Suspension System Diagnostics

Active Suspension Systems: Diagnosing Electronic Shocks And Struts

We've all heard the term "Active Suspension," but a shock or strut doesn't really go anywhere - so what makes it "active?" To meet the criteria for active, a shock or strut must be able to react to three critical streams of information.

We’ve all heard the term “Active Suspension,” but a shock or strut doesn’t really go anywhere – so what makes it “active?” To meet the criteria for active, a shock or strut must be able to react to three critical streams 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. These three inputs can be used to detect a rough road or an emergency situation in which 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 pistons have the same construction as passive units and, 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 the surfaces in the bore wear.

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 pistons in some units, however, may not have any valving.

The size of the orifices are controlled by electromagnetic solenoids that can modify 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 to the problem 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 and 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, for example.

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 tail lights. More sophisticated systems use more than two accelerometers mounted in various locations.

Control Module

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 front-wheel-drive 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.

You May Also Like

Ride Height Sensors

If one of these sensors is replaced, it must be calibrated after it is installed.

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, the linkage that connects the sensor to the suspension arm can also be damaged. The connector can be damaged and cause a short or open and a code will be set. If one of these sensors is replaced, it must be calibrated after it is installed.

GM Rattle or Creak Noise From Front Suspension

The noise may be originating from the front lower control arm front bushing. Here’s how to correct it.

Mercedes Airmatic Suspensions

Diagnostic tips to aid your repair efforts.

The Complete Shock And Strut Swap

One mistake to avoid when replacing struts is reusing the bearing plates or upper mounts.

ADAS Sensor Corrections

When performing a calibration, you are adjusting the connection between the sensor, vehicle and surroundings.

Other Posts

Ford F-250 Shock Replacement (VIDEO)

Follow along the step by step installation process of shocks on a Ford F-250. This video is sponsored by PRT.

Struts and Memory Steer

We can summarize the cause of memory steer in one word: binding.

Air Ride Conversion Kits

Air ride and active shocks and struts will eventually fail – succumbing to either damage to or dry rot of the air spring.

Curing Shock, Strut and Suspension Noise

As vehicles age, eliminating noise is Job 1. But ‘noise’ may mean different things to you and your customers.