Speed sensors – how complicated could they possibly be? They use only two or three wires, so what’s to learn? Well, imagine checking a wheel speed sensor with a scope and seeing the pattern in Figure 1 while the wheel is rotating. Then, imagine your surprise when you learn that the pattern is perfectly fine. Part one of this article discusses basic speed sensors that have been around for a while, and part two introduces the advanced speed sensors found in many modern vehicles. These advanced speed sensors are used for crankshaft, camshaft, wheel, and transmission shaft speeds in common units, such as the 8L90, 10L/R80, ZF9HP, and more. Before we cover these modern high-tech sensors, let’s take a trip down memory lane and see how the speed sensor has evolved.
PASSIVE SPEED SENSOR – VARIABLE RELUCTANCE (VR)
When considering a speed sensor, you might envision a simple two-wire sensor that generates an AC sine wave, as seen toward the end of Figure 2. 
That sensor, called a variable reluctance (VR) sensor, has been around for decades, and it is commonly used to measure the rotation of just about any component, such as the crankshaft, camshaft, various transmission shafts, wheel speed, and just about anything that rotates in a vehicle. The VR sensor’s construction is about as simple as it gets. It includes a permanent magnet wrapped with a thin coil of wire. It operates on the principles of “induction” – where a magnetic field induces current in a wire. 
With the VR speed sensor, when a magnet is next to a tone wheel’s tooth, its flux lines concentrate or contract toward the metal tooth, and when the tone wheel rotates a bit, an air space opens around the magnet, and the flux lines expand out. 
This expansion and contraction of these invisible flux lines cause current and voltage to be induced into the windings. A computer can count these pulses, interpret the results, and determine rotational speed. I cannot imagine anything simpler than that.
Now, in automotive electronics, for diagnostic purposes, there might be a voltage on the VR speed sensor circuit originating from a module, as shown in Figure 3. However, it’s not powering the sensor by any means; it exists to check the sensor for opens or shorts. If you ever check a circuit while the sensor is unplugged, you might find 5 volts across the circuit wires. With the sensor plugged in, you might see around 0.5v or less across the circuit wires because the sensor now shares that voltage with a resistor in the computer. If the computer detects either 0v (short circuit) or 5v (open circuit) on the signal circuit, it knows there’s a problem and will set the appropriate diagnostic trouble code. Figure 2 is the wheel speed sensor on a 2007 Acura MDX, and it shows the open-circuit voltage at 4.7 volts, a closed-circuit voltage of 135 millivolts, and the sine wave pattern of 433 mV AC, which rides on top of that 135 mV DC.
The VR sensor’s (Figure 4) major downside is that it loses signal strength at low RPM and is sensitive to electrical interference. Low-speed signal strength relies on the magnetic flux lines expanding and contracting across the sensor’s windings for AC induction. The flux lines don’t expand and contract fast enough across the windings at low speeds to generate a significant voltage. It’s also not as precise as a digital sensor. If position accuracy and low-speed sensitivity are necessary, manufacturers will opt for either a magneto-resistive type sensor or a hall-effect sensor. The VR sensor is also sensitive to electromagnetic interference (EMI) from outside sources, such as ignition, alternator, solenoids, relays, and injectors. The two VR sensor wires are often twisted between the sensor and the computer to negate any EMI that might be induced into the wires.
Common diagnostics practices include checking the resistance with a DVOM, measuring AC voltage while rotating the component, and using a scope to check signal quality. Many techs will opt to change the sensor if they suspect it’s bad simply because the VR sensor is often inexpensive, and purchasing a new one might be more cost-effective than spending time diagnosing it. 
If the sensor is expensive or not readily available, basic tests can confirm the VR sensor’s proper operation.
Using an ohmmeter to check the VR sensor resistance is the simplest test. Check service information for the resistance specification for the sensor. Around my shop, VR sensor resistance ranged from as low as 500 ohms in the 41TE to as high as 2400 ohms in an Allison 2000. If there’s no specification listed in the service info, follow a DTC routine for the sensor even if you don’t have a DTC. Often, the measurement specs are found within the diagnostic steps.
If the resistance test passes, test for AC voltage with the DVM and monitor the peak-to-peak voltage. Ideally, you would have another sensor to compare to, such as testing a different wheel’s WSS to verify the operation of the suspect WSS. The typical peak-to-peak voltage value must be strong enough for the computer to accept the signal. The computer won’t continue counting pulses if the voltage drops too low. AC signal strength can vary greatly between applications. 
The WSS signal in Figure 2 is less than 0.5v AC when spinning the wheel by hand, while the signal for the VSS in a 2006 Trailblazer (Figure 5) was over 5 volts AC when spinning the driveshaft by hand. Just keep in mind that signal amplitude (strength) matters when dealing with VR sensors.
A scope is going to provide the whole picture when diagnosing a VR speed sensor. Among the benefits of using a scope is the ability to see the quality of the pattern as well as the voltage. If there’s a damaged tone wheel tooth or if the sensor signal voltage fluctuates excessively, this can all be seen when reviewing the scope pattern.
In conclusion, the trusty VR sensor has been around for many years, and it’s still found in some applications, but it’s rare to find it on a modern vehicle. Most now are “active” speed sensors that output a digital signal. The digital signal works well at low speeds and accurately determines shaft position at all speeds.
ACTIVE SPEED SENSORS – HALL-EFFECT AND THE MAGNETO-RESISTIVE
A hall effect sensor typically has three wires: a power source (5v, 8v, 9v, or 10v is common), ground, and signal wire. The Hall-Effect sensor still uses the influence of a magnet, but it uses the magnetic field to trigger the sensor and toggle voltage on the signal wire, generating a square wave DC pattern that the computer can use to determine position and speed. Figure 6 shows the signal from the two Hall-effect cam sensors on the Cadillac CT5.
The three-wire Hall-effect sensor is a common engine speed sensor, and it’s also found in many transmissions. There are a few ways to evaluate the sensor electrically.
With a DVOM, you can check for supply voltage and excessive voltage drop on the ground circuit. You can also measure the voltage of the signal wire and slowly rotate the component to ensure that the voltage toggles from near zero to whatever pull-up voltage the system requires. 
For example, in Figure 7, this Nissan Sentra toggles the 5V sensor signal wire between 5V and 0.5V. This vehicle operated fine, but the fact that the sensor didn’t pull the signal closer to zero might raise some flags if the vehicle had an issue. The 0.5 volts on this Nissan was due to an added voltage drop on the sensor ground circuit, which is a bit excessive.
To check for intermittent failures, apply light heat with a heat gun and maybe introduce some soft tapping to see if the voltage breaks up on the signal wire. Because of the fast switching and the averaging nature of a DVM, you cannot effectively evaluate a Hall-effect sensor with a DVM on a running engine. Also, don’t expect to check this type of sensor with an ohmmeter because the integrated circuits will cause a very high resistance reading, and the results won’t lead to a reliable diagnosis.
You can check the running performance of a Hall-effect sensor using an oscilloscope. Reviewing back to Figure 6, on the cam sensor signals, you should see a switching voltage that drops to near zero (0.1V) and pulls up cleanly to the pull-up voltage (5V). 
An oscilloscope does a great job of showing noise and erratic signals, but don’t forget to confirm voltage levels. If component timing is essential, a scope will also allow you to compare multiple sensor signals. The Hall-effect sensor is very dependable and robust and can accurately measure rotation at all speeds, but it’s common to find a magneto-resistive sensor in place of a Hall sensor when sensor operating conditions might be a little rough.
To conclude part one, the VR and Hall-effect sensors have played a large role in engine, transmission, and stability control over the years. The VR sensor isn’t as common anymore, but the Hall effect is still widely used in modern vehicles. In part two, you will see a couple of advancements to the speed sensor that improve the accuracy, determines rotational direction, and even offers a “bit” of self-diagnosis. As a teaser, Figure 8 shows a pattern of a single pulse from a modern wheel speed sensor. In next month’s issue, look for part two, which explores these advanced sensors.
