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Active vs. Passive Wheel-Speed Sensors
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By Counterman
Most active suspension systems come in many styles with fancy names like airmatic, dynamic or advanced. And, it doesn’t matter if it is a BMW, Mercedes or Jaguar, an active suspension must be able to react to three critical pieces of information.
First, it must act on information from the ABS and stability control system. Second, it must measure body movement. Third, it must detect the extent and rate of suspension movement. With these three pieces of information, the suspension can actively adjust the compression and rebound of the shock or strut.
Why would an engineer or automaker include this feature on a vehicle? An active dampener allows for a ride without compromise. The three inputs can be used to detect a rough road or an emergency situation where body roll could change the stability of the vehicle.
Electronic Shocks/Struts
Electronically adjustable shocks and struts use conventional mono-tube and twin-tube oil-filled dampeners. The rods, gas chambers and piston have the construction of passive units. Like a passive unit, they can fail if they leak, the gas escapes or the rods are bent. They can also wear out like a conventional unit as the oil inside breaks down and surfaces in the bore wear.
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What makes these units unique are the valves with their variable orifices. These valves regulate the flow between the chambers on either side of the piston. The piston in some units, however, does not have any valving.
The size of the orifices controlled by electromagnetic solenoids can control the valves very quickly. The electrical connections and solenoids are typically found outside the body and act on the valves inside the unit using magnetism. The signal to the solenoid is pulse-width modulated and varies the voltage to change the size of the orifice.
The valves and solenoids can’t be serviced or separated from the shock or strut. If a problem is detected with the system, the valves go into a fail-safe position that is fixed, and the system becomes passive. The driver is then alerted with a message or light on the instrument cluster or message center.
Most systems will perform a circuit check when the system wakes up. This typically involves sending a signal to fully open and close the valve. If the system detects an open, short or a voltage outside of the specifications, it will set a code.
Measuring Wheel Movement
Ride-height sensors not only measure the position of the suspension, but also the rate of movement. They are supplied with a voltage of around 5 volts. The signal voltage is changed as a magnet moves past a coil. Most sensors have three wires – ground, power and signal.
Internally, it is difficult to damage one of these sensors. Externally, however, the linkage that connects the sensor to the suspension arm can be damaged. Additionally, the connector can be damaged and cause a short or open that sets a code. If one of these sensors is replaced, it must be calibrated after it is installed.
Ride-height sensors are sometimes called suspension-position or wheel-displacement sensors. The data from the sensor is used to measure the movement of the suspension. By knowing how far and fast the suspension is moving, the module can use the information to determine the size of the orifice in the dampener to control compression and rebound. These sensors should be calibrated if a sensor is replaced, a module is reprogrammed or if the battery dies.
Measuring Body Movement
Accelerometers mounted to the body measure changes in the ride. These accelerometers are typically mounted to the strut towers. These sensors output information as gravitational forces, or “G-force,” to a module. Changes in body roll due to cornering will produce lower G-force than a pothole would.
Information from the accelerometers is coupled with data from the ride-height sensor, steering sensor and other inputs by a computer processor in a module. The module can determine if the vehicle is going around a corner or traveling down a bumpy road. With this datastream, the valving inside the dampener can be adjusted in milliseconds for the best control and ride quality.
The accelerometers on the body differ from vehicle to vehicle. Some manufacturers mount the sensors under the headlights, on strut towers and near the taillights. More sophisticated systems use more than two accelerometers mounted in various locations.
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The control module for the electronic dampeners needs more than the movement of the wheels and body to determine the correct settings for the dampeners. The module uses and shares information with the anti-lock braking system, engine control module and instrument cluster. This information is typically shared on the high-speed CAN serial data bus. On some BMW 7 Series models, the information is shared on the fiber-optic Flex Ray bus.
With all this information, the module can do some amazing things with the adjustable dampeners. Problems like nosedive under braking, torque steer and understeer on FWD vehicles can be minimized. If the vehicle has air ride, the volume and pressure inside the air springs can also be tuned along with the valving in the dampeners to optimize ride quality and control.
Most active suspension systems will perform a circuit check when the system wakes up. The system will send 5 to 12 volts to the actuators and ride height sensors. The system is also looking at the resistance in the circuit, and the amount of voltage dropped. If the system detects an open, short or voltage outside of the specifications, it will set a code. Next, the control module will fully open and close the valves in the struts. If the system does not detect any irregularities, the system will go into an active mode.
Looking for these self-diagnostic signals can be performed using a meter. You may have to use a bypass harness or back probe the connector. If the system detects any problems, the system will go into a passive mode.
Sometimes servicing an active suspension is like rebuilding an engine with a new crankshaft and reusing the old bearings and valve springs. When a new active strut is reassembled with the old and tired spring and strut plate, the results can be less than desirable.
Upper strut mounts and bearings can be hammered to death. The upper strut mount essentially supports the vehicle weight and counters both braking and acceleration torque. Most mounts are sandwiches of rubber, metal and bearings. Over time, the rubber can lose its ability to isolate the suspension from the body. Bearings can also seize and bind, causing the vehicle to have steering problems.
Look up the ride height specifications and measure ride height front and rear, and on both sides of the vehicle. If ride height is less than specifications, the problem is most likely one or more weak springs that should be replaced. Springs should typically be replaced in pairs to maintain the same ride height side-to-side.
Weak springs also are more likely to fail. The springs on many late-model vehicles are thinner to reduce weight and have an outer plastic coating to protect the metal from corrosion. If this outer coating is cracked or damaged, corrosion can form a hot spot that eats into the spring, weakens it and eventually causes the spring to break.
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By Counterman
While it might not sound like it to the untrained ear, the orchestration of components to achieve the ideal combustion cycle is nothing short of a symphony.
For fuel-injected engines, two important instruments in this precise arrangement are the mass airflow (MAF) sensor and the manifold absolute-pressure (MAP) sensor.
The MAF sensor, typically situated between the air-filter housing and the intake manifold, might be considered the maestro. Also known as an air meter, the MAF sensor uses a heated element to measure the amount of air by weight that’s entering the engine. As the air cools the heated element, this cooling effect changes the electrical resistance of the element. The amount of cooling the element experiences is directly proportional to airflow, and the sensor conveys this information to the engine computer by way of changing voltages or digital frequencies.
The engine computer then uses this information – along with other inputs – to adjust the amount of air entering the engine.
Other inputs that help determine the proper air-fuel ratio include: oxygen sensors, which measure the amount of air in the exhaust gases; throttle-position sensors, which tell the computer if the throttle is closed, partially open or wide open; knock sensors, which monitor for signs of engine knocking; and (on some vehicles) MAP sensors, which measure the amount of pressure or vacuum in the intake manifold.
While most fuel-injected engines today utilize a MAF sensor to obtain a precise measurement of airflow, MAP sensors play a starring role in fuel-injected vehicles with speed-density engine-management systems. However, turbocharged engines often have both a MAF and a MAP sensor.
“In turbocharged engines, the partnership between MAP and MAF sensors isn’t just a technicality – it’s the secret behind the vehicle’s ability to harness forced induction with unparalleled precision,” Walker Products explains.
Let’s take a closer look at each type of sensor and what they bring to the table.
MAF Sensors
Air changes its density based on temperature and pressure. In automotive applications, air density varies with the ambient temperature, humidity, altitude and the use of forced induction (turbochargers and superchargers). Compensating for changes in air density due to these factors is essential for maintaining the optimal air-fuel mixture and efficient engine operation.
Consequently, MAF sensors are better-suited than volumetric-flow sensors to provide an accurate measurement of what the engine needs. MAF sensors offer a more direct and accurate measurement of the critical parameter for engine combustion: the mass of air. This facilitates better engine performance, fuel efficiency and emissions control compared to relying solely on volumetric-flow measurements.
There are two types of MAF sensors used in automotive engines: the vane-meter sensor and the hot-wire sensor.
The vane-type MAF was the first one out there, and it was used on import vehicles from the 1970s and 1980s.
“It didn’t have many actual problems,” Charles Dumont explains
link hidden, please login to view. “However, many of them were replaced, because back then the vehicles didn’t have onboard diagnostic capabilities. Usually after mechanics and DIYers had replaced all the other ignition parts and sensors, the MAF sensor was the last-ditch effort.” These days, you’re more likely to encounter the hot-wire style of MAF sensor. The hot-wire MAF sensor is smaller, faster and more accurate than the older vane-type MAF sensor, making it the preferred choice in most late-model vehicles.
Delphi provides a great explanation of the hot-wire MAF sensor
link hidden, please login to view. “Put simply, a MAF has two sensing wires,” Delphi explains. “One is heated by an electrical current, the other is not. As air flows across the heated wire, it cools down. When the temperature difference between the two sensing wires changes, the MAF sensor automatically increases or decreases the current to the heated wire to compensate. The current is then changed to a frequency or a voltage that is sent to the ECU and interpreted as air flow. The quantity of air entering the engine is adjusted accordingly.”
MAF sensors are pretty dependable, but there are a few things that can undermine their performance.
Any air or vacuum leaks downstream of the sensor can allow “unmetered” air to enter the engine. This includes loose fittings or clamps in the plumbing between the air-filter housing and throttle, as well as any vacuum leaks at the throttle body, intake manifold or vacuum-hose connections to the engine.
Anything that contaminates the surface of the sensor also can hinder its ability to respond quickly and accurately to changes in airflow. This includes fuel varnish and dirt deposits as well as any debris that might get past or flake off the air filter itself.
A frequent cause of MAF-sensor failure is directly related to the air filter. Low-quality or incorrectly installed air filters can allow paper particles or dirt to accumulate on the hot wire, effectively insulating it and affecting the reading of the sensor.
Oil-soaked air filters also can have an effect on MAF-sensor operation, so it’s important to warn someone of this possibility if they’re installing a performance high-flow filter. In some cases, modified intake systems can cause increased air turbulence, which can affect the performance of the MAF sensor as well.
A dirty MAF sensor can cause performance problems and, in some cases, trigger a diagnostic trouble code. You can recommend MAF-specific cleaners (any harsher solvents can ruin the sensor) and air filters as maintenance items before your customer spends the money on a replacement sensor.
Symptoms of a failing MAF sensor could include rough idling or stalling; RPM fluctuations without driver input; and a decline in fuel economy and engine performance. A problem with the MAF sensor often triggers a “Check Engine” light.
MAP Sensors
As the name implies, the primary function of a manifold absolute-pressure sensor is to measure the pressure within the intake manifold of an engine (usually a fuel-injected engine). Essentially, a MAP sensor is measuring the barometric pressure – the atmospheric pressure that’s pressing down on earth. Barometric pressure is influenced by changes in elevation, air density and temperature.
The pressure reading from a MAP sensor is an indicator of engine load, and it helps the engine computer calculate fuel injection for the optimal air-fuel mixture. The MAP sensor helps the engine adapt to different operating conditions, such as changes in altitude or driving up a steep incline, where air pressure can vary significantly.
A MAP sensor contains a sealed chamber that uses a flexible silicon chip to divide the sensor vacuum from the intake-manifold vacuum. As soon as the driver starts the vehicle, the MAP sensor is called into action, performing “double duty as a barometric-pressure sensor,” according to Delphi. With the key turned on but prior to the engine starting, there’s no vacuum in the engine applied to the MAP sensor, so its signal to the engine computer “becomes a baro reading helpful in determining air density.”
“When you start the engine, pressure in the intake manifold decreases, creating a vacuum that is applied to the MAP sensor,” Delphi explains on its website. “When you press on the gas accelerator pedal, the pressure in the intake manifold increases, resulting in less vacuum. The differences in pressure will flex the chip upward into the sealed chamber, causing a resistance change to the voltage, which in turn tells the ECU to inject more fuel into the engine. When the accelerator pedal is released, the pressure in the intake manifold decreases, flexing the clip back to its idle state.”
Typically, you’ll find the MAP sensor in the air cleaner, fender wall, firewall, intake manifold or under the dash, Standard Motor Products (SMP)
link hidden, please login to view. Given their location, MAP sensors commonly fail “due to the constant contact of the movable wiper arm over the sensor element and the exposure to the high underhood heat,” according to SMP. The high heat can melt or crack the electrical connectors. MAP sensors also are susceptible to contamination.
“If the MAP sensor uses a hose, the hose can become clogged or leak and unable to read pressure changes,” Delphi explains. “In some cases, extreme vibrations from driving can loosen its connections and cause external damage.”
A failing MAP sensor will compromise the engine’s ability to maintain the proper air-fuel ratio, leading to a number of potential symptoms. These symptoms could include noticeably poor fuel economy, sluggish acceleration and an odor of gasoline (signs of a rich air-fuel ratio); surging, stalling, hesitating, overheating and a general reduction in engine power (signs of a lean air-fuel ratio); higher emissions that can lead to a failed emissions test; erratic or unusually high idle; and hard starting or even a no-start condition. A faulty MAP sensor also can set off a “Check Engine” light.
Parting Thoughts
MAF and MAP sensors are small components that play a big role in modern fuel-injected engines. With turbocharged engines becoming more and more prevalent in some of the most popular models on the road today, these sensors should continue to play an important role in automakers’ fuel-economy and emissions-control strategies.
“As turbocharged technology evolves, understanding and optimizing the cooperative function of these sensors becomes the key to unlocking the full potential of modern turbocharged engines,” Walker Products explains.
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By Counterman
Continental has added eight new part numbers to its line of OEM knock sensors.
The sensors are the same part that the vehicle manufacturer uses and deliver the exact fit, form and function as the original part, ensuring an easy installation and long service life, according to Continental.
The eight new part numbers provide application coverage for some of the most popular domestic, European and Asian makes and models on the road today. The expanded line covers Chrysler, Dodge, Ford, Infiniti, Jeep, Lincoln, Mercedes-Benz, Mercury, Nissan and Ram models ranging from 2000 to 2023. The new sensors provide coverage for 28.8 million vehicles in operation (VIO) in the United States and 2.4 million vehicles in Canada.
“Our newly expanded line was developed to meet the growing need for reliable knock sensors on some of the most common vehicles on the road today,” noted Brendan Bachant, Continental product manager for engine management and fuel. “The original sensors can be prone to failure due to mechanical damage, excessive vibration, high engine temperatures, and corrosion. Continental has made these OEM sensors available to the aftermarket so that professional technicians can easily and confidently service the most common vehicles in the shop, like the Ford F-150 and Explorer, the Jeep Wrangler and the Nissan Maxima and Altima. Technicians can be confident when choosing the Continental knock sensor that they will avoid comebacks.”
Knock sensors are designed to detect engine ping caused by pre-ignition and relay the information to the electronic control unit to adjust engine timing and help keep the engine running smoothly. These sensors are an ideal repair for a rough-running engine with a timing and knock-sensor fault code and will help shops to restore the performance of their customers’ vehicles to OE specifications, according to Continental.
Continental knock sensors are built in ISO-certified facilities to deliver the highest level of dependability, the company noted.
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