Sensing wheel technology changes

Wheel speed sensors have evollved over the yers, but still are important undercar components.

wheel speed sensors speed sensors electrical systems repair shop training technician training A/C training automotive aftermarket As long as vehicles have been rolling, the speed of the load-carrying wheels has been an issue. When I was a young boy watching western movies in the 1960s, I could always tell when a wagon was going to crash, because the camera would focus on a wood-spoked, steel-tired wagon wheel that was obviously spinning faster than it should. Those wooden wheels had a hard wood hub spinning on a hard wood axle; there was no bearing at all except for the animal fat-based grease that was packed in between the components.

In the course of the story, a crisis situation would come about that caused the draft animals to bolt and carry the wagon too far too fast (usually hurtling along through boulder-strewn cactus country or on the rim of a 1,000-foot promontory). Then a wheel would come off, tossing the wagon's cargo and passengers far and wide amid splintered boards accompanied by a suitably exciting explosion of dust and debris.

I'd remember those scenes as I rode in the back seat of the family car on vacation trips while looking out the side window at the blur of the spinning wheels on the vehicles we were passing. And as I watched, I'd wonder how fast a car would have to go before the wheels came off the way those wooden wagon wheels did.

For years, vehicle road speed was measured at the output shaft except on old Volkswagen beetles, which had the speedometer cable fed through the hollow left front wheel spindle with its square end poking through a matching hole in the left front wheel bearing dust cap and a small snap ring to hold the end of the cable in place. The old VW engineers had a way of doing things like that: smooth and simple, easy to troubleshoot and easy to repair. Heck, the gas gauge on older bugs was even mechanical; it used a short cable-and-sheath affair that led from the float lever in the tank right up to the gauge.

We watch the speedometer when we drive and we make our decisions accordingly. There are laws, road conditions and special circumstances that must be considered if we're to handle vehicles safely and responsibly.

Then we have the inaccurate speedometers, those that tell us we're going slower than we actually are. I drew a repair order on a Ford Tempo back in the late 1980s with an odd write-up. The owner, a young girl, said her car seemed noisier at road speed than it should be.

When I headed up the four-lane highway on my test drive, I noticed that I was passing a lot of other cars even though the Tempo's speedometer was reading 65 and that was the speed limit. When I used the mile markers and my wristwatch to make calculations, I found the car was going 85 mph when the needle was pointing at 65.

When the wheel speed information on contemporary vehicles is utilized for purposes of vehicle dynamics (ABS, traction control, ride stability, active suspension, etc.), wheel speed information delivered to the electronic controller absolutely has to match reality. The decisions made by ABS systems must be instantaneous, thus the fastest CAN bus network is used to carry information to those modules.

Changing Technology

Wheel speed sensing seems to have begun on aircraft in the late 1920s. When pilots landed heavy planes, they had to slowly apply the brakes in order to find the skid point and then release the brakes enough to prevent the skid, which tended to destroy tires and raise safety concerns. (A blowout at 200 mph isn't a good thing no matter what the wheels are carrying.)

One source tags a French engineer named Gabriel Voisin with credit for the invention of anti-lock braking on aircraft that consisted of an ingenious flywheel and valve assembly that rotated at the same speed as the tires during normal braking. But it used the special valve to automatically re-route fluid to the master cylinder when the wheel speed decreased to a velocity that didn't match the flywheel speed. That was a totally mechanical wheel speed sensing system, but electronics began to evolve and pervade the automotive industry and Chrysler and Bendix introduced wheel speed sensing as a part of the anti-lock brake system on the 1971 Chrysler Imperial.

It isn't rocket science to wrap a few thousand turns of wire around a strong cylindrical magnet, encase that assembly in plastic with a protective metal sheath and place it near to a rotating wheel with some evenly spaced ferrous metal or magnetic teeth. The technical term is Variable Reluctance Sensor. Because the magnetic field shifts with the passing of each little tooth or window, that arrangement produces a sine wave that increases with amplitude and frequency as the teeth on the rotating component whirl past the sensor.

The same principle works when the magnets are in the teeth and the sensor core provides the shift. The output on these sensors is from 0.25 to 400 volts, and the tip of the magnet is generally from 0.25 to 2.8 mm depending on design and application of the system in question. Sensor resistance, depending on system design, can be as low as 200O or as high as 4,000O, and these sensors can handle temperatures from -40° to about 325° F.

What is rocket science is to build a specialized computer that can use that speed signal and do needful things with the information provided. Any electronic device that takes input has to have outputs in order to be useful; ABS systems have isolation and dump solenoids, a hydraulic pump and some warning indicators as their outputs. On some vehicles these space age computers convert the incoming vehicle speed sensor signals and send that information to the PCM, body computer, instrument cluster, cruise control, etc. Simpler ABS modules use the wheel speed information to control some simple valves that are used to prevent rear wheel lockup on pickups and vans (mandated by the government in 1987).

Ford and Dodge pickups used a rear axle sensor with 1,000 to 2,000 ohms resistance (wire wound sensor) reading off the ring gear, and Ford used that sensor (dubbed a Differential Speed Sensor) in the early 1990s to provide vehicle speed information. Those sensors failed so often that they got inexpensive really fast, even at the dealer (I've purchased them for $12). GM opted to use a sensor on the transmission output shaft to monitor rear axle speed.

Digital Sensors

This was all fine, but with the advent of new vehicle dynamics systems, the old wire-wound vehicle speed sensors just can't provide accurate input below a certain speed. For one thing, the sine wave flattens out dramatically as the wheel slows to parking lot speeds, making it harder to measure, because analog signals have to be converted to digital signals in order for the module's electronics to make decisions.

Over time, more accurate sensors became the order of things. Computers like crisp digital signals that don't have to be converted, and Hall Effect units filled the bill. In ignition systems, Asian manufacturers tended to use sensitive optical emitters and detectors in their distributors with a thin slotted metal disk whirling between them.

The Hall Effect sensor is based on the principle that when an electric current flows through a conductor that is saturated in a magnetic field, that magnetic field exerts a transverse force on the moving charge carriers that tends to push them to one side of the conductor. The Hall Effect conductor switches its signal very suddenly as the magnetic field comes and goes, so by spinning a cup with windows and vanes so that they pass either near the sensor or between the sensor and its magnet, a nice square wave signal is produced. The Hall Effect sensors we're comfy with today have a voltage feed, a ground and an output signal wire.

As Hall Effect units in distributors replaced the now ancient wire-wound stators of the '70s and early '80s — all three domestics started using distributor mounted Hall Effect sensors to input engine speed and position to their engine controllers in the early '80s — so engineers eventually opted for the same crisp, digital signal for vehicle speed sensors. Chrysler began using three-wire Hall Effect crank, cam and vehicle speed sensors in the early 1990s (with an 8-volt supply, a ground, and a signal output). This made controllers a little easier to build; the old chip that was used on older modules to convert analog to digital signals became unnecessary.

Magnetic Resistance Element (MRE) wheel speed sensors output a square wave signal, and the ones with two internal MRE elements can detect rotational direction of wheels along with wheel speed. Located in wheel bearings, each direction MRE sensor contains an integrated circuit consisting of 2 MRE's, a processing unit, and a magnet. The sensor rotor is integrated with inner race of hub bearing, and has north and south poles arranged in circle. Wheel rolling forward generates a distinct signal like a regular square wave, but a wheel rolling backward generates an odd square wave with a lower voltage dip in the center of the normal wave.

Troubleshooting

All MRE sensors reportedly can detect rotation speed down to less than 1 mph, but be aware that they can't be tested for resistance or output voltage. One of the wires feeding these sensors is a 12-volt power supply to the sensor, the other wire is a ground. Some sensors of this type have three wires, one 12-volt feed a ground and the signal output. Make sure you know what you have before testing.

The square wave output is transmitted back to the ABS control module on the 12-volt feed; on some vehicles this is a 7 to 14 milliamp square wave signal that is kind of hard to read if you don't know what to look for. To get that reading on a Dodge or a Jeep, disengage the 12-volt wire from its two-pin connector and wire your meter in series, setting it to read milliamps. You see, the controller is looking for milliamps, so conventional wisdom would call for us to look at milliamps, too.

Set up your meter, turn the wheel slowly and look for the switch. If you aren't using a scope, don't spin the wheel rapidly or you won't see the signal switch. Cool stuff. But hold the phone! Here's another trick you can use that doesn't require extracting the terminal but still provides useful info. You can simply backprobe the 12-volt side of the sensor terminal and measure millivolts with a sensitive meter – you can get a square wave that way, too.

The square wave pattern increases in frequency with wheel speed. The handiest way to read it is on your scan tool, but be aware that some scan tools don't know which sensor is which and may lead you to the wrong wheel.

At the Ford dealer, I had a couple of Ford Contours throw me for a loop because they were reporting the left and right front wheels transposed on the WDS screen. Years later, I had a Dodge Caravan that smacked me around the same way on the rear wheels, but in this case I was using the OTC Genisys (4.0 system). The Genisys wouldn't retrieve ABS codes on that Dodge, and secondly, when I accessed the datastream it was reporting the wrong rear wheel. It took a friend's DRBIII to get the truth out of that one: we had changed the wrong sensor.

In another case, a Ford Explorer was tossing codes for a dead left front wheel speed sensor, but that sensor had normal resistance and a nice AC analog signal was making it all the way to the module. In this case, the hotline engineers had me disengage the left front wheel speed sensor wires from the module connector and move the terminals from the right sensor into those cavities, thereafter inserting the left sensor terminals in the right sensor cavities. If the module threw a code for the opposite sensor we'd know it was the sensor. If the same code appeared, we'd know it was the module, which turned out to be the case.

Many (not all) ABS modules with Variable Reluctance Sensors will send a bias voltage out to one side of each sensor when the module first wakes up to see if that voltage comes back on the opposite leg. If it doesn't, a code is stored. But that doesn't always mean the sensor is at fault. The wires leading out to the front wheel speed sensors tend to break on high mileage vehicles (lots of bending when the wheels are turned), and more than a few wheel speed sensors have been replaced unnecessarily for that reason. On GM cars, the sensors are built into the bearings, and when the bearings start to fail, the sensors generally do, too.

When MRE sensors first came out on Jeeps, I drew a repair order on one that had a bad sensor, and when the warranty claim made it to Chrysler I got a snippy call from a tech rep who thought he had me boxed in. "What tool did you use to check that sensor and determine it was bad?" he asked, his REJECT stamp hanging over the warranty claim.

"I pulled a sensor off another new vehicle, installed it, and the code went away," I told him. I could hear him deflating as he mumbled an apology and ended the call. The claim was paid.

In another case I saw an uncommanded Traction Control event on a 2001 Pontiac Montana that turned out to be a wheel speed sensor falsely reporting 22 mph when the vehicle was actually traveling at about 3 mph. A new bearing and sensor assembly took care of that one, but not before TSBs led us to check tail light grounds and a lot of other stuff.

Finally, late 1980s and early '90s Thunderbirds, Cougars and Continentals had a pulse ring for the front wheel speed sensors that could loosen up and cause the sensor on that wheel to believe the wheel was skidding when in reality it wasn't, and the resulting symptom was total brake failure.

With the pedal bouncing under the driver's foot, one of those cars could go right through an intersection. It has happened more than once, so check those if you happen across a mid-1980s Ford with ABS.

Conclusion

First, understand which type of sensor you have, and second, have a reliable scan tool on hand. Wheel speed sensors are in a hostile environment doing a very important job, and sometimes they die. And if the sensor doesn't cost much on a job you're not sure of, replace it, but pay close attention to the pulse wheel or ring, and get rid of any iron filings that may be hanging around in there.

Use the scope of choice whenever you can to verify the integrity of the signal. And don't forget to check the wire harness, which is the most overlooked part of the system.

Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail Richard at [email protected].