ABS Hall Effect sensor testing methods
If you haven’t noticed, ABS systems are on practically every car
and truck on the road today with very few exceptions. Years ago,
it was just an option, and an option that was quite expensive too.
But, as time went on the purpose of the ABS system graduated
from a basic “controlled stop for icy conditions” type of scenario
to what it is today. Today, the ABS system is far more complex
than back then and has taken on so many other roles that I’m not
sure back when the ABS was first introduced that those engineers
and designers of that time could have even imagine the capabilities
the future held for the Anti-lock braking systems.
Today, ABS isn’t just for controlled stopping on wet or slippery pavement, it’s all about controlling the vehicle under any “out of control” conditions, or situations when the driver’s attention has been reverted to something other than the road, forward or backward. These days the ABS systems can monitor the pitch and roll of the vehicle, over all speed, and with the help of other various systems on the car the ABS is informed of potential hazards in front or behind the vehicle that may require the ABS to take over the vehicles braking system.
Which means, the once “Oh I’m not going to worry about fixing that expensive ABS system on my car. I can get by without it.” stories can’t exactly be shuffled off to the side because they’re not needed or too expensive. All this incorporated interfacing into the various other systems of the vehicle may inadvertently cause those other systems to malfunction, set a code, or in some cases cause those systems to be inoperative. So servicing the ABS has become a priority in the repair shop and not a secondary, “Oh by the way” type of repair.
The heart and soul of the ABS system is the speed sensors. Without the wheel speed sensors none of the magic is possible. But, the very item that makes all of this work is the same component that’s out in all the harsh conditions Mother Nature can dish out. Everything from the weather and road conditions to debris getting tangled up into the sensors. Then, in the salt belt areas you’ve got the impacted ice and snow to deal with that can snap a reluctor wheel like a twig causing even more expensive repairs. All these type of failures to deal with and we haven’t even hooked up a scan tool yet.
There are two types of speed sensors found on today’s cars, the active and the passive speed sensors. One sensor can read tire rotation at the sightless rotation while the other needs to get the wheel spinning before it can generate a signal back to the controller. Both of the sensors have their good and bad points, but for this article we’re going to focus on the Active speed sensor.
The active speed sensor or for a much simpler term, a Hall Effect sensor. It consists of a permanent magnet embedded into the plastic housing with a small coil of wire that is used to send a signal back to the ABS control unit. The magnet has to pass over a metallic object to create a magnetic inductance into the coil of wire. The magnetic field strength changes when the magnetism sensitivity is disrupted by a metal surface shape and by the speed that it passes by the magnetic material. That surface is the reluctor or what is sometimes referred to as the tooth ring, either mounted on the CV shaft or as an internal part of the wheel hub assembly. Either way the toothed pattern of the reluctor is nothing more than an “ON/OFF/ON/OFF” (continually changing) signal for the magnetic pickup in the speed sensor.
When the wheel is rotating, the toothed pattern is passed over the sensor and an electrical pattern is created in the shape of the actual reluctor teeth. For example, if you’re looking at the toothed pattern on a scope and one tooth has a chipped tooth, that seemingly endless and repeated saw tooth pattern on the screen will be interrupted and changed by that single deformed tooth on the reluctor. This pattern will duplicate as the toothed ring passes by the sensor. The frequency of the signal depends on the rotational speed of the wheel and the amount of teeth on the disk or ring.
The Active speed sensor is generally a two wire sensor, but there are a few instances where it can be a three wire. The two wire sensor generally has a 12 volt power supply lead and a signal return lead. The reluctors toothed pattern causes either a high or low current to flow while the gap between the teeth causes the opposite to happen. The voltage levels are different on a 3 wire sensor because of their lower current current requirements. The voltage levels can also change from system to system depending on current flow and the resistance values, but a clear square wave pattern should still be visible either way.
TESTING THE SENSOR
As with any other system, if you don’t know how a system works it's pretty impossible to know how to properly diagnose and or test for a problem. And, with the cost of parts and components these days the old “swap til’ ya got it” method of repairing things isn’t going to fit into your customer’s budget very well. These days, even more than ever before, proper testing along with above par diagnostic tool usage is essential in getting to the heart of the problem without firing off the proverbial parts cannon.
Lab scope hookups
On the two wire Hall Effect type active wheel speed sensors there’s actually two separate signals we can observe with our lab scope. One is around a 2 volt signal and the other can be as high as 20 v (Which I’ve never seen one that high, but in all the reference material it does mention it to be that high.) The 2 volt (ish) signal is unique in its function. I like to refer to the 2 volt signal (Or sometimes referred to as the BIAS voltage) as the watch dog of the ABS system. Since these type of sensors contain a coil of wire and a magnet they can’t do much in the way of reading low wheel speed, that is, until the car gets moving fast enough to induce a magnetic field into the sensors coil of wire. So, what the engineers have done is to create a small BIAS voltage signal that is low enough and weak enough to detect any deviation in the wire quality, sensor condition, and or return signal strength before the wheels even turn one degree.
This signal can be read with a lab scope a whole lot easier than with a multi-meter, although you could use a multi-meter. I prefer the scope myself since a lab scope lays the signal out on the screen as a flat continuous line, it does make it easier for me to see it.
What tools do I need?
You’ll need your lab scope, a set of probe leads, and a few settings adjusted on your scope. Back probe the connector (if possible) to the sensor with your leads, but be sure not to disturb or mar any of the insulation or weather packing while doing so. Polarity isn’t an issue, except for those diehards that can't stand seeing current ramping backwards on the scope screen. If that’s your hang up…flip the probe leads or go to your scope settings and reverse the screen. I try not to get hung up on which side of the zero volt line I'm seeing the 2v signal, just getrdone and move on.
With the leads connected, and the ignition OFF, you should be seeing a zero voltage reading on your lab scope. On some scopes you’ll have to make sure you’ve got the scope set to its normal screen mode and the trigger at the infinite (or OFF) level, just to make it easier to read things properly. If your scope has the ability to screen out erratic or undesirable signals I’d have that turned on as well. Now all you have to do is turn the vehicle to ON. At the very moment you turn the key to ON, a voltage of around the 2 volt mark should appear on the screen. (Some vehicles it can be as high as 5 or 8 volts, but check with the manufacturer’s specs. to be sure.) 2.5 is about that average value that I’ve seen over the years.
Now, with the key ON, and the wheel stationary this BIAS voltage should be present, although on some vehicles, the 2 volt signal will disappear as quickly as it appeared on the screen. Keep in mind this minute voltage is a check of the wire integrity and that watch dog is looking for any intrusion into the circuit that it may deem as a fault. Meaning, that old watch dog you just let loose (when you turned the key on), may start to bark thinking your lab scope doesn’t belong in his yard. If that’s the case, the 2 volt signal will be discontinued by the controller until the next key cycle.
The next thing to do is to rotate the wheel (with the key still ON). Now what you should see is that saw toothed pattern of the reluctor that increases and decreases with the wheel rotational speed. If you spin the wheel as fast as you can you’ll probably see something like a straight line near the average voltage height. This is where using a scope is far better than a volt meter, because you can adjust the time on the lab scope screen and spread the signal waves out so that you can see the individual voltage segments of the wave form.
The three wire sensor is slightly different. One leg is used as a ground sensing lead from the sensor to the controller, one is used as a positive voltage lead and the third lead as the actual sensor signal return lead. On these my preferred method is to use two active channels on the scope. One channel for the positive feed signal and the other channel as the signal return lead. Hook up your back probe leads in the same manner as the two channel sensor and follow the same setup procedures on the lab scope. The same tests can be performed as well. Stationary and with the wheel turning will give you the same basic readings as if it were a two wire speed sensor with the exception of the voltage values will be somewhat different. These values will change from manufacturer to manufacturer.
Obviously, a scope is my preferred method of observing the Hall Effect sensor type wheel speed sensors. But you can always try the old homemade tester using a 220 ohm resistor a diode, and a 9 volt battery. I’ve made a few of these over the years that worked great and blinked the diode on and off when hooked directly to the sensor (sensor is disconnected from the vehicle), but I’ve also had some of these homemade testers just stay on and never blink. It really depends on the sensors resistance values more than anything else. Your results may vary. You could also try playing around with the 220 ohm resistor or a different type of diode for that matter.
A good scanner is of course the ultimate in testing the wheel speed sensors. Being able to see all four wheels in action on one screen really does make your diagnostic work that much simpler. But, no matter the method you chose the results are no different. It’s all about how quickly can you reach the right answer to the problem with what tools you have at your disposal. The better you get with using your tools the more you can fit into a day, which means more in your pocket when you go home.
Speed and accuracy in diagnosing today’s cars comes from a lot of practice and understanding the fundamentals of the system you’re working on. The more you put into bettering your skills the more you’re bound to put into a paycheck. Keep testing, keep experimenting with the different tools at your shop and you’ll become a better mechanic. Remember practice doesn’t make you perfect, perfect practice makes you better.