Are the wheel speed sensors active or passive on this vehicle?
have the same problem?
Thursday, February 2nd, 2017 AT 11:08 AM
Not sure what you are asking. What kind of problem are you trying to solve?
Thursday, February 2nd, 2017 AT 11:56 AM
Active sensors receive battery voltage when the ignition is on and return a signal. Passive sensors are magnetic and generate current to return the signal. I need to know which this vehicle uses in order to determine the proper test procedure with a multi-meter. I am not sure what you found confusing about the original question.
Thursday, February 2nd, 2017 AT 12:10 PM
I have been in TV/VCR repair since the early 1970's, and specialized in auto electrical and suspension and alignment since the late 1970's. In nine years of teaching Auto Electrical, and twenty five years of attending numerous manufacturer's classes, the terminology has never come up. The terms are understood by very few experienced mechanics, so yes, I am confused as to why you need to know this. All you have to do is look at how many wires the sensor has, and you will find they almost always have two for wheel speed sensors. All position sensors are magnetic and generate a signal the same way when their magnetic field is disturbed. The two-wire sensor generates a voltage that is sent to the computer. The three-wire sensors generate the same voltage, but they also have internal circuitry that amplifies it before the voltage is sent to the computer.
The signal from a two-wire sensor will be a sine wave, (something else mechanics don't need to know), that gets higher in amplitude as the movement speeds up. The signal gets weaker as speed slows down, which is why a lot of GM vehicles have trouble with "false activation" when slowing to a stop. In a three-wire sensor, that sine wave signal switches the circuitry on and off, and the signal voltage switches between near 0 volts and near supply voltage, most commonly 5.0 or 8.0 volts. That is a square-wave signal that does not change in amplitude as long as that signal is being generated.
The most important and most often-overlooked characteristic of generating any kind of output magnetically is "movement". Regardless if it's a position sensor or a generator, we need a coil of wire, a magnetic field, and most importantly, movement between the two. That means to get a signal, the tone ring has to be moving, the crankshaft has to rotating, or the speed sensor has to be spinning.
A true "active" position sensor uses a Hall Effect switching transistor that turns on and off in the presence of a magnetic field. Since there's a lot of circuitry in these sensors, there has to be a voltage supply wire and a ground wire in addition to the signal output wire, therefore they also have three wires. The difference with this type of position sensor is movement is not a critical factor. A common example of these is the distributor pickup assembly on older Chrysler 2.2L and 2.5L engines.
To differentiate between what you call "active" and "passive", look at when they generate a signal. A magnetic sensor with a coil of wire requires something to be moving. In the case of a wheel speed sensor, you will get 0 volts of signal if the tone ring is not moving. It does not matter if a tooth or a gap is inline with the sensor. A Hall Effect sensor will switch to 5.0 volts or to 0 volts, depending on whether the movable magnet is close or far away, but it doesn't have to be moving. Many three-wire crankshaft position sensors are of the Hall Effect design. With those, we can watch the signal voltage change state as we slowly turn the crankshaft by hand. A three-wire magnetic sensor will not generate any signal voltage when turning the crankshaft by hand, and depending on the design of its internal circuitry, the signal voltage could sit at 5.0 or 0 volts.
As for testing sensors, mechanics do not have customers' time and dollars to waste on that. I saw a coworker get fired after being warned numerous times for doing worthless testing on trade-in cars. I made my students test sensors, rebuild generators and starters, and rebuild wheel cylinders and calipers, but I strongly explained that was so they would be able to do it if they ever had to, and to understand how the parts worked. They knew they would rarely be testing sensors on customers' cars. To add to the misery, all service manuals have charts or explanations listing the acceptable range of sensors, typically resistance readings. That makes many do-it-yourselfers, and even some inexperienced teachers, think those charts will help in diagnosing a problem. In fact, half of those sensors that measure outside that "acceptable" range will be found to be working just fine, and half of those that are not working properly will be found to be "in specs". Those charts in the service manual are only good for learning if a specific sensor will have around 5 ohms or 5,000 ohms. Once you see how a magnetic sensor is constructed, you will see it is nearly impossible for one to change its resistance from when it was new, except for when it is completely open. It is fine to measure a magnetic sensor that is believed to be open, to verify our diagnosis, but then we replace it without testing the new one.
That brings me to your wheel speed sensor. The ABS Computer does the testing for you. If there is a break in the circuit, there will be a diagnostic fault code referring to the circuit. The clue is that is detected while the computer runs through its six-second self-test procedure, and the code will be set and the yellow warning light will turn on right away. A different code referring to "loss of signal" will be set if the tone ring is missing or cracked, or the sensor's tip is blocked by metal chips. That will not be detected, and the warning light will not turn on until the vehicle is moving. Both conditions result in a loss of signal, but the fault codes are quite different. I was hoping you would have shared the fault code(s) or symptoms, but apparently that is a secret. Another clue is defects related to a weak sensor signal are detected at lower speeds. A cracked tone ring typically is not detected until the vehicle is driven up to a mile, and it is detected at higher speeds. It can take a while before the computer assumes you are no longer going around a sweeping curve and the wheels should be rotating at the same speed.
To test a two-wire magnetic sensor, you simply measure between the two terminals. The easiest place to do that is right by the wheel, but on many vehicles the connector is poked inside the body away from water and salt damage. More problems occur between the terminals, due to corrosion, and to include those in the test, you need to go right to the computer's connector. It only takes a light film of corrosion between two mating terminals to cause a loss of signal. It is real common to overcome the problem by unplugging the connector to take the reading, then scratching a temporary clean area when the terminals are reconnected. That has happened to me many times. Now the circuit works, and I have no idea if I accidentally did something else to solve the problem, or if it is going to act up again in a few weeks.
There is no easy way to test a three-wire magnetic sensor. In fact, to show how one operates, it can take a couple of hours to set up a classroom demonstration. That's fine for learning. It is totally unacceptable to charge a customer for that. Besides, the computer does that for you.
A voltage test is better than a resistance test because it also includes all of the mechanical requirements. In other words, a wheel speed sensor can be perfectly fine in every respect, but it is not going to generate a signal if the tone ring is missing. There is still a problem though. Digital voltmeters are only accurate when measuring a 60 Hz sine wave. Many tone rings have around 55 teeth on them, so the measurement is only accurate at around one rotation per second. At any other speed, faster or slower, the voltage displayed will decrease even though the actual voltage might be going up. Fortunately, voltage is not what is being measured by the computer. It is counting the frequency of the pulses to determine wheel speed. If you really feel the need to do some exciting tests, you can watch the signals on an oscilloscope. I have a half dozen from my TV repair days, but that was way more work than I had an interest in doing.
Now, given all that wondrous story, I will admit that I spent a few hours testing the front wheel speed sensors on my friend's Suburban. We knew what the solution was up front, but we wanted to understand why it was happening in hopes we could design something better than what the engineers came up with so this would not happen again. The symptom was false activation when approaching a stop. There were no fault codes because there was no defect. Both front speed signals were normal, as viewed on a scanner, at higher speeds, but at low speeds the left one dropped to "0" mph too soon while the wheel was still rotating. We tried measuring the two signal voltages at the same time, with two meters, but there was no way we could guarantee both wheels were spinning at the same speed with the vehicle jacked up in the garage. Since we already knew what was wrong, we assumed we had identified an important clue when the signals were 700 millivolts on one side and 800 millivolts on the other side. Those were much lower than what we expected.
The cause of the false activation was rust buildup under the sensors that pushed them further from the tone rings. Our fix, which has turned out to be real effective, was simply to clean them, then seal them with a little silicone gasket sealer. The problem is, to verify our brilliance, we rechecked the signal voltages after the repair, and we still found 700 and 800 millivolts! Given the inaccuracy of the meters, and the unequal wheel speeds, we proved that voltage testing had absolutely no value.
There were no fault codes, so no need for resistance tests on the sensors. The sensors were developing signal voltages, and are not going to change resistance values, so there was no thought of replacing them. Voltage tests showed no change before to after the repair. In fact, there are no tests of value for this symptom. The entire diagnostic procedure was to observe the wheel speeds on a scanner, during a test-drive.
If you want to do anything I described here, have a dandy time, but do it to learn how the system works or to learn how to diagnose the problem more efficiently next time, but do not look at these tests as a regular part of every ABS job. We do not have that kind of time to waste. The goal is to earn the same reputation for efficiency that your competitor down the road has.