FX-3 Selective Ride System

Beginning with the 1989 model year, the FX-3 Selective Ride Control System (SRS) Regular Production Option (RPO) was available. It has three driver selectable suspension setups and is a very useful accessory, particularly if you race your car or participate in rally events. 

RPO FX-3 was optional on regular C4 models and standard on the ZR-1. It was continuously available until the 1996 model when it was replaced with a more capable system.

Theory of Operation 

The Selective Ride Control system is controlled by it's own computer. As delivered, it used special Bilstein shock absorbers. The 4 shocks are only part of the system however. There are dampening adjustment motors and position feedback sensors at each shock that monitor the rotary position of an actuator shaft and if their outputs are not what the computer expects to see, you get a SRS trouble light.

The computer also looks at the vehicle speed sensor plus checks itself.  You get the SRS trouble light if there are problems with those systems as well.

The shocks have variable diameter orifices that control how fast the shocks can move up and down. The orifice size is controlled by adjusting a shaft that extends through the top of the shock absorber. The shaft is turned left and right by the actuator mounted on the top of the shock.  There is a dampener adjusting motor on each shock that cranks the shaft left or right plus a sensor that reads the position of the shaft and feeds that information back to the SRS system computer.

Based on the input from the driver's select ride switch, the vehicle speed sensor and the position feedback sensor, the computer sends battery voltage to the dampening adjustment motor to move it to the appropriate position.

Power on Checks and System Monitoring

Immediately after receiving power (when you turn the ignition key) the SRC computer does a power on check and if it finds a problem, it lights the Selective Ride System light on the DIC (Driver's Information Center). If all is OK, it does not light the light and presets the shocks to a 60 degree initial position (the full range is 0 to 160 degrees). If things go badly after the initial power on sequence, the light will come on when the problem occurs and go off when the computer is satisfied that things are OK.  A trouble code is stored for later retrieval if the problem is intermittent.

Suspension Control

As the speed changes, the computer looks at what kind of ride you have selected and cranks the shaft around to what it thinks is the proper match of shock travel and dampening versus speed based on the selection you made with the three position selective ride switch located within easy reach of the driver's position. 

The system is designed such that the dampening rate increases with vehicle speed and decreases as you slow down.  In effect, it monitors the degree of stiffness requested by the driver and the speed of the vehicle. It then sends either + or - battery voltage to the dampening motors on the shocks and monitors how the shocks are responding to the commands using the position sensor information.  

The result is a constantly changing suspension setup, tuned to the conditions present at the time, instead of a compromise solution tuned to one standard set of conditions.  In other words: an active suspension.

Troubleshooting the FX-3 Suspension

If you have a service manual, there is a section that explains how to retrieve any stored trouble codes from the SRS computer which is the first step in repairing the system plus there are troubleshooting charts that walk you through a logical repair procedure based on conditions observed or measured. 

On the the early model, all digital dash C4s (which were made through 1989), plus any other mid model year C4s that had a 12 pin ALDL connector, you ground pins A & B on the ALDL connector under the dash just to the right of the steering wheel and count the flashes on the SRS light to see if any trouble codes have been stored. On all C4's with a 16 pin ALDL connector (except the 1996 model), you ground pin G and the trouble codes for the SRS computer will be displayed on the LCD screen that doubles as your trip odometer and speedometer. The appropriate service manual describes exactly how to extract the codes for either system as well as explaining what the codes mean.

Without the computer codes, you are shooting in the dark if you try to troubleshoot a FX-3 problem but a couple possibilities are bad grounds or battery wires going to the dampening adjustment motors or bad ground/signal leads coming from the position sensors to the computer. So, first you should check all connectors associated with each shock and make sure they are not corroded or dirty. Wiggle each of them in turn and see if the problem goes away.  If it does, you have a bad connection to deal with.

Another possibility is the driver transistors in the SRS computer itself.  If the computer commands the dampening motor to move but the driver transistors are bad or leaky, nothing will happen (or not enough will happen), the expected results will not be fed back by the position sensors, and the SRS trouble light will come on.   Changing the actuators or shocks will not correct this sort of failure, you must repair or replace the computer itself.

Lastly, if you install a High Energy Ignition and coincidentally begin to have problems with your Selective Ride System, the wiring from the HEI is cross talking into the SRS wiring.  Reroute the wires from the HEI so that it does not run parallel to any OEM wiring and try relocating the HEI control box to a different location to minimize the problem.

One final thought: like most electrical problems with a C4, check the connectors first and make sure they are free of corrosion and making good contact before you go on to more exotic solutions.  In the absence problems caused by a HEI system, poor connections and/or grounds are where most of the SRS problems originate.