Steering Mounted Controls Communication Module: Description and Operation
MODULE-STEERING CONTROLDESCRIPTION
This vehicle is equipped with a Steering Control Module (SCM) (4) that houses an integral, internal clock-spring. On some vehicles, the SCM also includes an integral, internal Steering Angle Sensor (SAS). The SCM is secured near the top of the steering column below the steering wheel and is completely concealed beneath the steering column shrouds. The SCM is installed as a modular unit that supports the left (lighting) multi-function switch (1), the right (wiper) multifunction switch (2), the hazard switch (3) and the turn signal cancel cam. The controls for each of the switches extend through appropriate clearance holes provided in the steering column shrouds.
The microprocessor-based SCM utilizes integrated circuitry and information carried on the Controller Area Network (CAN) data bus along with several hard wired analog and multiplexed inputs to monitor both the right and left multi-function switches, the ignition switch, the horn switch, the speed control switches and the remote radio switches. In response to those inputs, the internal circuitry and programming of the SCM allow it to control and integrate many electronic functions and features of the vehicle through both hard wired outputs and the transmission of electronic message outputs to other electronic modules in the vehicle over the CAN data bus.
The SCM circuitry, the clockspring, and the SAS are all contained within a flat, round molded plastic case. The back (instrument panel side) of the case has an integral mounting bracket that is secured to the stationary steering column housing with two screws. The back of the case also features a total of five integral fixed connector receptacles. The three upper receptacles are direct interface connectors for the hazard and multi-function switches and are concealed when these switches are mounted. The two lower receptacles connect the SCM to the vehicle electrical system through two take outs and connectors of the instrument panel wire harness.
The face of the SCM case consists of the rotating clockspring rotor with two integral connector receptacles and two pigtail wires with connectors located near the top. One receptacle receives the connector from the remote radio switches, and the other receives the connector from the horn switch and the speed control switches. The two pigtail wires contain the multistage driver airbag squib circuits. The turn signal cancel cam (not shown) extends from the back of the SCM case but is secured to the hub of the clockspring rotor and is keyed to the steering shaft so that it rotates the clockspring rotor with the steering wheel rotation.
A service replacement SCM is shipped with the clockspring pre-centered and with a molded plastic locking pin installed. The locking pin secures the centered clockspring rotor to the SCM case during shipment and handling, but must be removed after the SCM is installed on the steering column and before the steering wheel is installed. The service replacement clockspring is also shipped in a standard configuration and must be electronically configured for optional equipment, including automatic headlamps, front fog lamps, rear fog lamps, and SAS before these features will be operational. Using a diagnostic scan tool, follow the programming steps outlined for Configure SCM under Miscellaneous Functions for the SCM/Steering Control Module menu item as appropriate.
The SCM has programmable memory that can be reprogrammed using a diagnostic scan tool and Flash reprogramming procedures. The SCM cannot be adjusted or repaired. If ineffective or damaged the entire SCM including the integral clockspring and, if the vehicle is so equipped, the SAS must be replaced. The left and right multi-function switches, the hazard switch and the turn signal cancel cam can be removed from and are serviced separately from the SCM.
OPERATION
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/or a diagnostic scan tool.
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 a horn switch status message over the CAN data bus. The Forward Control Module (FCM) in the engine compartment controls horn relay operation based upon the horn switch 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 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 a 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 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 a 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 Control Node/CCN) controls all interior lighting operation based upon the various 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 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 a Signal Not Available (SNA) message when a fault is detected. The radio receiver controls its own operation based upon the 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 multifunction switch at all times and monitors the status of the switches on return circuits. The SCM then transmits the appropriate 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 also detect a shorted, open or stuck switch in each of the switch controls by monitoring the return input and will transmit a 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 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 this data to other electronic modules in the vehicle over the CAN data bus. The SCM can detect an ineffective SAS and will transmit a 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. 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. The SCM can be diagnosed, and any stored DTC can be retrieved using a diagnostic scan tool.