Import ABS Diagnostics: Customer Input Can Help Pinpoint System Culprits

Import ABS Diagnostics: Customer Input Can Help Pinpoint System Culprits

The ABS warning light is on in your customer’s car and he wants you to check it out. So where do you start your diagnosis? Because the ABS warning light is on, you know the ABS system has detected some kind of fault. But are there other problems that may be affecting the ABS system? Start your diagnosis by asking your customer a few questions. How long has the warning light been on? Has the warning light been on before? Have you noticed any other problems with the brakes? Has the brake warning light or any other warning lights been on recently? Have you had the vehicle serviced recently?

Depending on what kind of answers you get, you may want to investigate some other things before you plug in your scan tool to read the ABS fault code(s). For example, if your customer says the brake warning light also came on a few times, it might be a clue that the fluid level in the master cylinder is low or that the brakes are leaking fluid somewhere. If the check engine light or charging system warning light has been on, the vehicle might have a low voltage problem that is affecting the ABS system. And if the customer says he had the tires replaced a few days ago, the ABS warning light might be on because of a change in tire size, mismatched tire sizes, a severely underinflated tire or even a damaged wire to a wheel speed sensor.

In spite of their complexity, most ABS systems are pretty reliable and cause few problems. But when the ABS light is on, the system needs to be checked to find out why. ABS problems typically start to occur as the vehicle accumulates miles and age. Over time, rust and corrosion inside the brake system can plug up filter screens inside the ABS modulator, and sometimes even the ABS solenoid valves, causing various ABS and braking problems. Electrical connections between the wheel speed sensors, ABS module, pump motor and power relay may become corroded or loose.

Most wheel speed sensors are magnetic and can become contaminated with metallic particles from brake linings or rotors (or wheel bearings in the case of integral wheel speed sensors that are part of a sealed hub and bearing assembly). Accumulators can develop leaks, pumps can wear out and relays can fail. Moisture can cause corrosion in the module and electrical connections (watch out for vehicles that have been flood damaged!).

As a rule, serious ABS faults (like a wheel speed failure, accumulator leak, pump motor failure, shorted or open ABS solenoid valve, etc.) will be detected by the ABS self-diagnostics, set a code and turn on the ABS warning light. This will usually cause the ABS system to disable itself. On most vehicles, this should not affect normal braking. But on vehicles that also use the ABS system for power-assisted braking or use brake-by-wire (such as the Toyota Prius), it will cause an increase in pedal effort when braking. If it’s a newer vehicle with traction control and/or stability control, those functions may also be disabled.

Reading fault codes on older import ABS systems (Toyota, Honda, etc.) typically involves connecting a jumper wire between certain terminals in the ABS diagnostic connector so the ABS light will flash out the code(s). On some of the 1980s vintage Bosch systems, the system doesn’t even store codes, so you’re on your own trying to diagnose a fault, unless you have a special Bosch ABS tester for these systems.

On most of the newer vehicles, you need a scan tool to access the codes in the ABS system. This usually means the factory scan tool or an aftermarket professional-grade scan tool that has the right software to access the ABS system. If your scan tool can’t access the ABS system, you’ll have to upgrade the software or buy a new scan tool.

Once you’re able to read an ABS code, write it down. Don’t clear it until you have made a note of what you found. The next step is to refer to the vehicle manufacturer’s diagnostic charts and wiring diagrams. Auto repair today often requires as much research and background data as it does hands-on diagnosis before you can pinpoint and fix a fault. It’s also a good idea to search for any technical service bulletins that may be available for the vehicle in question. If there is a TSB out on your particular problem, it can often save you hours of frustration if the fault turns out to be some kind of software or hardware problem that can only be fixed by replacing the original part with an updated part.

You can check virtually any magnetic wheel speed sensor by measuring its resistance with an ohmmeter, but you need to look up the exact specifications for the vehicle because they can vary significantly from one application to another. For instance, older Toyota Camrys and Porsches both use a similar Bosch 2 ABS system. Yet the specs for the rear-wheel sensors on the Toyota are 900 to 1,200 ohms versus 1,600 to 1,800 ohms for the Porsche. Why? Who knows. But if the wheel speed sensor is out of specification, it won’t produce an accurate signal that the ABS control module can read.

Magnetic 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. Even a tiny nick in the tone ring around the outer CV joint or on the rotor may be enough to disrupt the signal.

The distance or “air gap” between the end of a wheel speed sensor and its ring is 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 it would damage both. But you don’t want too much clearance either. 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 the service literature for the required air gap and adjust it to specs. Insert a nonmagnetic 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, insert it 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. Use a digital multimeter or a digital storage oscilloscope to check the sensor’s output.

If the voltage reading is low or nonexistent, check the sensor’s resistance (with the key off). You can do this at the wheel or by using a breakout box in the ABS module wiring harness. Measuring at the harness will also check the continuity of the circuit. If you get a low reading at the harness, check the wheel speed sensor. If the sensor is within specs (typically 800 to 1,800 ohms), the problem is in the wiring, not the sensor.

If you’re using a scope to check a wheel speed sensor circuit, a signal pattern that is flattened (diminished amplitude) or erratic 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 can also be caused by internal resistance in the sensor or its wiring circuit, or from loose or corroded wiring connectors.

Pumps generate pressure and accumulators store pressure. A bad pressure switch or pump relay can prevent the pump from running, as can a blown fuse, or loss of power or ground in the pump circuit. If the pump runs when the fuse is replaced, it may tell you the pump has been overworking because of a leaky accumulator that isn’t holding pressure.

Accumulators typically leak at the point where they screw into the pump or modulator assembly. They can also fail internally. The accumulator has a flexible rubber diaphragm inside that separates the high-pressure nitrogen gas charge from the brake fluid. As rubber ages, it loses some of its flexibility. If the diaphragm cracks, brake fluid will force its way into the nitrogen chamber and eventually displace most or all of the nitrogen.

Once this happens, the accumulator will no longer be able to function like a compressed spring and apply pressure against the fluid. When the brakes are applied, there’s no reserve pressure in the accumulator for power assist or fluid return. The pedal feels hard and the system will try to compensate by running the pump to recharge the accumulator. This will usually set a code for the accumulator.

Warning: Do not attempt to remove an accumulator or open up the ABS modulator until the ABS system has been completely depressurized. This is usually done by pumping the brake pedal 30 to 40 times with the key off, or until the pedal feels hard. The accumulator can then be safely replaced with ordinary hand tools in most cases.

Here’s something else to keep in mind: discharging an old accumulator before working on the system may cause it to fail. If the diaphragm has lost too much elasticity, it may overextend itself when the system is discharged, causing it to crack. For this reason, some brake experts recommend replacing old accumulators any time an ABS system that’s more than eight to 10 years old has to be opened up for service.

Some types of ABS problems may not set a fault code or turn on the ABS warning light. These include intermittent faults or problems that may be due to environmental factors (heat, cold or moisture), or other problems that are not in the ABS control electronics. These problems include dirty brake fluid that contains debris, a sticking, dragging or leaking brake caliper, warped rotor, variations in rotor thickness, wrong brake linings or contaminated brake linings, changes in wheel and/or tire sizes, electrical glitches in the wiring or charging system, etc.

If a vehicle seems to be having an ABS or brake problem, but there is no ABS light or brake light, make sure both warning lamps are not burned out. The ABS and brake warning bulbs should both come on for a few seconds when the ignition is first turned on. No light means a bulb has failed or there is a problem in the warning lamp circuit.

Another overlooked cause of ABS faults (and other driveability problems) can be a low battery and/or low charging output. Electronic modules need a reliable 12 volts to operate normally, so if voltage isn’t up to specs for whatever reason, it may cause the system to misbehave or act strangely.

If someone has replaced the original equipment wheels and tires with aftermarket wheels and tires that are a different size or aspect ratio, the change in tire diameter may upset the calibration of the ABS system.

If there’s no ABS warning light and no ABS codes, you can probably assume the ABS self-diagnostics are working and the system is functioning normally. But it may be misbehaving if there is a problem with the brakes. Grabby rear brake linings, for example, can trigger the ABS system into action to prevent the rear wheels from locking up when braking hard on a wet or slick surface, or a rough road.

To rule out basic brake problems, you need to inspect the entire brake system. Check the master cylinder, brake lines and hoses, proportioning valves, calipers, wheel cylinders, brake pads and shoes, rotors and drums.

If the brake system appears to be in good working condition, but the ABS system is operating erratically or abnormally (kicking in during normal braking or not preventing wheel lockup when braking hard on wet or slick surfaces), the most likely cause is a wheel speed sensor fault or a system calibration issue.

Some ABS systems are more sensitive than others to subtle changes in wheel speed. Every ABS system is calibrated for a specific vehicle application (a specific size and class of vehicle, a specific weight range, a certain tire and wheel size, and the individual braking characteristics of that vehicle).

Any modifications that may have been made to a vehicle that significantly alter any of these basic parameters may upset the normal operation of the ABS system. Replacing the brake linings with ones that have a higher or lower coefficient of friction than the original linings may affect the operation of the system. Raising or lowering the vehicle’s ride height can also change the chassis dynamics and loading on the front and rear brakes.

Carley has more than 20 years experience in the aftermarket, including experience as an ASE-certified mechanic, and has won numerous awards for his articles. He has written 12 automotive-related books and developed automotive training software, available at

Special Service Procedure Needed When Changing Brake Pads On Mercedes E-Class With Bosch Sensotronic ABS

The Bosch Sensotronic Brake Control (SBC) system on 2004 and newer Mercedes-Benz E Class, W211 and R230 (SL) models requires a special procedure to disable the ABS system before the brake pads can be replaced. The SBC system is an “active” electrohydraulic braking system that can apply the brakes with up to 140 bars (more than 2,000 psi!) of pressure. So the last thing you want is the system applying brake pressure when you’re in the middle of a brake job. Caliper pistons could become projectiles!

The deactivation procedure can be done with a factory scan tool or an aftermarket equivalent, such as the ATE SBS made by Continental Teves. The ATE SBS tool plugs into the vehicle’s diagnostic connector and tells the system to disable itself when a red button marked “OFF” on the front of the tool is pressed. This causes the system to bleed off any residual pressure in the calipers (which is done to preposition the pads so the brakes can be applied faster in an emergency), and prevents the system from applying full pressure to the calipers. The deactivation procedure takes about 60 seconds. After that, the system is safe to work on.

To reactivate the SBC system once all brake work has been completed, just press the “ON” button on the ATE tool. The system will rebuild pressure in about two minutes and erase any fault codes from the control module’s memory.

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