Diagnosing Wheel Speed Sensors: WSS, ABS Systems

Diagnosing Wheel Speed Sensors: WSS, ABS Systems
Wheel travel rapidly sensors ( WSS ) provide all-important roulette wheel accelerate information not entirely for anti-lock brake systems ( ABS ), but besides for traction control condition and constancy operate systems. Some vehicles tied compare the wheel speed detector readings against the fomite rush detector ( VSS ) to make surely all of these sensors are accurate and working correctly. Click here to Read More ad

In some situations, it is besides possible that the signal from a WSS may not be bad enough to set a fault code, but may be off adequate to upset the normal operation of the ABS, traction control condition or stability see systems. This might be the leave of an intermittent signal or a noisy sign that is confusing the system see module .
On older ABS systems, a fault code is typically set if there is a personnel casualty of a sign in a WSS lap ( shorted or capable ), or the understand from one WSS detector differs importantly from the rest. On some newer vehicles, “ plausibility ” codes may besides be set if the amphetamine read of any WSS detector differs from the vehicle speed detector by more than 3 to 5 miles per hour. ad Most scan tools will allow you to not only read and clear the ABS codes that have turned on the ABS warn light, but to besides look at the individual WSS inputs ( Performance Information Data or PIDs ). The scan joyride will display a amphetamine input in miles per hour ( miles per hour ) for each wheel focal ratio detector .
Diagnosing Wheel Speed Sensors: WSS, ABS Systems WSS SPEED INPUTS
Most wheel speed sensors ( except for the latest generation of “ active ” WSS sensors ) won ’ metric ton generate a clear signal until a vehicle is traveling 3 to 5 miles per hour or fast. You can spin each wheel by bridge player to see if the WSS is generating a sign, but that won ’ t tell you if the bespeak is accurate or how one WSS compares to another. therefore to check the WSS inputs, the vehicle needs to be driven in a square pipeline at 12 to 25 miles per hour while you or a benefactor watches the wheel accelerate detector PIDs on the scan tool .
If all of the wheel speed sensors are generating a speed signal, and they all agree, the mistake is not a regretful WSS, but something else ( possibly an intermittent wiring fault in one of the WSS circuits ). Since most WSS problems are due to broken wires or loose or corroded wiring connectors, visually inspect the wires and use an ohmmeter or continuity examiner to check the wire while wiggling the wires back and away. If the tour suddenly goes clear or shows a leap in resistance, you ’ ve found the fault. ad If one of the WSS PIDs reads zero miles per hour and the other three are reading fomite focal ratio, the fault is a bad detector or a mistake in the detector wiring circuit ( keep read for data on how to check the detector itself ) .
If all four WSS are indicating a speed reading, but one is reading a couple of miles per hour faster than the others, stop the fomite and check the tire size on the wheel that is reading faster than the others. Chances are person mounted a bore that has a smaller diameter than the others. If the front WSS sensors are both showing a different accelerate than the raise WSS sensors, this excessively may be the solution of person having replaced the OEM tires with ones that are a different size front and rear. ad Changing bore sizes will change the WSS inputs, which may upset the operation of the ABS, traction master or stability control systems. On some vehicles, the master module can be reflashed to accommodate a change in bore size. But on most vehicles this is not possible. So the vehicle owner has to stick with the lapp OEM tire and wheel size. If they want to replace their OEM wheels and tires with aftermarket wheels and tires, they can go with plus steering wheel size and lower visibility tires to keep the overall tire diameter the same as before .
If you find a individual WSS that is reading slower than the others, the cause may be one of two things : an outsize tire on that roulette wheel or the detector is not generating a pulse for every notch in the tonicity ring causing a lower than normal speed end product. Things to check for here ( besides the tire size ) would be hoary, corroded or damaged teeth on the tonicity ring, or an excessive air gap between the point of the detector and the gang. ad WSS CHECKS
Whether you have a scan creature or not, there are a numeral of checks that can be made to verify the operation of a rack speed detector .
The wheel speed sensors on most vehicles are magnetic and generate an understudy current ( AC ) bespeak that increases in frequency and amplitude with wheel speed. These are sometimes called “ varying reluctance ” ( VR ) or “ passive voice ” WSS sensors because they generate their own voltage bespeak when the vehicle is in apparent motion. They have two wires : signal and ground .
Inside a passive voice WSS detector is a permanent attraction congress of racial equality surrounded by bull wire windings. When the tooth on the tone ring revolve past the detector lean, it changes the magnetic discipline and induces a current in the detector windings. The solution is a classical sine wave current practice that changes with wheel speed. ad The potency of the signal can be affected by immunity inside the detector, resistance in the external wire and connectors, metallic debris from worn brake rotors, drums or semi-metallic brake linings sticking to the end of the detector, and the atmosphere col between the detector and tone resound ( which is mounted on the axle, CV joint, brake rotor, brake barrel or inside the hub ) .
A narrow-minded tune opening is normally necessity to induce a solid signal in the detector ’ s magnetic windings. Air gaps typically range from .016 in. to american samoa a lot as .050 in. ( 0.40 to 1.3 millimeter ) depending on the application. As a rule, there should be no more than .006 inches of runout in the tone resound because besides much runout may cause an erratic signal. A crack up note surround can besides cause an erratic sign.

ad With magnetic WSS sensors, you can check the integrity of the detector by connecting an ohmmeter across its terminals. The resistor specification will vary depending on the lotion, but most sensors should read 450 to 2200 ohm ( constantly look up the demand specifications because they can vary a great deal from one vehicle lotion to another ). If a detector reads open, shorted or is out of specifications, it can ’ metric ton generate an accurate signal and must be replaced .
You can besides test the detector ’ randomness output by spinning the bore by hand at a rate of about one rotation per second. With a voltmeter attached to the detector ’ s terminals, a good WSS detector should generate about 50 to 700 millivolts AC. ad If a detector ’ south output is moo, and the detector is obliterable, pull it out and clean the peak. besides, check the air out gap when the detector is installed ( consult to the fomite manufacturer ’ south specifications and alteration procedure ) .
If you have a digital storage oscilloscope, you can connect the leads to the WSS detector terminals and observe the wave form on the oscilloscope. A setting will show the voltage and frequency of the signal, angstrom well as any missing or distorted humps in the sine wave form. If one or more humps in the wave radiation pattern are importantly shorter than the others, it indicates one or more damaged teeth on the detector tone band. ad The amplitude of the signal is affected by the air gap between each tooth and the detector, so if a tooth is chipped it won ’ deoxythymidine monophosphate generate as strong a pulse as the other tooth. The cook would be to replace the damaged tone surround ( or the hub if the detector and tone resound are integral parts of the hub assembly ) .
Something else to watch out for is mismatched parts. To produce an accurate bespeak, the detector ’ s shade ring must have the right total of teeth for the application. Make certain the note resound on a surrogate rotor, drum, axle shaft or outboard motorboat CV joint is the same as the master if any of these parts have been recently replaced, or need to be replaced. ad ACTIVE WSS SENSORS
In late years, several new types of “ active ” WSS sensors have appeared. These are used on many 1999 and newer Chrysler, Jeep and Mercedes models with Teves Mark 20e and up Teves ABS systems. active WSS sensors are besides found on the Ford Focus, Toyota Tundra and early applications. This character of detector is becoming more common on late-model vehicles because of its greater accuracy at broken speeds .
active WSS sensors, which Chrysler refers to as “ magneto-resistive ” sensors, work like a “ Hall effect ” detector and generate a feather wave on-off digital bespeak that changes frequency with wheel travel rapidly. This type of detector detects the change in charismatic polarity ( flux ) as the tooth on the shade resound or minor magnets pass under it. small magnets may be built into the bicycle bearings or bearing cachet for the WSS detector to read. ad active sensors much have three wires : a electric potential reference book input ( 5 to 12 volts ), a bespeak output and grate. But some active sensors have alone two wires ( mention electric potential and signal return ) .
How can you tell a 2-wire active detector from a 2-wire passive detector ? Turn the ignition on and backprobe both wires with a voltmeter. If you see a voltage recitation ( 5 to 12v ), you have an active WSS detector. If you see no voltage ( 0v ), you have a passive WSS detector .
active WSS sensors besides have an integrated circuit within the detector itself that converts and amplifies the signal before it goes back to the control faculty. This allows the detector to read bicycle speeds all the direction down to zero miles per hour ( which improves ABS control at low speeds as the vehicle comes to a stop ). ad When the key is on, an active WSS detector has two states : gloomy and high. In the low state, it generates a fix 0.9 volt return signal of about 7 milliamps. In the high state of matter, it generates a signal of 1.65 volts and 14 milliamps. The rate at which it toggles back and away between its high and humble states corresponds to the speed of the wheel .
This type of detector must be connected to its wiring harness to test it, and the ignition key must be on so the control module can provide mention electric potential to the detector. If you forget and leave the key off, you won ’ deoxythymidine monophosphate get any output from an active voice WSS detector.

ad Because of the add electronic circuitry inside the detector, and active WSS detector can not be tested with an ohmmeter like a passive WSS detector. You have to look at its output bespeak when the bicycle is turning and the ignition is on. This can be done with a scan tool ( count for a miles per hour reading while spinning the tire or during a quiz drive ), with a voltmeter ( turn the wheel very slowly and look for a change in the electric potential output from high to low ), with a NOID light ( the LEDs on the NOID easy will flash when it detects a sign pulsation ), or a DSO. The wave form on the telescope should show a square roll convention that changes from gloomy ( 0.9v ) to high gear ( 1.65v ) as the wheel turns. The dispute in stature of the wave form should be 0.75 volts ( 1.65v subtraction 0.9v ) .

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