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The Problem With Living In The 'Now'

I once had a shop manager who concentrated on the “now.” Every day was a mad dash to complete the jobs at hand. He wanted to know who was working on what, where the parts were and when everything would be done. He was constantly reacting to a customer’s...

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ASE G1: Drive Belt Inspection, Replacement

The ASE G1 Certification test contains 55 scored questions, plus 10 unscored ­research questions, that cover a range of skills and knowledge related to maintenance and light repairs in engine systems, automatic transmission/transaxle, manual drivetrain...

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Amateurs and Hacks Provide Job Security For Automotive Service Professionals

Two cars pull up in front of my shop. The drivers didn’t come in, but I heard the commotion from my office window. The boyfriend opens the hood of his girlfriend’s car. They both stare at the engine; she tells the boyfriend that she was supposed...

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Acura: Clunk Noise While Turning

Model: Acura RSX 2005-’06 Symptom: The front suspension makes a ­clunking noise while turning. Probable Cause: The front springs are moving on the spring seats. Corrective Action: Replace both front springs and do a four­-wheel alignment. Diagnosis:...

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Inside Import Car Collision Warning, Automatic Braking Systems

Anything that moves under its own power also has to stop, so brakes have been a safety feature on cars since day one. Over the years, technical innovations such as antilock brakes (ABS) have ­improved the ability to stop with minimal skidding on...

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Intermittent Engine Misfire Analysis

Even for an experienced diagnostic technician, ­attempting to diagnose an intermittent misfire ­condition that occurs only under specific driving conditions can be a frustrating exercise. Let’s begin by getting the basics out of the way. As we know,...

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Analyzing the Cylinder Pressure Waveform from a Running Engine, Part 3

By Vasyl Postolovskyi and Olle Gladso Contributing Writers and Instructors at Riverland Technical and Community College in Albert Lea, MN   In Part 1 of this Maximizing Tools series, we discussed an alternative approach to diagnosing an engine...

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Mac Tools Is Wrenching for a Cure

In support of Breast Cancer Awareness month, Mac Tools is featuring a variety of Wrenching For A Cure products available for purchase in the Flyer 11 through Nov. 2. Featured pink products include clothing, accessories, flashlights, pint glasses, and...

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5 Tool Storage Tips

  As a technician, you likely own thousands of dollars worth of tools and equipment, and require tool storage capacity to hold them all, along with carts and accessories to help move those tools around your work area. Here are a few items...

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Battery Service and Diagnosis

A good battery with an adequate charge is absolutely essential for reliable cold starting. A weak battery, or one that is rundown, may not deliver enough amps to crank the engine when temperatures plunge and the oil thickens. Cold weather can be hard...

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Toughest Spark Plug Changes

We have all been there before: scratched arms, busted knuckles and an aching back caused by a difficult spark plug replacement job. If you think they are getting tougher every year, you are right. Every new engine design is putting the plugs deeper...

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Improving the Head Gaskets, Fasteners Relationship

The relationship between head gaskets and head bolts is an intimate one. The clamping load applied by the head bolts is what allows the head gasket to maintain its seal. For this marriage to last, there has to be constant tension – not too much,...

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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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