Can somebody explain wheel speed sensors to me

Because the unit has discrete electronics built into it used to produce a pulse, the output impedance will not give you an accurate measurement that can be used whether or not the unit is good. The testing to see if the unit is functioning is to produce the pulse, no pulse, bad. Pulse, good. If you want a more technical description, I can provide that.

 

So....How does the manufacturer expect us to test?

Jack up a wheel, spin it, and read the AC output?

Pull it down the road while hanging under it with a meter and two tiny holes to try to get the probes into?

No, they expect you to OHM it

Or, if you have more info on diagnosing, just tell us the correct method

 

Yes. Re: Bendix
Rotate wheel and verify a minimum of 0.25 VAC sensor output at ~ 0.5 RPS

 

And what does that tell you if you have no voltage?

 

Yes they expect you to hang out the door while the truck is going down the road to check it.


No they don't expect to "ohm" it, I never saw that.

Just read the next quote.

Yep there it is. There are two wire sensors which are self-exciting, the three wire sensors require a voltage.

Its broke.

 

I concur with DOSGATOS information he provided and it is correct. That is a an operational test, however it is not a one all be all means of diagnosing a wheel speed sensor. Being that Bendix has been refenced in the discussion, please see accompanying guide copied and pasted directly from Bendix's site concerning Electrical Diagnostic procedure for their sensors. They put not as stated in capps not me.

Normal resistance range across pins at room temperature should be 1500-2000 Ohms. (Note: Previous model WS-20™ sensor was rated at 2000-2500 Ohms.) Individually test the resistance of each pin to vehicle ground and note there is NO CONTINUITY. If the resistance readings are as shown, the wire harness leading to the modulator may require repair or replacement. Before attempting repair or replacement of the wire harness, refer to the test procedures specified for the antilock controller in use for possible further testing that may be required to substantiate the wire harness problem. If the resistance values are NOT AS STATED, replace the sensor. Resistance could be as low as 1100 Ohms or as high as 3300 Ohms if wheel end has recently been exposed to extreme temperature.

 

Nope Nope Nope... you guys are running down a rabbit hole.

Sensor is fine, reading correctly, tone ring is out of round.

It's a Freightliner. It's a common problem, the tone ring is out of balance on the hub.

Corrosion in between where the tone ring mounts to the hub. Sensor is fine, don't go buy one.

Pull the wheels, brakes, hub off offending axle, wire wheel were the tone ring sits on the hub, and replace the tone ring, $55 part at freightliner, $600 in labor.

Southern tire mart in Corsicana TX did my last one. $700 out the door.

 

If the wheel speed sensor can see a variance between the other axles, then the sensor ain't bad. No need to go Ohm'ing wiring harnesses.


It's what the sensor is reading, thats bad