Steering Control Module

The Steering Control Module (SCM) contains the clockspring and a microprocessor. On some vehicles, it also contains the Steering Angle Sensor (SAS). The SCM communicates over the Controller Area Network (CAN) data bus with other electronic modules in the vehicle and a diagnostic scan tool connected to the 16-way Data Link Connector (DLC).

The SCM is connected to a fused B(+) circuit and receives a path to ground at all times. These connections allow it to remain functional regardless of the ignition switch position. The driver airbag squib circuits of the clockspring, the speed control switch circuits and the hazard switch circuits pass through the SCM, but the SCM does not monitor, and has no control outputs related to these circuits. Any other input to the SCM that would cause a vehicle system to function but does not require that the ignition switch be in the ON position, such as turning ON the lights or sounding the horn, prompts the SCM to wake up and transmit on the CAN data bus.

The following paragraphs briefly describe the SCM responses to the various hard wired inputs it receives.

• Horn Switch  - The horn switch is an input to the SCM. The SCM provides a reference input to the horn switch at all times and monitors the horn switch status on a second circuit. When the horn switch is closed, the SCM transmits an electronic horn switch status  message over the CAN data bus. The Front Control Module (FCM) in the engine compartment controls horn relay operation based upon this status message from the SCM.
• Ignition Switch  - The ignition switch is an input to the SCM. The SCM provides a reference input to a multiplexed circuit within the ignition switch and monitors the switch status on a second circuit to determine the switch position. The SCM then transmits the appropriate electronic ignition switch status  messages to other electronic modules over the CAN data bus. The SCM can also detect a short or open in the switch by monitoring the return input and will transmit an electronic Signal Not Available (SNA)  message when a fault is detected.
• Left Multi-Function Switch  - The left (lighting) multi-function switch provides several inputs to the SCM. The SCM provides reference inputs to the turn signal, beam select, exterior lighting and interior lighting controls of the left multi-function switch at all times and monitors the status of the switches on return circuits. The SCM then transmits the appropriate electronic switch status  messages to other electronic modules over the CAN data bus. The SCM can also detect a shorted, open or stuck switch in each of the switch controls (except front fog lamps) by monitoring the return input and will transmit an electronic SNA  message when a fault is detected. In general, the FCM in the engine compartment controls all exterior lighting operation and the Electromechanical Instrument Cluster (EMIC) (also referred to as Cab Compartment Node/CCN) controls all interior lighting operation based upon the various electronic switch status  messages from the SCM.
• Remote Radio Switches  - The steering wheel mounted remote radio controls provide inputs to the SCM. The SCM provides a reference input to a multiplexed circuits within the switches and monitors the switch status on a second circuit to determine both switch positions. The SCM then transmits the appropriate electronic remote radio switch status  messages to other electronic modules over the CAN data bus. The SCM can also detect an open or stuck switch by monitoring the return input and will transmit an electronic SNA  message when a fault is detected. The radio receiver controls its own operation based upon the electronic remote radio switch status  messages from the SCM.
• Right Multi-Function Switch  - The right (wiper) multi-function switch provides several inputs to the SCM. The SCM provides reference inputs to the front wiper/washer and rear wiper/washer controls of the right multi-function switch at all times and monitors the status of the switches on return circuits. The SCM then transmits the appropriate electronic switch status  messages to other electronic modules over the CAN data bus. The SCM also provides two hard wired high side driver outputs to the rear wiper motor to control the rear wiper ON and DELAY functions. The SCM can detect a shorted, open or stuck switch in each of the switch controls by monitoring the return input and will transmit an electronic SNA  message when a fault is detected. The FCM in the engine compartment controls wiper on/off, wiper high/low and rear wiper relay operation based upon the electronic switch status  messages from the SCM.
• Steering Angle Sensor  - Vehicles equipped with an optional Electronic Stability Program (ESP) system have a Steering Angle Sensor (SAS) and a second, dedicated microprocessor integral to the SCM. The SAS monitors the steering direction/angle and the rate of steering wheel rotation, then the SCM transmits electronic messages with this data to other electronic modules in the vehicle over the CAN data bus. The SCM can detect an ineffective SAS and will transmit an electronic SNA  message when a fault is detected. When an SCM is installed into a vehicle without properly centering and locking the entire steering system, the SAS data does not agree with the true position of the steering system and causes the ESP system to shut down. This can also damage the clockspring without any immediate malfunction. Unlike some other DaimlerChrysler vehicles, this SAS never requires calibration. Refer to STANDARD PROCEDURE . Determining if the clockspring/steering angle sensor is centered is also possible electrically using the diagnostic scan tool. Steering wheel position is displayed as ANGLE  with a range of up to 900 degrees. Refer to the appropriate menu item on the diagnostic scan tool.

The SCM will store fault information in the form of a Diagnostic Trouble Code (DTC) in SCM memory if a malfunction is detected. Any stored DTC can be retrieved using a diagnostic scan tool.

The hard wired circuits between components related to the SCM may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information. The wiring information includes wiring diagrams, proper wire and connector repair procedures, details of wire harness routing and retention, connector pin-out information and location views for the various wire harness connectors, splices and grounds.

However, conventional diagnostic methods will not prove conclusive in the diagnosis of the SCM or the electronic controls or communication between modules and other devices that provide some features of the SCM. The most reliable, efficient, and accurate means to diagnose the SCM or the electronic controls and communication related to SCM operation requires the use of a diagnostic scan tool. Refer to the appropriate diagnostic information.