Electronic Control Modules: Overview Jeep Liberty I

DESCRIPTION

A Body Control Module (BCM) is concealed behind the driver side end of the instrument panel in the passenger compartment, where it is secured to the fuse panel side of the junction block (JB). The JB is the interface between the body, the instrument panel, and the headlamp and dash wire harnesses. The JB also contains the fuses and relays used for the interior electrical system of the vehicle. The BCM is enclosed in a molded plastic housing with two integral external connectors that connect it to the vehicle electrical system. The BCM also has an integral interface connector that joins it through a connector to the circuitry within the JB. This connector is referred to as the JB-BCM connector. The combined BCM and JB are sometimes referred to as the Junction Block Module (JBM).

Scheme 1

Scheme 2

There are two different versions of the BCM: base and premium. The base BCM is a subset of the components in the premium version. The base version BCM does not support the following features: electronic vehicle information center (EVIC), fog lamps (front and/or rear), remote keyless entry (RKE), remote radio switches, or vehicle theft security system (VTSS). Both versions of the BCM utilize integrated circuitry and information carried on the programmable communications interface (PCI) data bus network along with many hard wired inputs to monitor many sensor and switch inputs throughout the vehicle. In response to those inputs, the internal circuitry and programming of the BCM 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 PCI data bus. Any time the BCM is replaced or reflashed for any reason, there may be subsystems that may need to be reprogrammed as well, such as Cabin Equalization. Follow the DRBIII(R) scan tool menu screen for details. The electronic functions and features that the BCM supports or controls include the following

1. A/C Select Switch Status - The BCM monitors an input from, and transmits the status of the A/C switch on the heater-A/C control.
2. Audio System Cabin Equalization - Each time the BCM receives an electronic cabin equalization request message from the radio over the PCI data buss, it provides an electronic response to the radio containing the appropriate equalization curve information. Because there are numerous optional radios which are common to many platforms and available with various speaker architectures, each radio contains a Digital Signal Processing (DPS) microprocessor chip. This DPS chip uses the equalization curve information to optimize the radio's sound output for the unique cabin and speaker architecture found within the particular vehicle to which the radio has been installed.
3. Cargo Lamp Disable - The BCM monitors an input from the cargo lamp switch to provide an interior lighting disable feature.
4. Chimes - The chime tone generator is located on the ElectroMechanical Instrument Cluster (EMIC) circuit board, but the EMIC goes to sleep with the ignition switch in the Off position. The BCM provides a wake-up output to the EMIC based upon inputs from the key-in ignition switch or the exterior lighting switch, then sends electronic chime request messages to the EMIC for the headlamps-on warning and key-in ignition warning.
5. Door Lock Inhibit - The BCM monitors the key-in ignition switch and the driver side front door ajar switch to provide a door lock inhibit feature.
6. Exterior Lamp Load Shedding - The BCM provides a battery saver feature which will automatically turn off exterior lamps that remain on after a timed interval.
7. Exterior Lamp Status - The BCM monitors the status of the park lamp, low beam, high beam or Daytime Running Lamp (DRL - Canada only), front fog lamp (optional), and rear fog lamp (in required markets only) relays.
8. Exterior Lighting Control - The BCM provides exterior lamp control for standard head and park lamps, as well as Daytime Running Lamps (DRL - Canada only), front fog lamps (optional), and rear fog lamps (in required markets only). This includes support for features including optical horn (also known as flash-to-pass) and headlamp time delay.
9. Flip-Up Glass Control - The BCM monitors the tailgate cylinder lock switch, the tailgate handle switch, the Remote Keyless Entry (RKE) module inputs and the rear wiper switch to provide control for the rear flip-up glass actuator.
10. Fog Lamp Control - The premium BCM provides fog lamp control for front fog lamps (optional), and rear fog lamps (in required markets only).
11. Front Wiper System Status - The BCM monitors the status of the front wiper motor park switch.
12. Fuel Economy and Distance to Empty Calculations - The BCM calculates and transmits the fuel economy and Distance To Empty (DTE) data.
13. Headlamp Time Delay - The BCM provides a headlamp time delay feature with the ignition switch in the Off position.
14. Heated Rear Glass Control - The BCM provides control and timer functions for the heated rear glass feature and transmits the system status.
15. Ignition On/Off Timer - The BCM monitors and transmits the elapsed ignition. On timer data and monitors the ignition Off time.
16. Ignition Switch Position Status - The BCM monitors and transmits the status of the ignition switch.
17. Instrument Panel Dimming - The BCM monitors and transmits the selected illumination intensity level of the panel lamps dimmer switch.
18. Interior Lamp Load Shedding - The BCM provides a battery saver feature which will automatically turn off all interior lamps that remain on after a timed interval.
19. Interior Lighting Control - The BCM monitors inputs from the interior lighting switch, the door ajar switches, the flip-up glass ajar switch, the tailgate ajar switch, the cargo lamp switch, the reading lamp switches, and the Remote Keyless Entry (RKE) module to provide courtesy lamp control. This includes support for timed illuminated entry with theater-style fade-to-off and courtesy illumination defeat features.
20. Intermittent Wipe and Front Wiper System Control - The BCM monitors inputs from the front wiper and washer switch and the front wiper motor park switch to provide front wiper system control through the wiper on/off and high/low relays. This includes support for adjustable intermittent wipe, mist wipe (also known as pulse wipe), and wipe-afterwash features.
21. Key-In-Ignition Switch Status - The BCM monitors and transmits the status of the key-in-ignition switch.
22. Panic Mode - The BCM provides support for the Remote Keyless Entry (RKE) system panic mode feature.
23. Parade Mode - The BCM provides a parade mode (also known as funeral mode) that allows the interior Vacuum Fluorescent Displays (VFD) to be illuminated at full intensity while driving in daylight with the exterior lamps On.
24. Power Locks - The BCM monitors inputs from the power lock switches and the Remote Keyless Entry (RKE) module (optional) to provide control of the power lock motors through outputs to the lock, unlock, and driver unlock (RKE only) relays. This includes support for rolling door locks (also known as automatic door locks) and a door lock inhibit mode.
25. Programmable Features - The BCM provides support for several standard and optional programmable features, including: rolling door locks, headlamp time delay interval, Remote Keyless Entry (RKE) driver-door-only or unlock-all-doors, RKE optical chirp, and RKE audible chirp.
26. Remote Keyless Entry - The premium BCM provides the optional Remote Keyless Entry (RKE) system features, including support for the RKE Lock, Unlock (with optional driver-door-only unlock, and unlock-all-doors), rear flip-up glass control, Panic, audible chirp, optical chirp, and illuminated entry modes, as well as the ability to be programmed to recognize up to four RKE transmitters.
27. Rolling Door Locks - The BCM provides support for the power lock system rolling door locks feature (also known as automatic door locks).
28. Tailgate and Flip-Up Glass Ajar Status - The BCM monitors and transmits the status of the tailgate and rear flip-up glass ajar switches.
29. Remote Radio Switch Interface - The premium BCM monitors and transmits the status of the optional remote radio switches.
30. Self-Diagnostics - The BCM provides support for diagnostics through communication with the DRBIII(R) scan tool over the PCI data bus network. Each analog and digital input can be verified, and each output can be actuated through the use of this diagnostic protocol. The BCM also stores Diagnostic Trouble Codes (DTCs) to assist in troubleshooting this unit.
31. Tire Size - The BCM calculates vehicle speed based upon a programmed Tire Revolutions per mile (TIRE REV/MILE) value. The correct tire size must be programmed to the BCM using the DRBIII(R). Using the DRBIII(R), select Body Computer, then Program Tire Size. The BCM must be programmed with one of the available tire sizes in the DRBIII(R) menu or by programming the correct REV/MILE value.
32. Vacuum Fluorescent Display Synchronization - The BCM transmits panel lamp intensity data which allows modules with Vacuum Fluorescent Displays (VFD) to coordinate their illumination intensity.
33. Vehicle Speed System - ABS Equipped - The CAB provides 12VDC hard wire speed sensor supply to the Left and Right Wheel speed sensors and the Rear Wheel Speed Sensor the speed sensor output hard wire square wave signals back to the CAB. The CAB outputs a hard wire square wave back to the BCM. The BCM calculates vehicle speed based upon a programmed Tire revolutions per mile (TIRE REV/MILE) value. This calculation is based upon tire circumference revolutions per mile. This calculation must be programmed into the BCM. New service BCM's are shipped in default mode that prevents speedometer indication until a valid tire size is programmed. The BCM outputs a hard wired square wave Vehicle Speed Output signal to the PCM. The PCM transmits vehicle speed to the instrument cluster via the PCI bus.
34. Vehicle Speed System - NON-ABS Equipped - The BCM provides a 12VDC hard wire speed sensor supply to the Rear Wheel speed sensor. The speed sensor outputs a hard wire square wave signal back to the BCM. The BCM calculates vehicle speed based upon a programmed Tire Revolutions per mile (TIRE REV/MILE) value. This calculation is based tire circumference revolutions per mile. This calculation must be programmed into the BCM. New service BCM's are shipped in default mode that prevents speedometer indication until a valid tire size is programmed. The BCM outputs a hard wired square wave Vehicle Speed Output signal to the PCM. The PCM transmits vehicle speed to the instrument cluster via the PCI bus.
35. Vehicle Theft Security System - The premium BCM monitors inputs from the door cylinder lock switches, the tailgate cylinder lock switch, the door ajar switches, the tailgate ajar switch, the flip-up glass ajar switch, the hood ajar switch (in required markets only), and the Remote Keyless Entry (RKE) module to control the features of the optional Vehicle Theft Security System (VTSS).

Hard wired circuitry connects the BCM to the electrical system of the vehicle. These hard wired circuits are integral to several wire harnesses, which are routed throughout the vehicle and retained by many different methods. These circuits may be connected to each other, to the vehicle electrical system and to the BCM through the use of a combination of soldered splices, splice block connectors, and many different types of wire harness terminal connectors and insulators. See BODY CONTROL MODULES in SYSTEM WIRING DIAGRAMS article. The wiring information includes wiring diagrams, proper wire and connector repair procedures, further details on wire harness routing and retention, as well as pin-out and location views for the various wire harness connectors, splices and grounds.

Many of the electronic features in the vehicle controlled or supported by the BCM are programmable using a customer programming procedure or the DRBIII(R) scan tool. In addition, the BCM software is Flash compatible, which means it can be reprogrammed using Flash reprogramming procedures. However, if any of the BCM hardware components is damaged or faulty, the entire BCM unit must be replaced.

OPERATION

The Body Control Module (BCM) monitors many hard wired switch and sensor inputs as well as those resources it shares with other electronic modules in the vehicle through its communication over the Programmable Communications Interface (PCI) data bus network. The internal programming and all of these inputs allow the BCM microprocessor to determine the tasks it needs to perform and their priorities, as well as both the standard and optional features that it should provide. The BCM programming then performs those tasks and provides those features through both PCI data bus communication with other electronic modules and through hard wired outputs through a number of driver circuits, relays, and actuators. These outputs allow the BCM the ability to control numerous accessory systems in the vehicle.

The BCM operates on battery and ignition voltage inputs received through several fuses in the Junction Block (JB). This arrangement allows the BCM to provide some features regardless of the ignition switch position, while other features will operate only with the ignition switch in the On, Start, and/or Accessory positions. All of the battery voltage circuits are connected to the BCM through the JB/BCM connector. The BCM receives ground through five separate circuits. Three of these circuits are connected to the BCM through a connector of the instrument panel wire harness on three separate ground circuits, while the other two circuits are connected to the BCM through the JB/BCM connector.

The BCM monitors its own internal circuitry as well as many of its input and output circuits, and will store a Diagnostic Trouble Code (DTC) in electronic memory for any failure it detects. These DTCs can be retrieved and diagnosed using a DRBIII(R) scan tool. Refer to the appropriate diagnostic information.

HARD WIRED INPUTS The hard wired inputs to the BCM include the following

1. A/C On/Off Control
2. Door Lock Switch
3. Driver Door Ajar Switch Sense
4. Flip-Up Glass Ajar Switch Sense
5. Flip-Up Glass Release Switch Sense
6. Fog Lamp Switch Sense
7. Front Wiper Park Switch Sense
8. Front Wiper Switch
9. Front Washer Pump Driver
10. Fused B(+)
11. Fused Ignition Switch Output (Run-Acc)
12. Fused Ignition Switch Output (Run-Start)
13. Headlamp Switch
14. High Beam Switch Sense
15. Hood Ajar Switch Sense - Premium With VTSS - Markets Where Required Only
16. Key-In Ignition Switch Sense
17. Left Cylinder Lock Switch Sense - Premium With VTSS Only - Omitted In Some Markets As Required
18. Panel Lamps Dimmer Switch
19. Passenger Doors Ajar Switch Sense (Input From Three Ajar Switches Connected In Parallel)
20. Radio Control - Premium With Remote Radio Switches Only
21. Rear Courtesy Lamp Control
22. Rear Window Defogger Control
23. Rear Wiper Intermittent Driver
24. Rear Wiper On Driver
25. Right Cylinder Lock Switch Sense - Premium With VTSS Only - Omitted In Some Markets As Required
26. RKE Antenna (Two Circuits) - Premium With RKE Only
27. Tailgate Ajar Switch Sense
28. Tailgate Cylinder Lock Switch Sense
29. Vehicle Speed Sensor Signal

See BODY CONTROL MODULES in SYSTEM WIRING DIAGRAMS article for additional details.

HARD WIRED OUTPUTS The hard wired outputs of the BCM include the following

1. Courtesy Lamp Driver
2. Courtesy Lamp Load Shed
3. Door Lock Relay Control
4. Driver Door Unlock Relay Control - Premium With RKE Only
5. Flip-Up Glass Release Motor Driver
6. Front Fog Lamp Relay Control - Premium With Front Fog Lamps Only
7. Front Wiper High/Low Relay Control
8. Front Wiper On/Off Relay Control
9. Hazard Lamp Control
10. High Beam Relay Control
11. Horn Relay Control - Premium With RKE Only
12. Instrument Cluster Wake Up Signal
13. Low Beam Relay Control
14. Park Lamp Relay Control
15. Passenger Door Unlock Relay Control
16. Rear Fog Lamp Relay Control - Premium With Rear Fog Lamps In Markets Where Required Only
17. Rear Window Defogger Relay Control
18. RKE Supply - Premium With RKE Only
19. Tailgate Lock Driver
20. Tailgate Unlock Driver
21. Vehicle Speed Output
22. Vehicle Speed Sensor Supply (Non-ABS)
23. VTSS Indicator Driver - Premium With VTSS Only

See BODY CONTROL MODULES in SYSTEM WIRING DIAGRAMS article for additional details.

GROUNDS The BCM receives ground through five separate circuits, and also supplies a ground path to several switches through the following hard wired circuits

1. Door Lock Switch Ground
2. Headlamp Switch Return
3. Radio Control Return
4. RKE Ground - Premium With RKE Only
5. Tailgate Switch Ground

See BODY CONTROL MODULES in SYSTEM WIRING DIAGRAMS article for additional details.

COMMUNICATION Not including the two RKE antenna circuits (RKE antenna + and -), which merely pass through the premium BCM from the RKE module to the external RKE antenna in the instrument panel wire harness, the BCM has the following communication circuits

1. PCI Bus
2. RKE Program Serial Data - Premium With RKE Only
3. RKE Transmit Serial Data - Premium With RKE Only

See BODY CONTROL MODULES in SYSTEM WIRING DIAGRAMS article for additional details.

MESSAGING The BCM uses the following messages received from other electronic modules over the PCI data bus

1. Battery Temperature (PCM)
2. Electronic Vehicle Information Center (EVIC) - Premium Only
3. Coolant Temperature (PCM)
4. Distance Pulses (PCM)
5. Engine Speed (PCM)
6. Fuel Tank Level (PCM)
7. Fuel Used (PCM)
8. Intrusion Transceiver Module Commands (ITM) - Premium In Markets Where Required Only
9. Manifold Absolute Pressure (PCM)
10. OK to Lock - Rolling Locks (PCM)
11. SKIS Status (SKIM)
12. Vehicle Identification Number (PCM)
13. Vehicle Speed from CAB with ABS

The BCM provides the following messages to other electronic modules over the PCI data bus

1. A/C Select Switch Status (PCM)
2. Country Code (EMIC, PCM, EVIC)
3. Distance to Empty (EVIC) - Premium Only
4. Door Ajar Status (EMIC)
5. Exterior Lighting Status (EMIC)
6. Flip-up Glass Ajar Status (EMIC)
7. Fuel Economy (Average) (EVIC) - Premium Only
8. Hood Ajar Status (ITM) - Premium In Markets Where Required Only
9. Ignition On Timer (EVIC) - Premium Only
10. Intrusion Transceiver Module Commands (ITM) - Premium In Markets Where Required Only
11. Key-In Ignition Switch Status (EMIC)
12. Panel Lamp Intensity (EVIC, Radio)
13. Tailgate Ajar Status (EMIC)
14. Radio EQ Curve (Radio) - Premium Only
15. Radio Mode (Radio) - Premium Only
16. Radio Preset Scan (Radio) - Premium Only
17. Radio Seek Down (Radio) - Premium Only
18. Radio Seek Up (Radio) - Premium Only
19. Radio Volume Down (Radio) - Premium Only
20. Radio Volume Up (Radio) - Premium Only
21. Vacuum Fluorescent Display Synchronization (EVIC, EMIC, Radio)
22. Vehicle Theft Security System Status (PCM, ITM) - Premium Only

See BODY CONTROL MODULES in SYSTEM WIRING DIAGRAMS article for additional details.

DESCRIPTION - DATABUS

The Daimler Chrysler Programmable Communication Interface (PCI) data bus system is a single wire multiplex system used for vehicle communications on many Daimler Chrysler Corporation vehicles. Multiplexing is a system that enables the transmission of several messages over a single channel or circuit. All Daimler Chrysler vehicles use this principle for communication between various microprocesor-based electronic control modules. The PCI data bus exceeds the Society of Automotive Engineers (SAE) J1850 Standard for Class B Multiplexing.

Many of the electronic control modules in a vehicle require information from the same sensing device. In the past, if information from one sensing device was required by several controllers, a wire from each controller needed to be connected in parallel to that sensor. In addition, each controller utilizing analog sensors required an Analog/Digital (A/D) converter in order to "read" these sensor inputs. Multiplexing reduces wire harness complexity, sensor current loads and controller hardware because each sensing device is connected to only one controller, which reads and distributes the sensor information to the other controllers over the data bus. Also, because each controller on the data bus can access the controller sensor inputs to every other controller on the data bus, more function and feature capabilities are possible.

In addition to reducing wire harness complexity, component sensor current loads and controller hardware, multiplexing offers a diagnostic advantage. A multiplex system allows the information flowing between controllers to be monitored using a diagnostic scan tool. The Daimler Chrysler system allows an electronic control module to broadcast message data out onto the bus where all other electronic control modules can "hear" the messages that are being sent. When a module hears a message on the data bus that it requires, it relays that message to its microprocessor. Each module ignores the messages on the data bus that are being sent to other electronic control modules.

OPERATION - DATABUS

Data exchange between modules is achieved by serial transmission of encoded data over a single wire broadcast network. The wire colors used for the PCI data bus circuits are yellow with a violet tracer, or violet with a yellow tracer, depending upon the application. The PCI data bus messages are carried over the bus in the form of Variable Pulse Width Modulated (VPWM) signals. The PCI data bus speed is an average 10.4 Kilo-bits per second (Kbps). By comparison, the prior two-wire Chrysler Collision Detection (CCD) data bus system is designed to run at 7.8125 Kbps.

The voltage network used to transmit messages requires biasing and termination. Each module on the PCI data bus system provides its own biasing and termination. Each module (also referred to as a node) terminates the bus through a terminating resistor and a terminating capacitor. There are two types of nodes on the bus. The dominant node terminates the bus through a 1 KW resistor and a 3300 pF capacitor. The Powertrain Control Module (PCM) is the only dominant node for the PCI data bus system. A standard node terminates the bus through an 11 KW resistor and a 330 pF capacitor.

The modules bias the bus when transmitting a message. The PCI bus uses low and high voltage levels to generate signals. Low voltage is around zero volts and the high voltage is about 7.5 volts. The low and high voltage levels are generated by means of variable-pulse width modulation to form signals of varying length. The Variable Pulse Width Modulation (VPWM) used in PCI bus messaging is a method in which both the state of the bus and the width of the pulse are used to encode bit information. A "zero" bit is defined as a short low pulse or a long high pulse. A "one" bit is defined as a long low pulse or a short high pulse. A low (passive) state on the bus does not necessarily mean a zero bit. It also depends upon pulse width. If the width is short, it stands for a zero bit. If the width is long, it stands for a one bit. Similarly, a high (active) state does not necessarily mean a one bit. This too depends upon pulse width. If the width is short, it stands for a one bit. If the width is long, it stands for a zero bit.

In the case where there are successive zero or one data bits, both the state of the bus and the width of the pulse are changed alternately. This encoding scheme is used for two reasons. First, this ensures that only one symbol per transition and one transition per symbol exists. On each transition, every transmitting module must decode the symbol on the bus and begin timing of the next symbol. Since timing of the next symbol begins with the last transition detected on the bus, all of the modules are re-synchronized with each symbol. This ensures that there are no accumulated timing errors during PCI data bus communication.

The second reason for this encoding scheme is to guarantee that the zero bit is the dominant bit on the bus. When two modules are transmitting simultaneously on the bus, there must be some form of arbitration to determine which module will gain control. A data collision occurs when two modules are transmitting different messages at the same time. When a module is transmitting on the bus, it is reading the bus at the same time to ensure message integrity. When a collision is detected, the module that transmitted the one bit stops sending messages over the bus until the bus becomes idle.

Each module is capable of transmitting and receiving data simultaneously. The typical PCI bus message has the following four components

1. Message Header - One to three bytes in length. The header contains information identifying the message type and length, message priority, target module(s) and sending module.
2. Data Byte(s) - This is the actual message that is being sent.
3. Cyclic Redundancy Check (CRC) Byte - This byte is used to detect errors during a message transmission.
4. In-Frame Response (IFR) Byte(s) - If a response is required from the target module(s), it can be sent during this frame. This function is described in greater detail in the following paragraph.

The IFR consists of one or more bytes, which are transmitted during a message. If the sending module requires information to be received immediately, the target module(s) can send data over the bus during the original message. This allows the sending module to receive time-critical information without having to wait for the target module to access the bus. After the IFR is received, the sending module broadcasts an End of Frame (EOF) message and releases control of the bus.

The PCI data bus can be monitored using the DRBIII(R) scan tool. It is possible, however, for the bus to pass all DRBIII(R) tests and still be faulty if the voltage parameters are all within the specified range and false messages are being sent.

The Controller Anti-lock Brake (CAB) is mounted to the Hydraulic Control Unit (HCU) and operates the ABS system.

The CAB voltage source is through the ignition switch in the RUN position. The CAB contains dual microprocessors. A logic block in each microprocessor receives identical sensor signals. These signals are processed and compared simultaneously. The CAB contains a self check program that illuminates the ABS warning light when a system fault is detected. Faults are stored in a diagnostic program memory and are accessible with the DRB scan tool. ABS faults remain in memory until cleared, or until after the vehicle is started approximately 50 times. Stored faults are not erased if the battery is disconnected.

Scheme 3

Scheme 4

Scheme 5

1. Remove the negative battery cable from the battery.
2. Pull up on the CAB harness connector release and remove connector.
3. Remove the pump connector from the CAB.
4. Remove the CAB mounting bolts.
5. Remove the CAB from the HCU.

DESCRIPTION - DATA LINK CONNECTOR

The data link connector is located at the lower edge of the instrument panel near the steering column.

Scheme 6

OPERATION - DATA LINK CONNECTOR

The 16-way data link connector (diagnostic scan tool connector) links the Diagnostic Readout Box (DRB) scan tool or the Mopar Diagnostic System (MDS) with the Powertrain Control Module (PCM).

DESCRIPTION - PCM

The Powertrain Control Module (PCM) is located in the engine compartment.

Scheme 7

DESCRIPTION - MODES OF OPERATION

As input signals to the Powertrain Control Module (PCM) change, the PCM adjusts its response to the output devices. For example, the PCM must calculate different injector pulse width and ignition timing for idle than it does for wide open throttle (WOT).

The PCM will operate in two different modes

DESCRIPTION - 5 VOLT SUPPLIES

Two different Powertrain Control Module (PCM) five volt supply circuits are used; primary and secondary.

DESCRIPTION - IGNITION CIRCUIT SENSE

This circuit ties the ignition switch to the Powertrain Control Module (PCM).

DESCRIPTION - POWER GROUNDS

The Powertrain Control Module (PCM) has 2 main grounds. Both of these grounds are referred to as power grounds. All of the high-current, noisy, electrical devices are connected to these grounds as well as all of the sensor returns. The sensor return comes into the sensor return circuit, passes through noise suppression, and is then connected to the power ground.

The power ground is used to control ground circuits for the following PCM loads

1. Generator Field Winding
2. Fuel Injectors
3. Ignition Coil(s)
4. Certain Relays/Solenoids
5. Certain Sensors

DESCRIPTION - SENSOR RETURN

The Sensor Return circuits are internal to the Powertrain Control Module (PCM).

Sensor Return provides a low-noise ground reference for all engine control system sensor.

OPERATION - PCM

The PCM operates the fuel system. The PCM is a pre-programmed, triple microprocessor digital computer. It regulates ignition timing, air-fuel ratio, emission control devices, charging system, certain transmission features, speed control, air conditioning compressor clutch engagement and idle speed. The PCM can adapt its programming to meet changing operating conditions.

The PCM receives input signals from various switches and sensors. Based on these inputs, the PCM regulates various engine and vehicle operations through different system components. These components are referred to as Powertrain Control Module (PCM) Outputs. The sensors and switches that provide inputs to the PCM are considered Powertrain Control Module (PCM) Inputs.

The PCM adjusts ignition timing based upon inputs it receives from sensors that react to: engine RPM, manifold absolute pressure, engine coolant temperature, throttle position, transmission gear selection (automatic transmission), vehicle speed, power steering pump pressure, and the brake switch.

The PCM adjusts idle speed based on inputs it receives from sensors that react to: throttle position, vehicle speed, transmission gear selection, engine coolant temperature and from inputs it receives from the air conditioning clutch switch and brake switch.

Based on inputs that it receives, the PCM adjusts ignition coil dwell. The PCM also adjusts the generator charge rate through control of the generator field and provides speed control operation.

Note. PCM Inputs: A/C request (if equipped with factory A/C) A/C select (if equipped with factory A/C) A/C pressure transducer Auto shutdown (ASD) sense Battery temperature Battery voltage Brake switch J1850 bus (+) circuits J1850 bus (-) circuits Camshaft position sensor signal Crankshaft position sensor Data link connection for DRB scan tool Engine coolant temperature sensor Fuel level (through J1850 circuitry) Generator (battery voltage) output Ignition circuit sense (ignition switch in on/off/crank/run position) Intake manifold air temperature sensor Knock sensors (2 on 3.7L engine) Leak detection pump (switch) sense (if equipped) Manifold absolute pressure (MAP) sensor Oil pressure Oxygen sensors Park/neutral switch (A/T only) Power ground Power steering pressure switch Sensor return Signal ground Speed control multiplexed single wire input Throttle position sensor Transfer case switch (4WD range position) Vehicle speed sensor

Note. PCM Outputs: A/C clutch relay Auto shutdown (ASD) relay J1850 bus (+/-) circuits for: speedometer, voltmeter, fuel gauge, oil pressure gauge/lamp, engine temp. gauge and speed control warn. lamp Clutch pedal position switch override relay Data link connection for DRB scan tool EGR valve control solenoid (if equipped) EVAP canister purge solenoid Five volt sensor supply (primary) Five volt sensor supply (secondary) Fuel injectors Fuel pump relay Generator field driver (-) Generator field driver (+) Idle air control (IAC) motor Ignition coil(s) Leak detection pump (if equipped) Malfunction indicator lamp (Check engine lamp). Driven through J1850 circuits. Oxygen sensor heater relays Oxygen sensors (pulse width modulated) Radiator cooling fan relay (pulse width modulated) Speed control vacuum solenoid Speed control vent solenoid Tachometer (if equipped). Driven through J1850 circuits. Transmission convertor clutch circuit. Driven through J1850 circuits.

OPERATION - 5 VOLT SUPPLIES

Primary 5-volt supply

1. Supplies the required 5 volt power source to the Crankshaft Position (CKP) sensor.
2. Supplies the required 5 volt power source to the Camshaft Position (CMP) sensor.
3. Supplies a reference voltage for the Manifold Absolute Pressure (MAP) sensor.
4. Supplies a reference voltage for the Throttle Position Sensor (TPS) sensor.

Secondary 5-volt supply

1. Supplies the required 5 volt power source to the oil pressure sensor.
2. Supplies the required 5 volt power source for the Vehicle Speed Sensor (VSS) (if equipped).
3. Supplies the 5 volt power source to the transmission pressure sensor (certain automatic transmissions).

OPERATION - IGNITION CIRCUIT SENSE

The ignition circuit sense input tells the PCM the ignition switch has energized the ignition circuit.

Battery voltage is also supplied to the PCM through the ignition switch when the ignition is in the RUN or START position. This is referred to as the "ignition sense" circuit and is used to "wake up" the PCM. Voltage on the ignition input can be as low as 6 volts and the PCM will still function. Voltage is supplied to this circuit to power the PCM's 8-volt regulator and to allow the PCM to perform fuel, ignition and emissions control functions.

The Sentry Key Immobilizer Module (SKIM) is the primary component of the Sentry Key Immobilizer System (SKIS). The SKIM is located on the right side of the steering column, below the ignition lock cylinder housing and is concealed beneath the steering column shrouds. The housing for the SKIM has an molded plastic halo-like antenna ring. When the SKIM is properly installed on the steering column, the antenna ring is oriented around the circumference of the ignition lock cylinder housing. A single connector containing six terminal pins is located on the opposite end of the SKIM from the antenna ring. A stamped metal mounting bracket secured to the SKIM housing is used to secure the component to the right lower flange of the steering column jacket.

Scheme 8

The SKIM cannot be adjusted or repaired. If faulty or damaged, the entire SKIM unit must be replaced.

The Sentry Key Immobilizer Module (SKIM) contains a Radio Frequency (RF) transceiver and a microprocessor. The SKIM transmits RF signals to, and receives RF signals from the Sentry Key transponder through a tuned antenna enclosed within the molded plastic antenna ring. If this antenna ring is not mounted properly around the ignition lock cylinder housing, communication problems between the SKIM and the transponder may arise. These communication problems will result in Sentry Key transponder-related faults. The SKIM also communicates over the Programmable Communications Interface (PCI) data bus with the Powertrain Control Module (PCM), the ElectroMechanical Instrument Cluster (EMIC) and/or the DRBIII(R) scan tool.

The SKIM retains in memory the ID numbers of any Sentry Key transponder that is programmed into it. A maximum of eight Sentry Key transponders can be programmed into the SKIM. For added system security, each SKIM is programmed with a unique Secret Key code. This code is stored in memory, sent over the PCI data bus to the PCM, and is encoded to the transponder of every Sentry Key that is programmed into the SKIM. Therefore, the Secret Key code is a common element that is found in every component of the Sentry Key Immobilizer System (SKIS). Another security code, called a PIN, is used to gain access to the SKIM Secured Access Mode. The Secured Access Mode is required during service to perform the SKIS initialization and Sentry Key transponder programming procedures. The SKIM also stores the Vehicle Identification Number (VIN) in its memory, which it learns through a PCI data bus message from the PCM during SKIS initialization.

In the event that a SKIM replacement is required, the Secret Key code can be transferred to the new SKIM from the PCM using the DRBIII(R) scan tool and the SKIS initialization procedure. Proper completion of the SKIS initialization will allow the existing Sentry Keys to be programmed into the new SKIM so that new keys will not be required. In the event that the original Secret Key code cannot be recovered, SKIM replacement will also require new Sentry Keys. The DRBIII(R) scan tool will alert the technician during the SKIS initialization procedure if new Sentry Keys are required.

When the ignition switch is turned to the On position, the SKIM transmits an RF signal to the transponder in the ignition key. The SKIM then waits for an RF signal response from the transponder. If the response received identifies the key as valid, the SKIM sends a valid key message to the PCM over the PCI data bus. If the response received identifies the key as invalid, or if no response is received from the key transponder, the SKIM sends an invalid key message to the PCM. The PCM will enable or disable engine operation based upon the status of the SKIM messages. It is important to note that the default condition in the PCM is an invalid key. If no message is received from the SKIM by the PCM, the engine will be disabled and the vehicle immobilized after two seconds of running.

The SKIM also sends SKIS indicator status messages to the EMIC over the PCI data bus to tell the EMIC how to operate the SKIS indicator. This indicator status message tells the EMIC to turn the indicator on for about three seconds each time the ignition switch is turned to the On position as a bulb test. After completion of the bulb test, the SKIM sends indicator status messages to the EMIC to turn the indicator off, turn the indicator on, or to flash the indicator on and off. If the SKIS indicator flashes upon ignition On or stays on solid after the bulb test, it signifies a SKIS fault. If the SKIM detects a system malfunction and/or the SKIS has become inoperative, the SKIS indicator will stay on solid. If the SKIM detects an invalid key or if a key transponder-related fault exists, the SKIS indicator will flash. If the vehicle is equipped with the Customer Learn transponder programming feature, the SKIM will also send messages to the EMIC to flash the SKIS indicator and to generate a single audible chime whenever the Customer Learn programming mode is being utilized. See POWER DOOR LOCKS & REMOTE KEYLESS ENTRY SYSTEM article for Sentry Key Transponder programming.

The SKIS performs a self-test each time the ignition switch is turned to the On position, and will store fault information in the form of Diagnostic Trouble Codes (DTC's) in SKIM memory if a system malfunction is detected. The SKIM can be diagnosed, and any stored DTC's can be retrieved using a DRBIII(R) scan tool. Refer to the appropriate diagnostic information.

The Transmission Control Module (TCM) is a submodule within the Powertrain Control Module (PCM). The PCM is located on the left inner fender.

Scheme 9

The TCM is the controlling unit for all electronic operations of the transmission. The TCM receives information regarding vehicle operation from both direct and indirect inputs, and selects the operational mode of the transmission. Direct inputs are hardwired to, and used specifically by the TCM. Indirect inputs originate from other components/modules, and are shared with the TCM via the PCI bus.

Some examples of Direct Inputs to the TCM are

1. Battery (B+) voltage
2. Ignition "ON" voltage
3. Transmission Control Relay (Switched B+)
4. Throttle Position Sensor
5. Crankshaft Position Sensor (CKP)
6. Transmission Range Sensor (TRS)
7. Pressure Switches (L/R, 2/4, OD)
8. Transmission Temperature Sensor (Integral to TRS)
9. Input Shaft Speed Sensor
10. Output Shaft Speed Sensor

Some examples of Indirect Inputs to the TCM are

1. Engine/Body Identification
2. Manifold Pressure
3. Target Idle
4. Torque Reduction Confirmation
5. Speed Control ON/OFF Switch
6. Engine Coolant Temperature
7. Ambient/Battery Temperature
8. Brake Switch Status
9. DRB(R) III Communication

Based on the information received from these various inputs, the TCM determines the appropriate shift schedule and shift points, depending on the present operating conditions and driver demand. This is possible through the control of various direct and indirect outputs.

Some examples of TCM Direct Outputs are

1. Transmission Control Relay
2. Solenoids (L/R, 2/4, OD and UD)
3. Vehicle Speed (to PCM)
4. Torque Reduction Request (to PCM)

Some examples of TCM Indirect Outputs are

1. Transmission Temperature (to PCM)
2. PRNDL Position (to BCM)

In addition to monitoring inputs and controlling outputs, the TCM has other important responsibilities and functions

1. Storing and maintaining Clutch Volume Indices (CVI)
2. Storing and selecting appropriate Shift Schedules
3. System self-diagnostics
4. Diagnostic capabilities (with DRB(R) III scan tool)

Note. If the TCM has been replaced, the "Quick Learn Procedure" must be performed. See STANDARD PROCEDURE - TCM QUICK LEARN .

The heated seat module is also known as the Seat Heat Interface Module. The heated seat module is located under the left front seat cushion, where it is secured to a mounting bracket via two push-pin retainers. The heated seat module has a single connector receptacle that allows the module to be connected to all of the required inputs and outputs through the seat wire harness.

Scheme 10

The heated seat module is an electronic microprocessor controlled device designed and programmed to use inputs from the heated seat relay, the two heated seat switches and the two heated seat sensors to operate and control the heated seat elements in both front seats and the two heated seat indicator lamp Light-Emitting Diodes (LEDs) in each heated seat switch. The heated seat module is also programmed to perform self-diagnosis of certain heated seat system functions and provide feedback of that diagnosis through the heated seat switch indicator lamps.

The heated seat module cannot be repaired. If the heated seat module is damaged or faulty, the entire module must be replaced.

The heated seat module operates on fused battery current received from a fuse in the junction block. The module is grounded at all times. Inputs to the module include a resistor multiplexed heated seat switch request circuit for each of the two heated seat switches and the heated seat sensor inputs from the seat cushions of each front seat. In response to those inputs, the heated seat module controls battery current to the heated seat elements and sensors, and controls the ground for the heated seat switch indicator lamps (LED's).

When a heated seat switch (Driver or Passenger) is depressed a signal is received by the heated seat module, the module energizes the proper indicator LED (Low or High) in the switch by grounding the indicator lamp circuit to indicate that the heated seat system is operating. At the same time, the heated seat module energizes the selected heated seat sensor circuit and the sensor provides the module with an input indicating the surface temperature of the selected seat cushion.

The Low heat set point is about 36° C (96.8° F), and the High heat set point is about 42° C (107.6° F). If the seat cushion surface temperature input is below the temperature set point for the selected temperature setting, the heated seat module energizes an N-channel Field Effect Transistor (N-FET) within the module which energizes the heated seat elements in the selected seat cushion and back. When the sensor input to the module indicates the correct temperature set point has been achieved, the module de-energizes the N-FET which de-energizes the heated seat elements. The heated seat module will continue to cycle the N-FET as needed to maintain the selected temperature set point.

If the heated seat module detects a heated seat sensor value input that is out of range or a shorted or open heated seat element circuit, it will notify the vehicle operator or the repair technician of this condition by flashing the High and/or Low indicator lamps in the affected heated seat switch. Refer to DIAGNOSIS AND TESTING - HEATED SEAT MODULE for flashing LED diagnosis and testing procedures. Refer to DIAGNOSIS AND TESTING - HEATED SEAT MODULE for heated seat module diagnosis and testing procedures.