Part 1B
AIRBAG CONTROL MODULEThe Airbag Control Module (ACM) is an electronic module that monitors the airbag system for proper operation, stores diagnostic trouble code (DTCs), controls the airbag warning lamp and contains the sensor and actuator that is responsible for airbag deployment. There are no external impact sensors. The ACM is mounted on a special bracket that is fastened to the floor of the truck at the bottom of the instrument panel. It is located forward of the console. The ACM provides diagnostic information (DTCs) to the technician through the DRB III via the PCI bus. Some circuits are tested continuously; others are checked only under certain circumstances. The warning lamp is driven with messages relayed to the Electro/Mechanical Instrument Cluster (EMIC) from the ACM via the PCI bus.
The Airbag Control Module (ACM) is secured with four screws to the top mounting surface of a stamped steel bracket welded onto the top of the floor panel transmission tunnel forward of the instrument panel center support bracket and below the instrument panel center stack in the passenger compartment of the vehicle. Concealed within a hollow in the center of the die cast aluminum ACM housing is the electronic circuitry of the ACM which includes a microprocessor, an electronic impact sensor, an electromechanical safing sensor, and an energy storage capacitor. A stamped metal cover plate is secured to the bottom of the ACM housing with four screws to enclose and protect the internal electronic circuitry and components.
The ACM housing has integral mounting flanges on each side. Two of the mounting flanges, one on each side, have an integral locating pin on their lower surface. An arrow cast into the top of the ACM housing near the rear provides a visual verification of the proper orientation of the unit, and should always be pointed toward the front of the vehicle. A molded plastic electrical connector receptacle containing twenty-three terminal pins exits the rearward facing side of the ACM housing. These terminal pins connect the ACM to the vehicle electrical system through a dedicated take out and connector of the instrument panel wire harness.
The impact sensor and safing sensor internal to the ACM are calibrated for the specific vehicle, and are only serviced as a unit with the ACM. The ACM cannot be repaired or adjusted and, if damaged or faulty, it must be replaced.
The microprocessor in the Airbag Control Module (ACM) contains the front supplemental restraint system logic circuits and controls all of the front supplemental restraint system components. The ACM uses On-Board Diagnostics (OBD) and can communicate with other electronic modules in the vehicle as well as with the DRB III scan tool using the Programmable Communications Interface (PCI) data bus network. This method of communication is used for control of the airbag indicator in the ElectroMechanical Instrument Cluster (EMIC) and for supplemental restraint system diagnosis and testing through the 16-way data link connector located on the driver side lower edge of the instrument panel. (Refer to INSTRUMENT CLUSTER/AIRBAG INDICATOR - OPERATION).
The ACM microprocessor continuously monitors all of the front supplemental restraint system electrical circuits to determine the system readiness. If the ACM detects a monitored system fault, it sets an active and stored Diagnostic Trouble Code (DTC) and sends electronic messages to the EMIC over the PCI data bus to turn ON the airbag indicator. An active fault only remains for the duration of the fault or in some cases the duration of the current ignition switch cycle, while a stored fault causes a DTC to be stored in memory by the ACM. For some DTCs, if a fault does not recur for a number of ignition cycles, the ACM will automatically erase the stored DTC. For other internal faults, the stored DTC is latched forever. In standard cab models, the ACM also monitors inputs from the passenger airbag ON/OFF switch. On models equipped with optional side curtain airbags, the ACM communicates with both the left and right Side Impact Airbag Control Modules (SIACM) over the PCI data bus. The SIACM notifies the ACM when it has detected a monitored system fault and stored a DTC in memory for its respective side curtain airbag system, and the ACM sets a DTC and controls the airbag indicator operation accordingly.
The ACM receives battery current through two circuits, on a fused ignition switch output (run) circuit through a fuse in the Junction Block (JB), and on a fused ignition switch output (run-start) circuit through a second fuse in the JB. The ACM is grounded through a ground circuit and take out of the instrument panel wire harness. This take out has a single eyelet terminal connector secured by a ground screw to the left side of the floor panel transmission tunnel near the ACM in the passenger compartment. Therefore, the ACM is operational whenever the ignition switch is in the Start or ON positions. The ACM also contains an energy-storage capacitor. When the ignition switch is in the Start or ON positions, this capacitor is continually being charged with enough electrical energy to deploy the airbags for up to one second following a battery disconnect or failure. The purpose of the capacitor is to provide backup airbag system protection in case there is a loss of battery current supply to the ACM during an impact. The capacitor is only serviced as a unit with the ACM.
Two sensors are contained within the ACM, an electronic impact sensor and a safing sensor. The electronic impact sensor is an accelerometer that senses the rate of vehicle deceleration, which provides verification of the direction and severity of an impact. The safing sensor is an electromechanical sensor within the ACM that provides an additional logic input to the ACM microprocessor. The safing sensor is a normally open switch that is used to verify the need for a front supplemental restraint deployment by detecting impact energy of a lesser magnitude than that of the electronic impact sensor, and must be closed in order for the front airbags or seat belt tensioners to deploy. A pre-programmed decision algorithm in the ACM microprocessor determines when the deceleration rate as signaled by the impact sensor and the safing sensor indicate an impact that is severe enough to require front supplemental restraint system protection and, based upon the status of the passenger airbag ON/OFF switch input and the severity of the monitored impact, determines what combination of seat belt tensioner and front airbag deployment is required for each front seating position. When the programmed conditions are met, the ACM sends the proper electrical signals to deploy the seat belt tensioners and dual front airbags.
The hard wired inputs and outputs for the ACM may be diagnosed and tested using conventional diagnostic tools and procedures. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the ACM, the PCI data bus network, or the electronic message inputs to and outputs from the ACM. The most reliable, efficient, and accurate means to diagnose the ACM, the PCI data bus network, and the electronic message inputs to and outputs from the ACM requires the use of a DRB III scan tool. Refer to the appropriate diagnostic information.
AIRBAG WARNING LAMP
The AIRBAG warning lamp is the only point at which "symptoms" of a system malfunction can be observed by the customer. Whenever the ignition key is turned to the "run" or "start" position, the airbag control module performs a lamp check by turning the AIRBAG warning lamp on for 6-8 seconds. If the lamp remains off, it means that the ACM has checked the system and found it to be free of discernible malfunctions. If the lamp remains on, there could be an active fault in the system or the circuit that operates the lamp may be shorted to ground. If the lamp comes on and stays on for a period longer than 6-8 seconds, then goes off, there is usually an intermittent problem in the system.
CHILD TETHER ANCHOR
All models are equipped with child seat tether anchors. Models equipped with the 60/40 rear (second row) seat have three anchors that are integral to the rear seat back panels, two on the 60 percent seat back panel and one on the 40 percent seat back panel. Models equipped with a 40/20/40 rear seat have two loop-type anchors secured to the rear floor panel behind each outboard 40 percent second seat back, and two loop-type anchors secured to the liftgate opening sill panel behind the third seat back. The anchors integral to the seat backs cannot be adjusted or repaired and, if faulty or damaged, the rear seat back panel must be replaced. The loop-type anchors are available for service replacement.
See the owner's manual in the vehicle glove box for more information on the proper use of the factory-installed child seat tether anchors (standard cab) and anchor straps (quad cab).
Fig.8 Clockspring:
Fig.9 Turn Signal Cancel Cam:
CLOCKSPRING
The clockspring assembly is secured with two screws onto the multi-function switch mounting housing near the top of the steering column behind the steering wheel. The clockspring consists of a flat, round molded plastic case with a stubby tail that hangs below the steering column and contains two connector receptacles that face toward the instrument panel. Within the plastic housing is a spool-like molded plastic rotor with a large exposed hub. The upper surface of the rotor hub has a large center hole, two large flats, an engagement dowel with a yellow rubber boot, a short pigtail wire with connector, and two connector receptacles that face toward the steering wheel.
The lower surface of the rotor hub has a molded plastic turn signal cancel cam with two lobes that is keyed to the rotor and is secured there with four integral snap features. Within the plastic case and wound around the rotor spool is a long ribbon-like tape that consists of several thin copper wire leads sandwiched between two thin plastic membranes. The outer end of the tape terminates at the connector receptacles that face the instrument panel, while the inner end of the tape terminates at the pigtail wire and connector receptacle on the hub of the clockspring rotor that face the steering wheel.
Service replacement clocksprings are shipped precentered and with a molded plastic locking pin that snaps into a receptacle on the rotor and is engaged between two tabs on the upper surface of the rotor case. The locking pin secures the centered clockspring rotor to the clockspring case during shipment, but the locking pin must be removed from the clockspring after it is installed on the steering column. (Refer to ELECTRICAL/RESTRAINTS/CLOCKSPRING - STANDARD PROCEDURE - CLOCKSPRING CENTERING).
The clockspring cannot be repaired. If the clockspring is faulty damaged, or if the driver airbag has been deployed, the clockspring must be replaced.
The clockspring is a mechanical electrical circuit component that is used to provide continuous electrical continuity between the fixed instrument panel wire harness and the electrical components mounted on or in the rotating steering wheel. On this model the rotating electrical components include the driver airbag, the horn switch, the speed control switches, and the remote radio switches, if the vehicle is so equipped. The clockspring case is positioned and secured to the multi-function switch mounting housing near the top of the steering column. The connector receptacles on the tail of the fixed clockspring case connect the clockspring to the vehicle electrical system through two take outs with connectors from the instrument panel wire harness.
The clockspring rotor is movable and is keyed by an engagement dowel that is molded onto the rotor hub between two fins that are cast into the lower surface of the steering wheel armature. A yellow rubber boot is installed over the engagement dowel to eliminate contact noise between the dowel and the steering wheel. The two lobes on the turn signal cancel cam on the lower surface of the clockspring rotor hub contact a turn signal cancel actuator of the multi-function switch to provide automatic turn signal cancellation. The yellow sleeved pigtail wires on the upper surface of the clockspring rotor connect the clockspring to the driver airbag, while a steering wheel wire harness connects the two connector receptacles on the upper surface of the clockspring rotor to the horn switch and, if the vehicle is so equipped, to the optional speed control and remote radio switches on the steering wheel.
Like the clockspring in a timepiece, the clockspring tape has travel limits and can be damaged by being wound too tightly during full stop-to-stop steering wheel rotation. To prevent this from occurring, the clockspring is centered when it is installed on the steering column. Centering the clockspring indexes the clockspring tape to the movable steering components so that the tape can operate within its designed travel limits. However, if the clockspring is removed from the steering column or if the steering shaft is disconnected from the steering gear, the clockspring spool can change position relative to the movable steering components and must be re-centered following completion of the service or the tape may be damaged. Service replacement clocksprings are shipped pre-centered and with a plastic locking pin installed. This locking pin should not be disengaged until the clockspring has been installed on the steering column. If the locking pin is removed or damaged before the clockspring is installed on a steering column, the clockspring centering procedure must be performed. (Refer to ELECTRICAL/RESTRAINTS/CLOCKSPRING - STANDARD PROCEDURE - CLOCKSPRING CENTERING).