Last month, we explored the tried and tested VR and Hall-effect sensors that have provided a speed input to the engine, trans, and ABS controllers for decades. In the past, it seemed those two sensors would handle almost anything the automobile would need. The inexpensive VR sensor worked great if the module only needed information on rotational speed, and the Hall effect worked great if position accuracy and low-speed operation were necessary. Modern vehicles have become so advanced that they require even more precise sensors that measure low speed, high speed, and even rotational direction. With these additional requirements, the modern advanced speed sensor was born.
MAGNETO RESISTIVE SENSORS
The MR sensor is an active speed sensor that typically has two wires – a power and a ground circuit. These sensors often confuse technicians since they resemble a VR sensor. They will fail common VR tests such as measuring for resistance or checking for an AC voltage with a voltmeter or an oscilloscope.
When MR sensors first became mainstream in stability control systems, their identifying characteristic was that the output signal had a milliamp change when triggered instead of a toggling voltage, like in a Hall-effect sensor or an AC voltage signal (like in a VR sensor). The MR sensor changed the amperage on the circuit from about 7mA to about 14mA. The sensor’s “ground” wire would also often change slightly when monitoring voltage, but the logic in the PCM is monitoring the amperage change and not counting the voltage pulses.
You can test the active speed sensor in a few ways. One very simple method is rapidly shorting the wires at the speed sensor connector and observing the PID on the scan tool. You should see an RPM or speed input. For example, Figure 1 shows a Mitsubishi Outlander’s wheel speed when rapidly shorting the terminals of the computer side of the WSS connector. This test may or may not be included in the service information, but it’s a quick way to check the integrity of the computer and the wiring.
Visually inspect the sensor and tone wheel. Many MR sensors will read magnets embedded into the component, often called an encoder ring. For example, wheel speed sensors often read the tiny magnets embedded in an encoder ring along the wheel bearing seal (Figure 2). Excessive rust or damage might affect the sensor output, and bearing failure can also lead to a poor signal.
As mentioned previously, MR sensors have no resistance spec, but you can monitor the amperage with a DVM. You need to measure amperage in series, so you’ll need to use proper jumper wires and connect your DVM in line with the sensor. Rotate the wheel, and you should see the amperage change between 7mA and 14mA.
If you t-pin into the circuit while it’s connected, don’t expect to see a great voltage change. You can see in Figure 3 of this 2011 Cadillac SRX that the WSS voltage only changes around 20mV. The voltage change results from the MR circuit increasing the amperage, but the computer isn’t looking for this voltage change.
You can also witness this voltage with an oscilloscope (Figure 4). If you place a 10-ohm resistor in series and measure the voltage drop across the resistor while rotating the wheel, you can get a crisp digital signal, like with this 6T40 input speed sensor. If you want to get fancy, you can plug the voltage values into your ohms law calculator (V=I*R) and calculate the amperage. The 10 ohms added to the circuit won’t affect the circuit’s operation since it has such low resistance and the circuit hardly pulls any current. In Figure 4, a math channel was added to show the amperage change, and it’s right on the money at 7mA and 14mA.
SENSORS
Many modern vehicles utilize advanced speed sensor technology. These advanced sensors are still two-wire and output an amperage change, but the sensor can either be a Halleffect or Magneto Resistive design. As for diagnostics, it doesn’t matter. The sensor internally does the processing and outputs a signal, and it’s the signal that makes this sensor advanced. There are two main types of advanced speed sensors – one that changes pulse width to indicate direction and the other that sends a binary message after each pulse to provide even more information.
The direction of rotation is very important with modern vehicles that need to know if the vehicle is rolling backward at a stop (hill hold feature), or if a transmission shell or shaft is spinning forward or reverse, or if the engine is spinning clockwise or counterclockwise. Yes, you read that correctly. Auto manufacturers are trying to hone an engine’s start/stop feature so that it restarts with as little rotation as possible.
Imagine this: when a vehicle approaches a stop, and the engine shuts off, the engine winds down to a stop. Since some pistons are pushing up on the compression stroke, compression will rebound the engine backward a bit. We’ve all seen this effect when you shut an engine off. It stops and then backs up a bit. If the crank sensor cannot measure how far in reverse the engine spun, the starter would have to crank until the PCM sees its signature signals from the camshaft to know where it’s at. 
Now, with these advanced speed sensors, it can see exactly how far the engine rebounded and allow for a quick start when the driver leaves the stop light. Figure 5 shows a 2.0-liter 4-cylinder engine that slows down to a stop, then there’s a gap, then a few more pulses. Figure 6 shows the zoomed-in pattern of a pulse during slowdown and then a pulse when the engine rotates in reverse. You can see that the pulse width changed from about 80 microseconds when spinning clockwise to about 160 microseconds when spinning counterclockwise. This advanced speed sensor internally processes the pulse and then precisely sends a signal out, accurately identifying the position and rotational direction.
The other advanced speed sensor sends a digital signal it picks up from a magnetic encoder ring. Figure 7 shows that the sensor signal contains a large pulse (relatively speaking – it’s only 27 milliamps) and then a series of 7mA to 14mA pulses. These pulses make up a binary message that follows the AK protocol. This protocol defines information on signal strength, direction, and air gap. Not only does this sensor inform on rotational speed and position, but it also informs on signal strength, direction, and air gap. In Figure 7, the binary pulses differ when the wheel is rotated forward vs. reverse.
Service manuals don’t explain the operation of these sensors, nor do they provide details on diagnostics. As you can see, it would be easy to misdiagnose one of these sensors if you were expecting to find a typical hall-effect square wave pattern or a voltage that toggles between 0v and 5v. When testing a suspect sensor, if you don’t see a toggling voltage, zoom in and do a deep analysis of the pattern. The signal might be there. It just might not be what you are expecting.
