DanMarinucci

dmarinucci@motor.com

Foreign Service

14 September 2013

Is it September already? Yes, thecrunch of football helmets hittingpads and the roar at the stadium say itis. Matter of fact, it won’t be long be-fore we hear another crunchingsound—fallen leaves underfoot.

Last month I described the ABS wheel speedsensor and sensor triggering system found onpopular Honda vehicles; I’ll continue the discus-sion here. Fortunately, there are fairly simpleways to test these sensors, and these checks mayspell the difference between scoring a diagnosisand fumbling the job. I’ll recap last month’s de-tails first and then dig into the sensor tests.

Traditionally, ABS systems had a rotating ironor steel component to activate a wheel speed sen-sor. This toothed or gear-shaped part, mountedon a drive axle, may be called a reluctor, reluctorring, tone ring, tone wheel, trigger wheel, etc.However, the popular Honda ABS systems nolonger use this component. Instead, a magneticencoder triggers the speed sensor. The magneticencoder is a band or ring of mini magnets mount-ed on the inboard side of the wheel bearing.

Some technicians have botched a wheel bear-ing R&R here by installing the bearing back-

Last month, Dan described Honda’s magnetic encoder wheel

bearings and ABS wheel speed sensors. Here he offers helpful tips

for identifying these components and testing their operation.

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wards. If you install the bearing with the magneticencoder facing outboard, there’s nothing to acti-vate the wheel speed sensor (refer to page 12 ofthe August 2013 issue of MOTOR). If this happens,an uninformed tech will wonder why replacing awheel bearing caused one or more wheel speedsensor trouble codes. Furthermore, reversing awheel bearing is an expensive mistake becausethe bearing is usually destroyed during removal!

ABS Sensor Test TipsSuppose you need to check a suspect speedsensor on a popular Honda and your scan tooldoesn’t show detailed ABS data on Honda ve-hicles. Don’t punt away this diagnosis yet. Ifyou’re careful, you can test the sensor with abasic scope, a digital voltmeter and a smallmagnet. Here’s what I’ve observed.

Imagine that the right front wheel speed sensorhas set the trouble code(s). Safely raise the vehicleon a lift that allows you to spin the right front tirefreely. Manually spinning a tire usually is ade-quate for speed sensor testing.

First, see if the sensor produces a signal. Turnthe ignition switch on, engine off and shift thetransmission into Neutral. Next, trace the speedsensor wires until you find its harness connector.Usually, this gray, two-wire connector is on theframe, not far from the sensor. Also, both sensorwires going into the connector are black. But thetwo wires coming out of this connector—headingtoward the ABS control unit—have specific colors.

I’ve found that checking the sensor with a digi-tal storage oscilloscope (DSO) is very effective.Preferably, use a scope with at least two channels.You have to measure battery voltage on one sensorwire and about 2.00 volts on the other, so set upthe scope voltage scales accordingly. Then careful-ly connect the scope to the terminals in the sen-sor’s harness connector. I usually make the con-nection with a pair of backprobes lubed with di-electric grease to prevent damage to the connec-tors. If you don’t have a schematic handy, you havea 50-50 chance of probing the correct wires on thefirst try! Next, vigorously spin the right front tire.

continued on page 16

Foreign Service

16 September 2013

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For your information, the screencapture on page 14 was produced bya 2007 Civic wheel speed sensor. Thetop pattern is a typical Honda speedsensor signal. Note that it neatly andconsistently switches between about.70 volt (low state) and 1.40 volts(high state). Because this digital sig-nal is “low” about half the time andthen “high” about half the time, it hasa 50% duty cycle. A healthy speedsensor produces a consistent signallike this one.

The bottom pattern in the screencapture, which measures a bit morethan 10.00 volts, is the power supply tothe speed sensor. My testing indicatesthat the ABS control unit usually applies10.00 to 11.00 volts to this wire. Pre-dictably, the speed sensor quits workingif this power supply circuit fails; verify-ing power supply to the sensor is a vitalstep in wheel speed sensor diagnosis.

No scope? Then grab a digital volt-meter and confirm that the powersupply to the speed sensor is good. Ifit is, connect the voltmeter to theother speed sensor wire and spin thetire again. Now if you watch the me-ter display closely, you should be ableto see if the signal is switching highand low. Of course, the digital meterresponse doesn’t rival the clarity andaccuracy of a scope. But at least themeter tells you if the sensor is pro-ducing any signal whatsoever.

Okay, suppose there’s a good powersupply going to the speed sensor, butthe sensor’s not generating any signal.In that case, either the magnetic en-coder failed or the sensor itself quitworking. Experience shows that thering of mini magnets on the back ofthe wheel bearing is very reliable. Vi-sually inspect the vicinity of the wheelbearing for physical damage of anykind; possibly refer to the photographsof a new bearing in last month’s col-umn. (The side with the magnetic en-coder easily attracts a common paperclip or a thin, steel feeler gauge.)

You can play the percentages andreplace the sensor, or test the sensorfirst. To test the sensor, remove theretaining bolt and pull the sensor outof its bore. If the sensor doesn’t slideout readily, you may be able to grip

its flange with a large pair of pliers.Then try to carefully twist the sensorloose from its bore.

There’s a flat surface on the end ofthe sensor that faces the wheel bear-ing (see photo above). Place a magnetfirmly and squarely against this flat fora few moments and then remove it.I’ve used common “pick-up” magnetsfor this check—the kind usually usedto retrieve nuts and bolts. Anyway, re-peat this procedure while watchingthe voltage on the sensor’s signal out-put wire. Triggering a good speed sen-sor with a pick-up magnet shouldmake the signal toggle between ap-proximately .70 volt and 1.40 volts.

Sources have told me it’s prudent tolubricate the upper “barrel” of a speedsensor before you install it. Some techsuse a film of general-purpose grease,others use synthetic caliper grease. Thepoint is that you may be the personwho disassembles that part the nexttime. Corrosion inside the bore canmake speed sensor removal difficult.

Last but not least, pay close attentionthe next time you service a brake systemon an ABS-equipped vehicle. If youcan’t find an obvious example of a reluc-tor or tone ring, then you can bet thereare magnets on the wheel bearing.

Meanwhile, tune in next month fortips on installing Honda speed sen-sors equipped with O-rings. I’ll lookfor you then.

This is an example of a Honda digitalwheel speed sensor. The flat surfaceof the sensor I referred to in thetext is on the left side of the sensor“barrel,” starting at the tip.