Your customer just wants a smooth ride and that’s not going well for them right now. You see wires to the shocks and struts and wonder if the problem is electrical or mechanical. You might even wonder how it even works for that matter. Then there’s the service manual acronym alphabet soup. There is CVRSS (GM’s Continuously Variable Road Sensing Suspension), KDSS (Toyota/Lexus Kinetic Dynamic Suspension System), BMW EDC (Electronic Damper Control), VW DCC (Dynamic Chassis Control) and on and on. How do they work? Are there really that many systems to master? What goes wrong when they fail?
1. Manually Selectable Suspension
2. Semi Adaptive Suspension
3. Fully Adaptive Suspension
Suspension Review
Before getting too deep, let’s review suspensions in general. Suspension system springs must compress and rebound with bumps and holes in the road surface. The softness or firmness of a spring is known as spring rate. Shock absorbers are used to return the suspension to its natural position quickly and smoothly. Not only do shock absorbers control the compression of the spring, but the rebound as well.
Shock absorbers dampen the movement of springs by using fluid or gas forced through holes in the shock absorber’s piston. The size of the holes determines the damping effect of the shock. As the shock absorber compresses, a piston inside moves through oil or hydraulic fluid. Because energy never goes away, it just only changes states, the mechanical energy of the spring oscillations is turned into heat energy via the action just described within the shock absorber to give you that 1 ½ compression/rebound cycles you’re familiar with when you move each corner of the vehicle up and down and then step back to watch the shock’s dampening performance.
Springs and shocks are matched to vehicle weight. Sometimes they are chosen to correspond with the weight of optional equipment added to individual vehicles as they are built. This is referred to as a tuned suspension. Because the lines of differentiation have blurred in the last few years between shocks and struts, for the purpose of this article from here on out we’ll just refer to their generic term of damper.
Some of today’s suspension systems include electronically controlled dampers that can produce a soft, medium or firm ride. The ride and handling of the car can be changed for different road conditions or to suit individual driver preferences.
Semi-Adaptive Suspension
The major components of an electronically controlled semi-adaptive suspension system include:
• Body position sensors
• Steering position sensors
• Body acceleration sensors
• Suspension component acceleration sensors (active suspensions)
• Vehicle speed sensors
• Controlled dampers/suspension actuators
• Electronic control module
Systems as early as the 1990s included inputs from the PCM to tip off the electronic suspension control module was going to need to apply a greater duty cycle to the dampers in order to reduce lift from acceleration. Your customer’s foot on the accelerator (cable operated or throttle by wire) results in a TPS rate of change long before the engine speeds up and applies torque to the wheels via the transmission. I say “long before” in terms of computer speed.
Going to the faster CAN data buses a few years ago helped to accelerate the sharing of information between systems all the more. The same is true with applying the brake pedal, especially if the vehicle has an advanced braking system that looks at the rate of brake pedal apply as opposed to a simple discreet brake switch input. This input to the suspension module helps it to prevent dive from braking.
For stability control and/or vehicles equipped with variable effort steering, the steering angle sensor is also given double duty as an input to the suspension module. As the driver gives a more sudden input to the steering wheel, the vehicle will tend to roll as it begins to turn. This can be compensated for with variable damping. Lateral and yaw sensors almost round out the inputs to the suspension module on vehicle equipped with stability control systems. I say almost round out because the most important sensors inputting to the suspension module would be the body position sensors
Road Texture Detection – Direct and Indirect Body Position Sensors
Any electronically controlled suspension system must be able to detect the road surface in order to determine if the damper calibration needs to change in order to maintain a smooth and controlled ride. This is accomplished through one of two methods: Direct and Non-Direct Sensing.
Some TPMS systems as well as some ride control systems rely on the ABS wheel speed sensor to determine tire circumference via wheel speed variations between wheels. For example, when three out of four wheel speed sensors are reading 30 mph and the fourth wheel speed sensor is reading 30.5 mph, the natural assumption is that the fourth tire is either the wrong size (mismatched tires) or low on tire pressure. A tire that is low on air will be a tire that is lower in circumference therefor spinning slightly faster than the others.
If this goes on for a sustained period of time, a passive TPMS system can light a low pressure tire telltale on the IPC. This was a fairly common method of low tire pressure detection until the TPMS mandate of 2007, which required pressure sensors mounted in each wheel. If that fourth wheel speed sensor, however, had trends of slight sudden acceleration and deceleration that trend would indicate the tire dropping into a hole (expanding/changing speed) followed by the return of all the weight onto that tire as it hit the bottom of a hole (compressing/changing speed), the ABS module would conclude that a rough road is detected.
WSS variations that point to rough road surfaces are also now calculated by more sophisticated software in the ABS module to prevent ABS activation that may occur on rough roads. Active (Hall Style) WSS in recent years have helped this this technology tremendously. This rough road virtual sensor in the form of WSS variations also can be used in the non-direct versions of electronic ride control systems. GM’s Body Damping Control (BDC) used in midsized vehicles a few years ago was one such non-direct sensing electronic ride control system. You drove the vehicle on rough roads, the ABS module detected irregular WSS variations and passed that data on to the suspension electronic control module, which then in turn electronically changed the damper stiffness at each wheel to compensate for the rougher road surface. This compression/expansion of the tire on a rough road and the related exchange of information takes time — about 30 mS to be precise.
For this reason, most manufacturers have opted to spend a little more money and drop a suspension to frame sensor at each of the vehicle’s four corners. These sensors, sometimes called Body Position Sensors, acting like potentiometer equipped throttle position sensors. They typically utilize a rod on a pivot between the sensor arm and a suspension control arm or other moving component of the suspension. As the vehicle runs over smooth surfaces or bumpy surfaces the signature analog signal from each sensor is monitored by the electronic ride control module.
MagneRide
MagneRide has been around for since the late 1990s. Used in non-automotive vibration reducing applications such as washing machines, this innovative liquid that changes it nature at the flip of a switch has even seen promise in the field of personal body armor that can flex with the wearer under normal conditions then become as stiff as a metal plate when a fire fight ensues. At the root of the technology is a Magneto-rheological fluid containing oil and somewhere between 20 and 40 percent tiny iron particles. These iron particles are between three to 10 microns which is very small. The thickness of a human hair is 10 microns. When the fluid is not exposed to a magnetic field the particles are in disarray. Used in a suspension damper it would provide a very good resistance to coil oscillations for rough road conditions. Exposing the fluid chamber to a magnetic field (via an electrically charged coil) causes the iron particles to line up like soldiers in formation changing the shear stress of the liquid.
Simply put, this alignment reduces the resistance of the fluid as it is pushed by the piston in the damper’s cylinder. Creating a variable duty cycle give you a range of damper stiffness with less than 10 mS for the transitions. Because there is no movement of a valve into the solenoid as is the case with non-MR electronically controlled dampers, this makes for a very quiet and controlled ride. GM was the first to use the technology developed in house by their chassis division that later became part of Delphi corporation. The first application was seen on the 1997 Cadillac’s as standard equipment. It was an option on the Corvette that year and MR ride control systems are still in use by some new GM applications as well as other OEMs. Porsche and others use the technology in their engine mounts as well.
Fully Active Suspension
Care must be taken to fully understand the difference with these systems. Any system that is not fully active reacts to road forces by responding with resistance. Fully active suspension systems have been attempted in production vehicles for many years by numerous manufacturers. The volume has been historically low and the lifetime of the systems short due to system costs and problems.
Compared to conventional electronically controlled manual or semi-active systems, fully active suspension has a much greater degree of complexity. Take Mercedes ABC – Active Body Control system as an example. In addition to body position movement inputs from the stability control system yaw/lateral sensors and lift/dive inputs from the PCM/ABS module, the Mercedes ABC incorporates a complex active cylinder inside the strut assembly. A sensor in each cylinder monitors its exact position. A high pressure hydraulic pump allows for proactive (rather than reactive) fluid pressure/discharge to take place to counter the forces on the spring/damper from the road. The ride is more luxurious and sporty at the same time. With a high pressure system and not one but two electronic modules working the system, complexity extends to the service bay in the highest degree.
Service Issues
A damper is a damper, as mentioned before whether in the form of a shock or strut and whether coupled with the spring or outside the spring. Therefore the same woes of age and abuse apply. Visual inspection still is in order after a road test/corner bounce test. If there are any signs of fluid leaks around the damper, physical damage to the damper body, worn or broken mounts/bushings, cupped tire wear, or wiring harness/connector damage you are ready to sell some dampers.
On the road test observe for any unusual shaking, poor steering response, stiffness or noises when steering, excess dive from braking, swaying or leaning into curves and of course the classic overly bouncing nature resembling the clowns’ car in the circus. Electronically controlled dampers might exhibit excessive noise while adjusting damping rates on the fly. This often is the sign of the electronic damper needing replaced.
Damper solenoids can go open or short out like any other solenoid so an ohms check and/or current draw check would be appropriate along with a voltage drop test of the harness. However, you should (as always) first search for TSBs related to your suspension symptom. Considering the price of an electronic damper compared to a manual one, it would be well worth your effort. There might even be a software update for the suspension control module.
If the vehicle is a General Motors Product, always go the extra step (with any module) to Google “GM CAL ID” and enter the VIN number into the appropriate field in that website’s first page. As your progress through the pages of this website (no subscription required), you will find an electronic suspension module if the vehicle is equipped with such a system. As you select that module from the list of programmable modules, you’ll see if there is a calibration update. The reason for the update almost always is listed. If the listed reason matches your vehicle’s ride symptoms, you then either program that module with a factory scan tool or universal J2534 or sublet that job out to someone who is already into the module programming game.
Recently I had a dealer friend with a customer complaint of a high pitch noise while driving on rough roads. The tech also noticed the lift from acceleration seemed a bit much for this car. (2011 Cadillac STS) The search for TSBs didn’t turn up anything exciting so he entered the customer’s VIN into the GM CAL ID website and discovered a calibration update to fix a high pitched noise from the electronic ride control system and “power hop at launch.” Needless to say, the tech fixed the car with a quick re-flash and not a new set of struts!
Body Position Sensors/Misc. Connections
These are the weak links to these systems as most readers already know. They are basically potentiometers (like TPS sensors) in a harsh environment with fragile linkages and mounts. If there is any single most likely cause of a DTC or other ride control complaint on one of these systems it would be these sensors. Also in the harsh environment of the under carriage are the damper connectors which can become disconnected, damage or experience corrosion.
Communications/Other Systems
Outside of the dampers having problems, broken body position sensors/hardware, the occasional module hardware/software issues and the usual “Read the DTC, follow the chart” always look at issues that pertain to the other systems that share info with the suspension control module. Examples being the PCM, ABS, Variable Effort Steering, etc. Not only would the sensors themselves be possible suspects in the ride control diagnostic equation, but the primary modules they would report into would as well. This means taking a serious look at all other non-electronic suspension system DTCs including those U-Codes for communications issues.
Today’s ride control system is heavily reliant on information from other systems. That means before you even set the rack and start zapping that impact wrench on the shock mounts on a vehicle equipped with electronic ride control, connect an OE level or factory scan tool to the DLC and give the total vehicle a thorough DTC check. A little high tech checkup may go a long way towards keeping your suspension diagnostics riding along smoothly!
