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South Dakota Tech Program Graduates In High Demand

The automotive repair industry, in certain pockets of the country, has a labor force problem. There either isn’t enough technicians to go around, or not enough properly trained technicians to go around. This isn’t the case in Sioux Falls, SD. What...

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Automechanika Frankfurt 2014 Wrap Up: Photo Gallery

Aftermarket News Managing Editor Mark Phillips represented the Babcox Media team at Automechanika Frankfurt 2014. You can catch up on a lot of last week's news from the show at Aftermarket News or the Counterman Twitter account or just take a look...

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Auto Care Marketing: Price Sensitive vs. Fear Sensitive

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Volkswagen: Engine Does Not Start

Vehicle: 2001 Volkswagen Passat GLX, 2.8L Complaint: The customer says the vehicle will not start. Cause: Confirmed the customer’s complaint and found the vehicle did not start. Connected a scan tool and found the following codes: • 17978...

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Audi: Vibrates at Idle

Complaint The customer says that the check engine light is on and that the vehicle vibrates at idle. Cause The customer’s complaint is confirmed, with the vehicle vibrating at idle, but the vibration was not felt at high speeds. A scan tool found...

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Step-by-Step Lexus Fuel Pump Replacement

We will use a third-generation 2006 Lexus LS430 as our case study for this fuel pump assembly replacement. First, discharge the fuel system pressure, disconnect the negative battery terminal and drain the fuel. Be sure to remove the rear seat cushion...

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Midtronics Launches Informational Microsite

  Midtronics, Inc. has launched an informational microsite — or mini website — to provide customers with detailed information about the company’s newest diagnostic platform — the DSS-7000 Battery Service Diagnostic System. The microsite...

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German Tool Specialist KC Tool Announces Addition of Gedore Tools

As of August 1 KC Tool became the first certified reseller of Gedore Tools in the United States already adding more than 2,500 Gedore tools to its selection of German made tools. KC Tool plans to add an additional 6,000 Gedore Tools over the next few...

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United Stationers Expands Into Auto Aftermarket

United Stationers announced Sept. 11 that its wholly owned subsidiary, United Stationers Supply Co., signed an agreement to acquire MEDCO, a U.S. wholesaler of automotive aftermarket tools and supplies, and its affiliates including G2S Equipment de Fabrication...

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Driveability Diagnostics and Direct Injection

Direct injection is becoming standard on more and more late-model vehicles. These systems can be a diagnostic challenge, but with the right foundation, problems can be solved profitably. Years ago, all a low-level technician needed to diagnose a fuel...

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A/C Oil Service: Avoid Compressor Failure With Proper Maintenance

HVAC systems often fail because the air conditioning compressor fails. Not following the proper service procedures or compressor oil recommendations could be factors that contribute to premature compressor failure. Refrigerant Since the mid-‘90s,...

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How Ford's New Grille Shutters Control Airflow

Some new Ford vehicles feature active grille shutters — a new system that optimizes aerodynamics by using vents to control airflow through the grille to the cooling system and engine compartment. If air is required to cool the engine, the vents...

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Home Brakes Bearings Wheel Speed Sensors

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According to a recent BRAKE & FRONT END survey, the most commonly replaced ABS items are wheel speed sensors and wheel bearing hub units with wheel speed sensors. Pulling a wheel speed sensor code does not mean that you should immediately pull and replace the sensor. If you do not confirm the source of the code, you are doomed to have a comeback. 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 wheel speed sensor 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.

One new piece of hardware that is being used on certain Chrysler ABS systems (trucks mostly, but LH cars in 2004) as well as the Lincoln LS is an “active” Hall effect wheel speed sensor. Most wheel speed sensors are magnetic (variable reluctance), but the new Hall effect sensor generates a stronger square wave signal that allows the system to more accurately monitor wheel velocity at low speeds. The sensors have two wires and use the chassis as ground.

My Light Is On
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). 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.

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Larry Carley

Larry Carley has more than 30 years of experience in the automotive aftermarket, including experience as an ASE-certified technician, and has won numerous awards for his articles. He has written 12 automotive-related books and developed automotive training software, available at www.carleysoftware.com.
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