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The Key Differences Between 4WD and AWD Systems
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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
The fuel system, as a whole, is responsible for delivering fuel from the tank to the engine, then metering it into the combustion chamber. It consists of the tank, the lines, the pump and the metering device. If only it was as simple as it sounds. The challenge lies in the continual changes over the last century, and how the frequency of changes has increased over recent decades.
The heart and identity of any fuel system is the metering device or system that controls the flow of fuel into an engine. As a counter professional, you’re going to hear it all, and you’ll have to answer it all, so here’s a rundown on the major changes and differences over the years.
Carburetion Systems
A carburetor is a basic mechanical device, and the primary metering device used on the earliest automobiles. Carburetors held their ground until the late 1980s, when the last examples were eventually replaced by fuel injection. The job of a carburetor is to not only meter the fuel but also to properly mix it with the air flowing into the engine through the process of atomization.
As the automotive industry began to migrate to fuel injection, a knee-jerk reaction opposing fuel injection ensued. We were familiar with carburetors, and liked the fact that they were mechanical devices that could be repaired and rebuilt using basic hand tools, and there were no electronics involved. Regardless of who made the carburetor or what style it was, an experienced technician could diagnose and repair a problem without the need for service information, scan tools or electronics.
Though considered “simple,” carburetors are more complicated than they seem, with multiple different circuits to manage all aspects of engine operation. “Tuning” a carburetor – the art of balancing performance, efficiency and drivability – takes a considerable knowledge of engine operating principles, and the patience and precision to get it right.
The majority of carbureted vehicles utilize mechanical fuel pumps, driven off the engine. This too adds to the attraction of these vehicles, as again there were no electronics involved. The drawback to carburetors came in their lack of ability for precise fuel control. They simply couldn’t keep up with the tightening noose of emission and fuel-economy standards that was in full force by the 1970s. As the end of their use in production automobiles came near, some electronics were incorporated into them, but ultimately proved ineffective.
Today, any professional will admit – regardless of complexity – that fuel injection is simply superior and necessary. However, carburetion is still popular on old vehicles, partly because of its relative simplicity, but also due to the popularity of restoring old cars to their original state. While far from commonplace, carburetor rebuild kits aren’t going away anytime soon.
Fuel-Injection Systems
The advantage of fuel injection is the ability to precisely control fuel delivery under all operating conditions. Not only is this a necessity for emissions and fuel economy, but it also has a major advantage in drivability – an operational attribute that goes hand in hand with efficiency and performance.
Attempts at fuel injection are as old as the internal combustion engine itself, but in the early days, too many bugs made it undependable. By the 1950s, substantial engineering efforts were applied to develop fuel injection, both in the United States and Europe. One of the more well-known systems was the original Rochester fuel injection developed by Chevrolet for the 1957 Chevrolet and Corvette.
The idea behind developing this fuel injection wasn’t in the interest of horsepower or emission control. It was drivability, with the goal to eliminate the undesirable and unavoidable attributes of a carburetor, including fuel slosh in the fuel bowl and the transition between primary and secondary circuits. As you may expect, racers played a substantial part in all this, and the best part is they were very successful, and it unlocked horsepower as well!
The Rochester fuel-injection system was available from 1957 through 1965, but it ultimately failed for only one reason: cost. It was an expensive option, and with the muscle-car wars in full force and much higher-horsepower carbureted engines available for a fraction of the cost, nobody was buying.
By the late 1970s, fuel injection was better-developed, and this time emissions and fuel economy played a strong part. It began its rise to the top, and thanks to the advancements in electronic and computer technology, it got there quick. By the early 1990s, carburetion was all but gone from production automobiles.
Fuel-injection systems can be separated into multiple categories and types, and since you’ll hear multiple terms, here’s how to tell them apart.
Mechanical Fuel Injection
Early gasoline fuel-injection systems were mechanical. The pumps were mechanical, and fuel was delivered directly to nozzles located in the intake manifold. The pressure of the fuel caused the fuel injectors to open. A type of air meter was necessary, but early systems relied primarily on vacuum signals or mechanical linkage between the air meter and fuel-distribution meter to determine the proper amount of fuel. Very minimal if any electronics were involved in these systems.
Early diesel fuel-injection systems were purely mechanical as well, but the difference was the required fuel pressure. It doesn’t require much pressure to inject fuel into an intake manifold, but it requires extremely high pressure to inject fuel directly into a cylinder (such as is necessary for a diesel). Diesel-injection pumps housed a mechanical high-pressure pump to feed the fuel to the injectors.
One of the most common gasoline fuel-injection systems to become popular beginning in the late 1970s was the Bosch Continuous Injection System (CIS). This, too, was overall a mechanical system, but an electric pump supplied the fuel, and minor electronics played a part in cold-start functions as well as fuel-mixture control.
Electronic Fuel Injection
Electronic fuel injection was a terminology that became well-known in the 1980s and was often indicated by the letters “EFI” on the back of a car. It seemed revolutionary at the time, and it indicated that the systems were now completely electronically controlled. It was this point in time when fuel pumps found their way into the gas tank; injectors were basically solenoids that opened the injector upon command from a computer; and the computer – along with a myriad of sensors – controlled everything surrounding the operation of the system.
Even though EFI was an early term that would now be as redundant as saying you have antilock brakes on a new car, it’s technically still an accurate term. It’s just not used often because it’s assumed – and correctly – that everything on a new car is tied to electronics. EFI is a term that can include many different types of fuel injection.
Throttle-Body Injection
Throttle-body injection (TBI) refers to the fuel injector(s) being located in a throttle body that looks almost like a carburetor at a glance. This was done by design, as it was the most efficient and quickest way for auto manufacturers to make the change to fuel injection, while utilizing many of the same components they already had such as the same intake manifolds and air cleaners. TBI was most common in the 1980s and early 1990s.
We’ve always loved fancy names. Have you ever heard of cross-fire injection? It was two throttle bodies at opposite corners of the intake manifold.
Port Fuel Injection
TBI was at a disadvantage because airflow was interrupted by the injector, and port injection was the next advancement in line. Port, or multi-point injection injects fuel into the intake runner just before the intake valve for each cylinder. The advantage is the ability to precisely control the fuel delivery and balance the airflow into each cylinder, leading to increased power output and improved fuel economy.
Early mechanical fuel-injection systems were port-injection systems, sans electronic control. Seem confusing? Many fuel-injection terms cross over from new to old technology. There are just so many manufacturer-specific names that it can be confusing! Like EFI, port injection was widely advertised as the latest greatest advancement, with tuned port injection topping the performance charts. Port injection still is the most common type of fuel injection used today, but when was the last time you saw it called out? Nobody really says it anymore because it’s not new. But there’s another technology that we’re not done talking about, and that’s direct injection.
Direct Injection
Direct means the fuel is injected directly into the combustion chamber. Direct injection has been around for years in the diesel world, but it’s still relatively new for gasoline engines. The challenge with this type of injection is injecting the fuel into the high compression of the combustion chamber. Just like a diesel, it requires extremely high fuel pressure, and gasoline direct injection utilizes a typical electric pump to supply fuel to the rail, plus a mechanically driven high-pressure fuel pump to supply the necessary pressure for injection.
The primary advantage of direct injection is that there’s less time for the air/fuel mixture to heat up since the fuel isn’t injected in the cylinder until immediately before combustion. This reduces the chance of detonation, or the fuel igniting from the heat and pressure in the cylinder. This allows a direct-injected engine to have higher compression, which itself lends to higher performance.
There are additional advantages of reduced emissions and better fuel economy, but there also are some now-familiar drawbacks, including carbon buildup on the backs of the intake valves, low speed pre-ignition and limited high-rpm performance. For this reason, many manufacturers are combining both direct- and port-injection systems.
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