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engine controls & fuel systems

THE NEW GENERATION OFWHEEL SPEED SENSORS

Wheel speed sensors [WSS] were usually ofthe same design regardless of where the carwas manufactured. For decades they wereall some variation of a PM Gen, or perma-nentmagnet generator style of sensor. Alsoknown as variable reluctance [VR] sensors,these two-wire devices were fairly simple todiagnose. Active sensors are not new bydesign, you may know them as Hall-effectsensors. Hall-effect sensors provide infor-mation to the PCM such as: crank speed,cam position, driveshaft speed, and vehiclespeed. Hall devices require a power supplyand have three wires. Active WSS’s are rel-atively new devices that also require apower supply to function, but they only havetwo wires. This is extremely important,because if you don’t know that the vehicleuses active sensors, then your diagnosticscanmislead you into replacing a sensor thatis perfectly functional.

ActiveWSS’s use something called an ASIC.An ASIC is a small integrated circuit thatwas designed from the ground up to serveone specific purpose. The name ASICmeans Application Specific IntegratedCircuit, and these devices have becomemore popular in the automotive world inrecent years. These differ from other inte-grated circuits that can be used inmany dif-ferent applications such as a 555 timer or a741 amplifier. ASICs are found in manydevices, but we will be looking at just oneapplication: the active wheel speed sensor.Active WSS’s use “magneto-resistive” tech-nology that is far more accurate than theolder VR or Hall devices. This enables ABScontrollers to precisely detect wheel speedsright down to zero mph.

While we do not need to know exactly howthey work internally, we do need to knowhow to tell if it’s really bad. Vehicles withactive sensors will initially look the same,

but there is more that is different than theinternal parts of this sensor. VR sensorsmust be mounted directly over a toothedring or ‘tone wheel’, easily visible when thesensor is removed. When you remove anactive sensor you may not see any teeth,because the ring has nowbeen incorporatedinto the hub or wheel bearing, the surfacewill be perfectly smooth. VR sensorsrequire no power supply, but if you probethe harness connector for an active sensorwhile the key is on you should find close to12v on one circuit. Since it’s electronic, orsolid state, measuring resistance will notdetermine anything. If you do, you shouldsee something over 10k ohms, if not, thedevice may be shorted internally. The sen-sor ground is at the mounting point so ifyou remove it, you can’t check it withoutproviding a ground.

The signal produced by an active sensor isdifferent, as is the way it’s produced. VRsensors generate an AC voltage pattern.Hall sensors will toggle a supplied referencevoltage, usually 5v, to ground and thenreleases this ground to create the familiar 5vsquare wave. An active WSS signal forwheel speed has two states: low and high.Toggling between these two states is whatindicates the rotational speed of the wheel.In the low state the sensor produces 0.9vand 7mA of current as an input to the ABScontroller. In the high state the signal volt-age leaps all the way to 1.65v and 14mA.This translates to a square wave of 1.65volts minus 0.9 =0.75v. This is easy to seewith a voltmeter, but you will have to turnthe wheel slowly. An oscilloscope is thebest way to verify that the tiny 0.75v squarewave signal is present and clean. The con-troller checks upon eachWSS every 7 milli-seconds to verify the connection-that’sabout 143 times every second.

The first hint that the vehicle has active sen-sors will be the reading on your scanner.Older VR sensors didn’t produce a signalthe controller could use until the vehicle

reached about 3 mph. This is why you maysee speed values displayed with the vehiclein park and the engine off. An ABS, tractioncontrol or vehicle stability system withactive sensors should display zero mphunder the same conditions.

The Teves Mark 20 e system used inMercedes Benz vehicles from 1999 is oneexample of a system that uses active sen-sors. Ford’s 2000 Focus has them alongwith tone rings that are actually part of thewheel bearing assembly. This is importantto know since many vehicles can be pur-chased without ABS. In the case of theFocus there are two different wheel bearingassemblies. The one for a non-ABS vehiclehas a RED bearing grease seal on bothsides. An ABS equipped Focus needs abearing with one RED and one BLACK seal.Just make sure that you do not install itbackwards. This type of sensor is becomingmore popular as time progresses, so thereis a good chance that if you work on ABSsystems, you will eventually see one.

Jeff Auerbach - Top Gun Technician