IMPORT ABS SYSTEMS:

DIAGNOSE AND CORRECT CONVENTIONAL BRAKE PROBLEMS BEFORE TROUBLESHOOTING A SUSPECT ABS PROBLEM

Servicing import brake systems today means working on a variety of different ABS systems. Over the years, there have been successive generations of ABS systems from Bosch, Kelsey-Hayes, Teves and others. So it’s important to know what kind of ABS system is on the vehicle and how it works if the ABS warning light is on, or if you suspect an ABS-related problem (see Import ABS Application Chart on page 24).

Regardless of who makes the system, most ABS systems work pretty much the same. The anti-lock function should come into play only when braking on a slick surface (rain, ice, snow or mud) or when making a hard, panic stop. The rest of the time, ABS is just along for the ride and should have no effect on normal braking.

All anti-lock brake systems control tire slip by monitoring the relative deceleration rates of the wheels when the brakes are applied. If one wheel starts to slow at a faster rate than the others, it means the wheel is starting to slip and is in danger of losing its grip, locking up and skidding. The ABS system responds by momentarily isolating the brake circuit for that wheel, releasing hydraulic pressure and then reapplying the brake in rapid sequence until traction is regained or the vehicle comes to a halt.

Electrically operated solenoid valves in the ABS modulator hold, release and reapply hydraulic pressure to each brake circuit. This produces noise and a pulsating effect that can be felt in the brake pedal. This is normal and helps the driver realize that the ABS system is doing its job - but some motorists who are unfamiliar with ABS may think there’s something wrong with their brakes.

SYSTEM CONFIGURATIONS
All ABS systems keep track of wheel deceleration rates with wheel speed sensors. Some have a separate wheel speed sensor for each wheel (four-channel systems) while others use a common sensor in the differential or transmission for both rear wheels (three-channel systems). The common sensor for the rear wheels reads the combined or average speed of both rear wheels.

Another variation is the "single channel" rear-wheel only ABS system that is used on many rear-wheel-drive pickups and vans (Kelsey-Hayes RWAL and Toyota Rear-Wheel ABS). The front wheels have no speed sensors and only a single speed sensor mounted in the differential or transmission is used for both rear wheels. Rear-wheel anti-lock systems are typically used on applications where vehicle loading can affect rear wheel traction, which is why it’s used only on pickup trucks and vans.

INTEGRAL OR NON-INTEGRAL?
Most import ABS systems are "non-integral" (add-on) systems because they use a conventional master brake cylinder and vacuum power booster with a separate ABS modulator. Some also have an electric pump for ABS braking (to reapply pressure during the ABS hold-release-reapply cycle), but do not use the pumps for normal power assist. Non-integral systems do not have to be depressurized or deactivated prior to working on the brakes. Non-integral (add-on) systems include Bosch 2, 2E, 2U, 5.0 and 5.3 ABS, Kelsey-Hayes 4WAL and EBC325 ABS, Honda ABS, Nippondenso ABS, Sumitomo ABS and Teves Mark 4 and Mark 20 ABS.

The only integral ABS system used on import applications is the Teves Mark 2, found on 1987 and up Saab 9000, 1988 to 1989 Merkur Scorpio, 1988 to 1991 Peugeot 505, 1991 and up Saab 900, and 1991 and up Volkswagen Passat models. The Teves Mark 2 ABS system combines the master brake cylinder and ABS modulator, pump and accumulator into one assembly. Teves Mark 2 does not have a vacuum booster for power assist and relies instead on pressure generated by the electric pump for this purpose. The accumulators contain more than 2,700 psi, so they must be depressurized prior to working on the brakes by pumping the brake pedal 25 to 40 times while the key is off.

WHEEL SPEED SENSOR DETAIL
Wheel speed sensors consist of a magnetic pickup and a toothed sensor ring (sometimes called a "tone" ring). The sensors may be mounted in the steering knuckles, wheel hubs, brake backing plates, transmission tailshaft or differential housing. On some vehicles, the sensors are an integral part of the wheel bearing and hub assembly. The sensor rings may be mounted on the axle hub behind the brake rotor, on the brake rotor itself, inside the brake drum, on the transmission tailshaft or inside the differential on the pinion shaft.

Wheel speed sensors generate an alternating current (AC) signal that increases in frequency and amplitude as wheel speed increases. Because the sensors are magnetic, they can attract metallic debris from semi-metallic brake linings and rotors that stick to the tip and interfere with the signal. If the ABS module doesn’t see a clean WSS signal, it may think there’s something wrong and set a wheel speed sensor code.

The distance or "air gap" between the end of a wheel speed sensor and its ring is also critical. A close gap is necessary to produce a strong, reliable signal. You don’t want metal-to-metal contact between the sensor and its ring since this would damage both. But neither do you want too much clearance. An air gap that’s too wide may produce a weak or erratic signal or, worse yet, no signal at all. So if a wheel speed sensor is adjustable (many are not), refer to a shop manual for the required air gap and adjust it to specs. Insert a non-magnetic brass or plastic feeler gauge between the end of the sensor and ring, and then tighten the set screw that locks the sensor in place.

Some sensors come with a piece of paper or plastic over the end that provides just the right gap when the sensor is installed. To install this type of sensor, it is inserted until it just touches the sensor ring, then backed off just enough so the ring will turn without rubbing against the spacer. Tightening the set screw locks it in place.

A good wheel speed sensor will generally produce an AC voltage reading of 50 to 700 MV when the wheel is spun at a speed of about one revolution per second.

If the voltage reading is low or non-existent, check the sensor’s resistance (with the key off). This can be done through a breakout box with a DVOM. Checking resistance through the breakout box will tell you if the sensor’s wiring harness is okay. If you don’t get the specified value (typically between 800 and 1,800 ohms), disconnect the sensor from its wiring harness and check the sensor’s resistance by attaching the DVOM test probes to the sensor leads. A resistance reading that’s now within range tells you the problem is in the wiring, not the sensor. If the sensor has too much internal resistance (opens) or too little resistance (shorts), the sensor is defective and needs to be replaced.

Grounds or shorts in the wheel speed sensor cables can be found by checking continuity between the wiring connectors. If a defect is found in the wires that run between the sensor and the chassis, replacing the wires with new ones is a better repair choice than trying to fix or splice them. These wires undergo a great deal of flexing every time the suspension encounters a bump, so new wires will hold up better than ones that have been soldered, spliced or taped.

Another way to check wheel speed sensors is with an oscilloscope. By displaying the sensor’s output pattern graphically on the scope screen, you often can see problems that are difficult to find otherwise. A missing or damaged tooth on a sensor ring, for example, may not produce a noticeable change in the sensor’s output voltage. But, the fluctuation that occurs in the signal every time the missing tooth passes under the tip of the sensor may be enough to affect the operation of the ABS module.

The scope connection can either be made through the breakout box or it can be hooked up directly to the wheel speed sensor. A scope pattern for a wheel speed sensor should show a classic sine wave alternating current pattern that changes both in frequency and amplitude with wheel speed. As the wheel is turned faster, signal frequency and amplitude should both increase. Damaged or missing teeth on the sensor ring will show up as flat spots or gaps in the sine wave pattern. A bent axle or hub will produce an undulating pattern that changes as the strength of the sensor signal changes with every revolution.

If the scope pattern produced by the sensor is flattened (diminished amplitude) or is erratic, it usually indicates a weak signal caused by an excessively wide air gap between the tip of the sensor and its ring, or a buildup of metallic debris on the end of the sensor. A weak signal also can be caused by internal resistance in the sensor or its wiring circuit, or loose or corroded wiring connectors.

Another important thing to keep in mind about wheel speed sensors is that they are affected by the size of the wheels and tires on the vehicle. A tire with a larger overall diameter will give a slower speed reading than one with a smaller diameter. Because the ABS system is calibrated to a specific tire size, vehicle manufacturers warn against changing tire sizes. So if somebody has installed a set of oversized rims with ultra low-profile tires on a car, it may upset the operation of the ABS system if the tire diameter is significantly different than the OEM tires.

TROUBLESHOOTING TIPS
First, don’t blame the ABS system for ordinary brake problems. Remember, the only time an anti-lock brake system does anything at all is when a wheel speed sensor tells it one or more wheels are slowing too quickly and starting to skid. The rest of the time the ABS system is passive and does nothing. Except for power assist on integral ABS systems, it has no effect on normal braking. Consequently, if the brakes are grabbing, pulling, dragging, making noise, leaking, etc., the problem isn’t the ABS system. Conventional brake problems must always be diagnosed and corrected before attempting to diagnose a suspected ABS problem.

Basic brake service procedures on ABS-equipped vehicles are essentially the same as those without ABS - with two exceptions. You may have to use a somewhat different bleeding procedure or sequence on some ABS applications because air can become trapped in the ABS modulator solenoids. You must also depressurize the accumulator(s) if the vehicle has a Teves Mark 2 ABS system.

The operation of the ABS system can often be affected by electrical problems in the vehicle, as well as the ABS system itself.

Underlying conditions that may cause trouble include:

• Low battery charge - A low charge can interfere with the operation of the anti-lock control electronics.
• Blown fuses - Check the brake control module fuse, main relay fuse and pump motor fuse.
• Corroded/loose connectors - The main relay, pump motor and motor relay, pressure switch, main valve, valve block, fluid level sensor and control module connectors all must be tight and correctly installed.
• Bad grounds - Check body grounds, especially on the modulator and ABS module.

If the ABS warning light remains on after you’ve started the vehicle or it comes on while driving, it usually means the system has self-diagnosed a fault and has deactivated the ABS system. Normal braking should be unaffected - unless the problem is related to the pump or accumulator on a Teves Mark 2 integral ABS system, in which case power assist may be lost. In any event, the cause of the ABS warning light should be investigated as soon as possible.

A brake warning lamp that remains on or comes on while driving usually indicates a problem with the hydraulic system, not the ABS system. The vehicle may have a fluid leak or loss of pressure, either of which pose a serious safety hazard. The vehicle should not be driven until the cause is diagnosed and repaired.

SCANNING FOR ABS FAULT CODES
Whether the ABS warning light is on or off, always do a bulb check when turning on the ignition. A burned-out bulb or problem in the warning lamp circuit may prevent the bulb from coming on when a code is set. The bulb should come on and remain on for a few seconds when the ignition is first turned on, then go out if no initial problems have been detected.

On Teves Mark 2 integral ABS systems, the ABS warning light will remain on while the pump motor builds up accumulator pressure. This may take up to 30 seconds if the accumulator has become discharged because the vehicle hasn’t been driven for a number of days, or because the accumulator has become discharged by pumping the brake pedal while the ignition was off.

If the ABS warning light goes out, as it should after a few seconds, it doesn’t necessarily mean all is well. Many key system self-checks like wheel speed sensor inputs and solenoid cycling are not performed until the vehicle is in motion. So a short test drive may be necessary to verify whether or not the light remains off or comes back on.

If the ABS light is on, you’ll probably have to use a scan tool to read the codes because many import ABS systems don’t have "manual flash codes." The earliest Bosch 2 systems don’t even have any self-diagnostic codes! You have to use a special Bosch ABS tester (Mercedes-Benz p/n 126-589-09-21-00) to check the operation of the system.

When using a scan tool to read ABS codes, you’ll either plug into a diagnostic connector for the ABS module or into the vehicle’s data communications link (DCL) connector. Once the link is established, follow the scan tool prompts to read and clear the codes.

Once you have a code, you have a place to start your diagnosis. Refer to the diagnostic chart or procedure for the code, and follow the steps to isolate the fault. This will usually involve measuring voltage or resistance within various wiring circuits or components. You’ll need a multimeter and possibly a breakout box to make these checks.

FALSE CODES
ABS systems will sometimes generate "false" codes. Such a code would seem to indicate a fault has occurred, but there’s really nothing wrong. You can waste a lot of valuable time chasing ghosts and replacing parts needlessly in an attempt to fix this kind of code. These kinds of problems are sometimes described in technical service bulletins (TSBs) issued by the vehicle manufacturers.

But sometimes a false code may have been triggered by the last guy who worked on the car. For example, rotating one wheel while the key is on may trigger a wheel speed sensor code on some applications.

False wheel speed sensor codes as well as ABS performance problems also can be triggered by changes in tire size. If replacement tires or wheels have a larger or smaller diameter than the original equipment tires and wheels, it will affect the wheel speed sensor readings - especially if there’s a difference in tire size between the front and rear axles. For this reason, the vehicle manufacturers do not recommend changing tire sizes on most ABS-equipped vehicles.