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Second-rate parts mean second-rate results

When it comes to diagnosing a problem, one of the biggest mistakes is thinking that the problem is gone after you’ve installed a new part. I’ve had vehicles brought in countless times with the same old story attached to them. The customer will say:...

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GearWrench Launches Street Team, a New Mobile Driver Program

GearWrench, a premier hand tool brand from Apex Tool Group, has announced Street Team, a new program that gives independent mobile distributors the support of an established tool brand without the restrictions of a franchise. "Response to the program...

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AUTO 7 Named Approved Vendor For Automotive Parts Associates

Steven Kruss, president of Auto 7, which supplies Korean-made, OEM-quality automotive parts to distributors across North America, announced that Auto 7 has been named an approved vendor of Automotive Parts Associates (APA), one of the nation’s...

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Internal Engine Oil Consumption Diagnostics

Due to the variables in engine design and ­operating conditions, internal engine oil ­consumption complaints are often the most difficult to solve. In some cases, oil consumption might be more severe under low-speed operation, in other cases, high-speed...

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Hyundai Fuel System, Emissions Diagnostics

Hyundai has done a good job of improving its ­offerings over the years from both an aesthetic and mechanical viewpoint. Complemented by a strong warranty and good value, the carmaker has been able to increase its market share year over year. If you aren’t...

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Curing Volvo Manual Transmission Rattle

In an effort to increase fuel efficiency, today’s engines produce more torque so they can be ­driven at extremely low rpm. ­Reduced viscosity engine and gearbox oils, less vehicle weight and improved aerodynamics also contribute to better fuel economy....

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Victor’s Cash Back Program Makes Select Welders and Plasma Cutters More Affordable

Victor Technologies has launched its Cash Back Rebate program for customers in the United States and Canada. Between July 1 and Dec. 31, 2014, purchasers of select Tweco and Victor Thermal Dynamics systems can receive between $50 and $450 cash back....

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AVI To Provide Technicians with $1 Million in Free Training

AVI (Automotive Video Innovations) is giving back to the automotive community that has given so much opportunity to the Florida-based video company. To carry out that mission, AVI reports that it is pledging to give away $1 million worth of training...

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GearWrench Launches Street Team, a New Mobile Driver Program

GearWrench, a premier hand tool brand from Apex Tool Group, has announced Street Team, a new program that gives independent mobile distributors the support of an established tool brand without the restrictions of a franchise. "Response to the program...

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The Why, Where, When of TPMS Sensors

To understand any TPMS relearn procedure, you have to understand this: Sensors only transmit, they do not receive. No vehicle asks a sensor for information on how it is doing. I know you’re thinking a sensor does receive signals when the vehicle...

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Most Common Causes of Misfire Codes

A flashing check engine light and a P0301 to P0312 diagnostic trouble code (DTC) is a surefire indication that one or more cylinders are misfiring. Occasional misfires may pass unnoticed, but a steady misfire is hard to miss. The engine usually feels...

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Pattern Failures of MAF and MAP Sensors

Pattern failures are those failures that happen over and over again — and the same applies to how customers describe these failures. Customers might unknowingly give you the answer to their problem without needing to open the hood. In this article,...

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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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