Suspension Control ( Automatic - Electronic ): Description and Operation

GENERAL DESCRIPTION
The Speed Dependent Damping (SDD) system controls the firmness of the car's ride by automatically controlling an actuator in each of the four struts. The system has three damping modes - Comfort, Normal and Firm. Damping mode selection is controlled by the SDD electronic control module according to vehicle speed conditions, lift / dive input, and any error conditions which may exist. When two demanded positions conflict, the system goes to the firmest damping position.

SYSTEM COMPONENTS

CONTROL MODULE






The SDD control module is a computer used exclusively to control and monitor the SDD system. It is located on a bracket behind the left quarter trim panel behind the driver's seat. The control module receives speed and lift / dive input and determines the optimum mode strut valving for current operating conditions. The control module also receives feedback from the struts to determine proper system operation. If the control module receives an incorrect feedback signal an error code will be set in SDD memory and a "SERVICE SDD SYSTEM" message will be displayed on the Driver Information Center. For vehicles with 1992 control modules, two successive ignition cycles are required to display the "SERVICE SDD SYSTEM" message. For diagnostic purposes, grounding the Diagnostic Enable circuit will cause a Light Emitting Diode (L.E.D.) on the control module to flash error codes.

STRUT






The strut contains an electrical actuator which rotates a selector valve to a specific location commanded by the control module. The selector valve contains different sized orifices which will increase or decrease the damping rate of the strut to control ride firmness. Actuators are an integral part of the strut and are not serviceable separately from the strut.

SYSTEM INPUTS AND OUTPUTS






SPEED INPUT
Ride firmness is selected according to the following speed conditions:
Speed increasing and below 5 mph (Stationary) - Firm mode.
Speed increasing and between 5 and 40 mph - Comfort mode.
Speed increasing and between 40 and 60 mph - Normal mode.
Speed increasing and above 60 mph - Firm mode.
Speed decreasing and above 55 mph - Firm mode.
Speed decreasing and between 55 and 35 mph - Normal mode.
Speed decreasing and between 35 and 2 mph - Comfort mode.
Speed decreasing and below 2 mph - Firm mode.

LIFT / DIVE INPUT
Lift / Dive input is received from the Powertrain Control Module (PCM). The system goes to the Firm mode during wide open throttle to provide anti-lift. The system will also go to the Firm mode when a high deceleration rate is seen to provide anti-dive.

LATERAL ACCELERATION SWITCH INPUT
A lateral acceleration switch is not used on the Allante'. A shorting wire is in the lateral acceleration switch connector to prevent a code from setting. If there is an open in this circuit a code 32 will set.

FEEDBACK VOLTAGE
Each actuator provides a feedback voltage signifying actuator position relative to a "Home" position. A single 3.0 volt pad signifies the home position which correlates to the first Comfort damping position of the actuator, while subsequent 0 - 1.0 volt pads signify the remaining damping positions. Each damping position is separated by a 5.0 volt pad.

ERROR LAMP






When the ignition switch is turned on the system performs a self test which lasts for 7.5 seconds. The error lamp will be on for this period. After the self test, the control module may be in one of the following modes:
Normal operating mode - the error lamp is on below 10 mph and goes off above 10 mph to indicate proper operation of the SDD system and to verify that the control module is receiving speed information.
Error mode - the error lamp is off below 10 mph and turns on above 10 mph.
Reset Mode - the error lamp is turned off to indicate a SDD control module problem.
Any system error will cause a "SERVICE SDD SYSTEM" message to be displayed on the Driver Information Center. For vehicles with 1992 control modules, the error must be active for two successive ignition cycles in order to display a "SERVICE SDD SYSTEM" message. The message will be displayed as long as the error code is active. If the fault corrects itself, the "SERVICE SDD SYSTEM" message will turn off, but the error code will be retained in the control module memory.
Error codes are stored in the control module in non-volatile memory. In diagnostics, the error codes are read via the L.E.D. by grounding the Diagnostic Enable circuit and counting the L.E.D. flashes.
If a code is set, a one second self diagnostic test is performed at three minute intervals. The error lamp will be on during this test and will go out if the fault is corrected. If the fault still exists, the lamp will be off below 10 mph and on above 10 mph.