Easing a stiff Corvette

Jan. 1, 2020
Last July, I received a call from an upset owner of a 2003 Chevrolet Corvette convertible. He explained that he had just purchased the 50th Anniversary Edition 'Vette with only 7,500 miles on it and was not happy with the ride or handling. In his wor

A horse and buggy rides smoother than this car.

Motor Age Garage 2003 Corvette rough ride suspension fixing vehicle repair shop training technician training automotive aftermarket

Last July, I received a call from an upset owner of a 2003 Chevrolet Corvette convertible. He explained that he had just purchased the 50th Anniversary Edition ’Vette with only 7,500 miles on it and was not happy with the ride or handling. In his words, “It rides like a log wagon and handles like one.” He explained the ’Vette would skip to one side or the other at highway speed when hitting an expansion joint. Hitting a small bump at slow speed would cause a hard thump, so he thought the shocks were bad.

He said it had the F55 suspension option, referring to a Regular Production Option (RPO) code, so it should ride smooth and handle extremely well. His concern was that if these dampeners (shocks) were bad, replacing them would run more than $1,200 a piece for the front and $875 each for the rear. He was not fond of experimenting with replacements just to see if the ride improved. He asked about buying simulators to fool the computer and installing regular shocks to correct the issue.

I became interested because this RPO code designated GM's Magnetic Selective Ride System, which meant it has suspension dampeners (shocks) filled with magneto-rheological fluid. This special fluid predictably reacts to a magnetic field by becoming thicker as the field strength increases. A dedicated computer module controls the strength of the magnetic field by managing the current flow through coil windings incorporated in the shock using a variable duty cycle. This varies the fluid's viscosity for instantaneous control of wheel to body movement by allowing for soft or hard compression/extension of the shocks.

The suspension module receives information at high speed over the vehicle's serial data communication network from the PCM for driver power demands and the ABS module for wheel speed, steering wheel position and vehicle motion from the yaw and lateral accelerometers. When working correctly, this sophisticated suspension control system can allow the vehicle to ride smooth, but respond to any vehicle movement for handling characteristics far beyond the capabilities a regular shock suspension can deliver.

Preliminary Inspection

After driving just a few blocks and hitting small bumps, the vehicle felt as though it was hitting the suspension's rubber stops. The problem with that idea is these vehicles do not have conventional suspension travel stops. They are incorporated into the top of the suspension dampeners. Hitting small potholes or dips resulted in the suspension abruptly becoming solid, giving the 'Vette poor ride and handling. Selecting either Touring or Sport mode through the driver operated traction/suspension control switch on the console, did not affect the ride.

Once out of the vehicle, I performed a bounce test. The shocks seemed weak, but that is normal, because the system is inactive when parked. I raised 'Vette for a visual inspection, looking for anything the suspension might be hitting and for anything loose, missing or broken. After the car was back on a level floor, I measured the suspension trim height at each corner.

To perform this measurement on the front, you bounce the vehicle and then lift the vehicle by hand up about an inch and a half, then gently let go. This settles the suspension for proper measurements. GM recommends you do this three times before measuring from the bottom of the ball joint to the center of the front side's pivot center point of the lower control arm mount. Record that measurement and then do the three bounce procedure again, but push down an inch and a half this time before releasing. Record that measurement and then average them to find the "Z" trim height. Specification is 1.73 inches and this 'Vette measured 1.7 inches using my crudely built tool consisting of a level and dial caliper. The procedure is the same for the rear suspension to find "D" trim height with a specification of 4.25 inches.

After nothing obvious was found wrong, it was time for some preliminary research of service information and system operation. Not finding a TSB relating to these particular symptoms, it was time to hook up a Tech 2 scan tool. Located in the ABS data section are options to view Variable Effort Steering (VES), Traction Control System (TCS) and Vehicle Stability Enhancement System (VSES) data. A sub-section in the VSES is Real Time Dampening (RTD). This is the ESC's data, which looked normal according to the service information. Plus no DTCs, current or history, were found for any system.

System Testing

The first test performed was bouncing the Corvette and using the scan tool's special ESC function for duty cycling of the coils. This test shows if the ESC can operate the coils and if the dampeners will firm up. In this mode, the ESC varies the amperage to each coil so you can detect the shocks ability to change responsiveness to joust and rebound. Each shock's integral electric coils produce a magnetic field instantaneously and linearly changing the viscosity of the magneto-rheological fluid. The ESC module's ability to control this fluid is the key to the MSRS ride and handling characteristics. As each corner was bounced, the dampeners stiffened up allowing no movement, even with two of us pushing down. Figure 2 shows the left rear shock solenoid's one ohm coil being 95 percent duty cycled.
Next, the suspension position sensors were tested. Figure 3 shows the sensors' normal average of about 2.6 volts with the vehicle at rest, so a test drive was now in order.

The Tech 2's Live Plot mode was used to record the left and right front suspension sensors as the vehicle was driven over a series of small bumps at about 30 mph. In Figure 4, the dashed line represents the left suspension sensor voltage, while the thick line represents the right sensor. Both show normal fluctuating voltages as the suspension moved. The suspension sensor voltages typically measure between .4 with suspension travel extended and 4.7 collapsed.

Then a Live Plot was recorded of the left front suspension position sensor and the shock solenoid's duty cycle percentage with normal reactions upon playback. Nothing in the ESC's input or output looked suspicious.

The scan tool was changed to view VSES data looking at the Yaw and Lateral accelerometers. The Yaw sensor provides vehicle body turning motion and the Lateral sensor provides side motion of the body to the ESC. Using both of these sensor inputs, the ESC can determine any vehicle body movement in any direction. The ESC monitors the suspension sensors, which are mounted on the frame and attached to the suspension control arms, for body to wheel position.

The steering wheel position input tells the ESC the intended direction of the vehicle and instantaneously any movement of the steering wheel. The dual steering wheel position sensors are combined to calculate steering wheel angle in degrees as seen in Figure 5 . Utilizing all these sensor inputs, the ESC can detect any vehicle body motion in any direction along with vehicle inertia to determine the proper compression or rebound rate of the shocks to deliver the best ride and handling.

Driving the vehicle revealed normal fluctuating accelerometer voltages above or below the stationary 2.5 volt measurements with typical reactions from the ESC. The Yaw sensor voltage remains steady while the Lateral is more active when encountering bumps. GM did issue a bulletin (PIC3844) in 2006 about unwanted Active Handling activation caused by a loose or missing Yaw sensor bracket.

This can allow the sensor to vibrate excessively when hitting a bump. The ESC might interpret the rapidly fluctuating voltage as excessive vehicle movement causing sudden stiffening of the shocks making for a rough ride. This easily can be tested using a digital storage oscilloscope (DSO). As observed in Figure 6, the Lateral sensor voltage changes little while the Yaw sensor remains steady. If either accelerometer was mounted loosely, the scope would show rapid voltage changes.

The four channel DSO was used to directly measure voltage at the suspension position sensors and amperage applied to the dampeners' coils. Utilizing the four channels allows for voltage and amperage monitoring and comparison between side-to-side or front to rear. Resistive or intermittent connection in the sensor or coil circuits can be easily detected with the scope connected at the ESC connector.

Remember that amperage is proportional to resistance so an intermittent open or poor connection would show no or decreased amperage flow where duty cycling voltage still would be displayed. The 2003 Corvette's ESC module is conveniently mounted in a carpet covered compartment in the left rear of the luggage area. The scope was connected easily using an inductive amp clamp around the output wire to the left front shock coil and back probe connections for monitoring the suspension position sensors.

The ABS module (GM's Electronic Brake Control Module or EBCM) is located low in the left front of the engine compartment between the engine and radiator. This makes measuring Yaw, Lateral, Steering Wheel position and Wheel Speed sensors more challenging.

The Fix

The diagnostic information was there all along, but overlooked. The scan tool snapshot plot illustrates the problem the best. The maximum suspension position sensor voltage of 2.59 was seen in Figure 4. This means the suspension was not allowed to fully compress. Looking at all the plots of hitting different bumps, none of the left front sensor voltage readings went above the 2.59 reading while the right front never went above 2.88 volts, no matter how sharp the bump. Something was stopping the control arms from moving upward. Remember when I said the suspension stops are designed between the top of the shock and its upper mount?

Repair Procedures

The "fix" required jacking the 'Vette up with the suspension hanging down to its fullest extension. This allows the shock body to extend down to reveal a yellow nylon strap peeping out from the shocks boot. The strap looks like the ones used to keep gas charged shocks collapsed for shipping, but it serves a different purpose on these shocks.

Pushing up on the dust boot until the top of the shock body could be seen brought to light the Corvette's problem. As seen in Figure 8, GM installed a temporary plastic stuffer between the top of the shock body and the top shock mount. This was there for shipping purposes only. Someone at the dealership had failed seven years ago to follow the New Vehicle Preparation bulletin No. 02-03-11-002A for RPO code F55 optioned vehicles. All four plastic stuffers were still installed. After removing the stuffers, the Corvette rode and handled extremely well.

The other diagnostic problem overlooked was if the ESC detects a fault it will display one of three messages in the instrument panel. Depending on the effects to handling "SHOCKS INOPERATIVE," "SERVICE RIDE CONTROL" or "MAXIMUM SPEED" may be displayed with the latter causing the PCM to limit maximum speed to 80 mph. This 'Vette had no DTCs or messages displayed.

Tim Janello is an assistant professor for Southern Illinois University in Automotive Technology teaching baccalaureate students for the past seven years. He has 40 years of experience in the automotive repair field with Master ASE certification and L1.

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