Aside from building vehicles with increased gas mileage and lower emissions, car manufacturers are also taking great strides to make them more comfortable, especially in the luxury sector. One example of this is BMW’s Dynamic Drive, an adaptive way to improve straight line driving comfort while maintaining high cornering performance. To do this, BMW Dynamic Drive utilizes unique variable stiffness sway bars. To find out if you have BMW Dynamic Drive, plug the last 7 digits of your VIN into this website. The code you are looking for is S229A.
The use of a standard passive stabilizer bar (often called sway bars or anti-roll bars) keeps the body of the car from tilting too much during cornering, and greatly improves the handling of a car. Connecting both wheels together, as you go around a turn, the bar twists like a spring. This resistance keeps one side of the body from traveling too far in the opposite direction of the other. The downside is it makes the suspension stiffer when driving in a straight line, or if one wheel were to jounce to the side, for example by hitting a bump.
BMW Dynamic Drive introduces a two-piece active sway bar to combat this effect. While driving straight, the system is loose and forgiving. When the car turns, the sway bar tightens in the opposite direction of tilt, keeping the vehicle cornering as flat as possible. Reduced body roll improves driving comfort, safety, stability, tire traction, and tire wear.
How-To Tell If You Have Dynamic Drive
This system can be equipped as an option in 5, 6, and 7 series along with the X5 and X6 starting in 2001. One way to find out is to put your VIN into the website listed above, you should get a code that mentions dynamic drive. Taking a peak under the vehicle to identify the sway bar being used may be the most concise way. There is an obvious difference that stands out, and that is the swaybar itself. You will notice the Dynamic Drive motor housing in the center. Other ways to tell under the hood is to get a peak at the power steering pump, which will have a small black light-bulb like pressure accumulator on it, and the power steering reservoir will be a long, tall cylinder instead of a standard reservoir to accommodate the extra fluid.
This system system is fairly complex, but hopefully in a minute you can walk away with a good understanding of the function and purpose of Dynamic Drive, and how to diagnose potential faults.
There are a few critical components that allow this concept to work. Here is a breakdown of the individual components.
Lateral Acceleration Sensor: This sensor supplies all the important information about what the car is doing to the control unit. Think of it as the sensor in your smart phone that detects shake, tilt, and other movement. When the car tilts when going around a corner, the lateral acceleration sensor is what detects it.
Dynamic Drive Control Unit: The electronic computer that converts the input supplied by the lateral acceleration sensor, DCS module, steering angle sensor, and others into workable actions for the sway bars. It is able to control the function of the sway bars by means of an electro-hydraulic valve block.
Hydraulic pump: The hydraulic pump serves a dual purpose. It uses hydraulic pressure hoses to power both the power steering as well as the dynamic drive valve body. By keeping constant high pressure on the valve body, the system can have a quick response time when diverting fluid to and from the sway bars. The pump is also equipped with a small accumulator bulb in order to supply pressure immediately to the system.
Dynamic Drive Valve Block: This is the heart and soul of the system. Working in a similar way to an automatic transmission valve body, the valve block uses solenoids and valves to control fluid flow to the sway bars, resulting in stiffer or softer sway bar operation.
Fluid Reservoir and Level Sensor: These are pretty basic, supplying an ample amount of fluid for the system to work with, and a way for the computer to alert you if the level drops too low. This is also your power steering reservoir. Use only Pentosin CHF-11s in these systems.
Hydraulic Valves: There are a number of hydraulic valves that aren’t located on the valve block. These include pressure control valves on the front and rear sub-frames to adjust actuation pressure, a directional valve that specifies the direction of the high pressure fluid, a vent valve on the front sway bar to allow air into the pump to prevent cavitation on really rough surfaces, and the fail-safe valve, which shuts off the sway bar adjustment mechanism effectively converting it to a standard sway bar. Keep in mind that if the system runs dry, the sway bar will offer zero stabilization, as the hydraulic fluid itself is a stressed member.
Active Sway Bars: This is what all the components above are working to control. One half of the BMW Dynamic Drive sway bar connects to a hydraulic oscillating motor, and the other half connects to the motor housing. The motor (which is more like a hydraulic ram that twists instead of expanding) is actuated by the valve block and pressure valves, and pressurized off the power steering pump. Because the two halves of the bar turn in opposite directions, the housing is forced upwards on the outside of a curve and downward on the inside of a curve. This set up improves driving comfort by having the sway bar disconnected and not providing any resistance when it is not needed, and providing very strong anti-roll dynamics when cornering. Additionally, the system can tighten or soften the front and rear independently, resulting in the computer being able to make constant adjustment to avoid under/oversteer, maintaining a flat, neutral handling vehicle.
Now when going through a corner the system uses the steering angle sensor, lateral-acceleration sensor, and the Dynamic Stability Control Module to determine how much pressure to apply to the sway bar, Voila! BMW Dynamic Drive.
Servicing and Diagnosing BMW Dynamic Drive problems
Nearly all problems with the system will trigger a light, but some more serious issues will activate ‘failsafe mode’. When in failsafe mode, check valves will close the hydraulic inputs and outputs from the hydraulic motors, effectively converting the sway bar into a solid passive unit. The sway bars don’t tend to suffer many issues, but can be in need of replacement if any of the internal seals are defective. In one notible case, one of our customers noted his BMW Dynamic Drive sway bar actually tore itself off of the chassis.
A faulty valve body aka valve block will sometimes make popping noises as the valves fail to successfully divert high pressure fluid. This is the most common failure point with the system other than leaks. Another component that will cause a failure is the lateral acceleration sensor. With no input for the computer to work with, the sway bars will go into fail-safe mode and the sway bars will become passive.
The front sway bar includes a venting system that travels through a series of hoses and check valves, and ends up in the fender. When driving on very rough surfaces, the hydraulic motors can cavitate, causing a rattling noise. The vent on the front bar allows air in and silences this cavitation. Engineers deduced that this cavitation cannot be audibly heard on the rear sway bar, so that assembly lacks this venting system.
While not technically part of the Dynamic Drive system, a key input the control module uses is the steering angle sensor. This helps the vehicle to determine the angle of the turn to provide the necessary stabilization required. A faulty steering angle sensor will trip a code and can effect the performance of the stability system.
As with any hydraulic system, BMW Dynamic Drive can be prone to leaking. It’s always best to fix a leak at the first sign since it’s only going to get worse, and always make sure the fluid is topped up. In the event that all fluid is lost, a warning indicator will appear on the dash and vehicle stability will be nearly nonexistent, effectively failing the fail safe mode. We assume this was a problem with German translation and not the function of the system.
Control module programming and hydraulic bleeding needs to be done by a technician with a factory scan tool, generally at the dealership.
Servicing can be costly, but eEuroparts hopes to ease some of that pain by offering genuine parts at a greatly discounted price over the dealer. These parts are available used, but because of the cost and labor of installation, installing used parts that are occasionally prone to failure may end up costing you more in the long run. Contact our customer service department today for any of your BMW part needs.
PS this is a cool video where a service tech, plugged into the vehicle computer, is able to control the system independently of sensor readings to run a diagnostic program.