Scoping out ABS wheel speed sensors: What could go wrong?
Anti-lock brake systems. Traction control. Electronic stability control. Great safety enhancements, but none of them could exist without wheel speed sensors. Even variable assist power steering and variable ride height systems rely in part on information from the wheel speed sensors.
There are two basic types of wheel speed sensors, the magnetic sensors and the Hall Effect sensors which have become more popular in the past few years.
Since vehicles use CAN (controller area network) multiplexing, the wheel speed sensor information is often integrated with the vehicle speed sensor information to control various systems.
Along with an accelerometer input, the wheel speed information is essential to stability control. Even some vehicles without anti-lock brake systems (ABS) will use wheel speed sensors as the vehicle speed input for the engine control module and transmission control module.
The traditional wheel speed sensor consists of a tone wheel (sometimes called a trigger wheel) plus a magnetic pickup. Inside the sensor itself is a permanent magnet surrounded by a coil of copper wire.
The magnetic field changes as the teeth of the tone wheel pass, creating an alternating voltage (AC) signal the changes in both frequency and strength based on speed. It is a typical sine wave. The sensor has two wires: ground and signal.
This type of wheel speed sensor is sometimes called a passive sensor because there is no power going to it.
A magnetic wheel speed sensor does not start working until the vehicle is travelling about 5 mph or faster.
The tone wheel may be mounted on the axle, the brake rotor, the drum or the constant velocity joint.
The sensor is stationary. The gap between the two is critical and, although some sensors may be adjusted, most can’t. You need a non-ferrous (brass) feeler gauge to measure the gap. If it is out of spec, the tone wheel or wheel speed sensor usually need replacement. A worn, cracked or warped tone ring won’t produce a usable signal.
Even a small chip in the tone wheel can cause problems so you should not use a pry bar or hammer to remove a half-shaft from a steering knuckle.
Despite the proper gap, signals from the sensors will be incorrect if the wheel bearings are worn or loose. The ABS module needs both signal amplitude and frequency to determine if brake application is necessary. Even a small difference can affect ABS operation, particularly at slower speeds, usually below 10 mph.
This is when false activation of the ABS system usually occurs. Since the system interprets the inputs from each sensor individually, any difference may be interpreted as wheel slip and that will trigger the ABS to kick in, despite the fact that the wheel is not slipping. False activation can also be caused by erratic, inconsistent signals such as those due to worn wheel bearing movement.
Some replacement sensors come with a piece of paper or plastic covering the tip. Leave this in place during installation to create the right gap. As soon as the wheel turns, the cover is knocked off by the tone wheel teeth.
It is common for dirt to accumulate on the sensor which inhibits its ability to generate a signal. Also, filings from an on-car brake lathe can collect on the sensor’s magnet resulting in an erratic or no signal. The sensor’s winding can break due to excessive vibration.
Although sensors can go bad, the most common problem is faulty wiring or corroded electrical connections. Use an ohmmeter to test the sensor’s winding and compare it to the service specs. Typically, the readings are between 450 and 2,200 ohms. Replace any sensor that shows an open or internal short. Visually inspect all connections.
The best way to test the circuit integrity is with an ohmmeter after unplugging the connector at the ABS module so that the entire circuit will be tested. Wiggle and shake the wires and connectors (and gently tap the sensor) watching for the resistance reading to drop out indicating a problem. Of course, any sensor that shows an open or ground with your ohmmeter test should be replaced.
When replacing parts, make sure the replacements match the original parts exactly. The tone wheel must have the same number of teeth and be the same diameter, for instance.
The same goes for tires. Mismatched tires rotate at different speeds. This causes mixed signals to be sent. It is best that your customers stick to the original size tires as even a slightly larger or smaller diameter can have an impact.
If they want plus one tires, they need to get plus one wheels. When they’re shopping for tires, it is best to check the overall diameters as found in the tire catalogs, especially if different brands may be involved.
You can test a sensor for output using a voltmeter set to the millivolt range. Turning the wheel by hand should produce an AC voltage, but that reading means nothing more to you than that the sensor is working.
You can also test wheel speed sensor with a digital oscilloscope. Connect its leads to the sensor terminals or at the ABS module and observe the waveforms as you turn the wheels. Any distorted signals indicate a problem with the tone wheel and/or sensor.
Sealed hubs with integral wheel speed sensors and tone rings have become more common. They are designed to keep out the elements, but that is not always the case where they use lots of road salt in the winter. Though uncommon, salt could get in and disrupt the signal enough to affect low speed ABS activation.
The latest advance in wheel speed sensors are the so-called active sensors. They provide better accuracy, especially at low speeds, and can even differentiate between forward and reverse as well as full stop.
Active wheel speed sensors work on the Hall Effect creating a digital, square wave. (That is why they are sometimes called Hall Effect switches.) It does not change amplitude, only frequency. Active wheel speed sensors rely on the change in magnetic polarity (north vs. south) as the tone wheel passes. On some, the magnet is built into the sensor. On others, such as the system used on Harley-Davidsons, the magnets are in the wheel bearings.
There are usually three wires leading to the sensors. One supplies the reference voltage (VRef), similar to many other automotive sensors. The other two are the signal and the ground. Sometimes only a reference voltage and a signal wire may be used.
Most active wheel speed sensors contain solid state module inside the housing that processes the signal before sending it to the controller. This is why it can even read no wheel movement when the vehicle is stopped.
The sensor actually sends two signals, a high and a low, as the trigger wheel (or tone wheel) passes. The rate at which the voltage switches from high to low corresponds to wheel speed.
Problems can be caused by loose or corroded connectors or wiring faults. You can check it with a scan tool or digital storage oscilloscope on which you will see a square wave pattern.
The following additional information was provided courtesy of Identifix:
Wheel speed sensors are simple devices, so what could go wrong?
After working on a system for a while, the question may change to, “How can something so simple be so difficult to diagnose?”
Wheel speed sensors are an important part of both the ABS and the Traction Control System.
In the past several years, we have seen wheel speed sensors cause a number of codes and possibly as many non-code failures/problems.
The most common is the now-classic “ABS false cycle.” The action is an ABS event, or activation on non-skid deceleration braking below 10 mph, although it can happen on deceleration braking from as high as 20 to 30 mph and not set a code.
Wheel speed sensors are permanent magnet signal generators. They produce an AC voltage proportional to rotational speed of the trigger wheel. Some have measured as low as 0.2 volts AC on very slow rotation, such as by hand. If a sensor has a glitch or drop out, however brief, the ABS controller can see the condition but may not be able to set a code due to the timer in the controller not timing out for that code’s requirements. Code parameters are set in one or two seconds — and that’s an eternity to a processor. The frequency of the glitch can cause cycling or ABS activation. On some models this can mean the brake pedal will go to the floor during this condition.
How do you find a problem that happens so quickly?
Scan/diagnostic equipment seldom show anything. Voltmeters also fail to show these problems since they average voltage, and a momentary problem will not last long enough to change a voltage average. A frequency meter might show the condition. Even the most accurate method, a waveform/lab scope, can fail to spot what we suspect is a speed sensor glitch.
It takes a scope with a high update/sampling rate to see the problem. The system needs to be tested by back-probing at the ABS controller. Back-probing, or otherwise testing the sensor while connected, is necessary due to a potentially weak sensor signal that does not show up on a disconnected sensor test.
To test: You need to drive the vehicle to test signals. Testing done in the shop means suspension is unloaded and drive axles/hubs are in a relatively smooth rotation. Many problems simply cannot be recreated on a hoist or with wheels off the ground. The first step is to recreate the problem — identify the sensor or sensors failing.
Many technicians say they have tested sensor resistance, and that the sensor tests good. Usually the tests were done at the disconnected sensor. Though this is a good basic test, even resistance tests should include all wiring, and are best done at the disconnected ABS controller harness. This test may spot a static failure, which should set a code for a system self test failure. Also ohmmeters apply the lowest amount of current to a circuit by design, so wires and connection may go unnoticed.
When testing with a scope remember to verify setup procedures, ground, etc. Even with a scope it may take several tries to catch a speed sensor problem.
We get many questions about waveforms, amplitude, frequency and AC voltage. What are they supposed to be? There is no absolute answer for specs. The best way to diagnose is to look at it from the controller’s point of view. It is meant to compare, more than anything else. Front sensors are expected to be the same or similar to each other. Rear wheel speed sensors have the same relationship to each other, but front and rear sensors are not necessarily the same. Look for erratic, inconsistent patterns compared to the other sensor on the same end of the vehicle.
When a sensor has been identified, often the tech replaces the sensor. Sometimes it fixes the problem, sometimes it does not. Remember the wires, connections, even the hub/bearing/tone ring can create these types of problems.
Sometimes the wrong sensor applications are installed. Or a replacement sensor has the wrong physical dimensions, air gap, etc. And there are bad new parts. Retest, verify application, INSPECT. Your eyes are one of the best inspection devices available. Test at the sensor, move the wiring. Remember wiring movement from vibration and heat is very subtle, not much at all. And that is when the problem is happening.
More than a few times we see strange or repetitive problems such as a 1998 K10 pickup with a consistent code for right front wheel speed sensor open or shorted to ground. Repeated tests showed no resistance problem, even at the Electronic Brake Control Module (EBCM).
The code set immediately — a result of system self test failure done as soon as the ignition is turned on.
The code is based on passing a current through a fixed resistance. The diagnostics led us to an EBCM failure, resistance was correct and nearly identical to the other side. Here’s what the technician did:
1) Replaced EBCM and the same code came back immediately.
2) Repeated tests, and tried a second EBCM ... same code returned.
3) Swapped the speed sensor wires at the EBCM. The code was gone, no other codes set.
4) Swapped the wires back to the original cavities of the connector. Code reset immediately.
5) Retested several times. Finally the resistance went to over limit and stayed there no matter what the tech did to the wiring.
6) Replaced the wheel speed sensor harness. The code did not reset.
7) Test drove the vehicle, retested several key cycles.
8) Buttoned up the vehicle. Same code set again, intermittently now, not every key cycle.
This had gone well past the point of being ridiculous. Retesting the sensor and harness yielded nothing but good values. Out of desperation, the technician replaced the wheel speed sensor and the problem vanished. The only explanation is — the current that passed through the sensor by the EBCM was more than an ohmmeter applies, and it was missing a subtle sensor failure. This goes back to ohmmeters not catching all problems.
A 1997 C20 had a code for erratic/intermittent operation at 40 to 60 miles per hour. The tech had tested frequency and AC voltage. All tests led to a failing sensor. The opposite side sensor was reading nearly three times the frequency on a scope as the sensor identified by the code.
Both sensors seemed to have no obvious signal problems, just one side substantially lower. The tech replaced the sensor, then a second sensor. The tech tried a hub/bearing assembly and got the same results.
In fact, each time the tests were run, he got the same results. The inspection of the sensor and tone ring always looked normal. We finally decided to look at the opposite side sensor and hub. There were cracks in the sensor and the tone ring was OK. Replacing the sensor solved the problem.
Now the output from each side was similar at all speeds. We are used to chasing the weakest indication, but signal generators can produce higher output with cracked or damaged sensors or trigger rings.
A 1999 T10 with an automatic transfer case was setting vehicle speed sensor codes in ABS and PCM, and the speedometer was erratic below 10 miles per hour. Anything over 10 mph was normal operation. When the technician tested it at the transfer case, vehicle speed signal was normal, with no drop outs or erratic output.
The tech tested the sensor repeatedly — it was always good, always had codes and the same problem. Turns out the component locator was wrong — the technician was testing the wrong sensor. This unit has three speed sensors on the transfer case. Be sure you are on the correct sensor. Try to verify with wire colors if possible.
The information in this article may seem basic and redundant. Basic is good, redundant is how we get good.
Wheel speed sensor diagnostic trouble codes
Right front wheel speed sensor
Left front wheel speed sensor
Right rear wheel speed sensor
Left rear wheel speed sensor
(For any or all of the above, check the appropriate wheel sensor, speed sensor circuit and the speed sensor rotor.)