch that eliminates the problem.