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Home >> Dodge or Ram Truck >> 1998 >> Grand Caravan FWD V6-181 3.0L SOHC VIN 3 MFI >> Repair and Diagnosis >> Powertrain Management >> Computers and Control Systems >> Description and Operation >> On-Board Diagnostic System >> System Description

System Description

Fig 1 Data Link (Diagnostic) Connector:






SYSTEM DESCRIPTION
The Powertrain Control Module (PCM) monitors many different circuits in the fuel injection, ignition, emission and engine systems. If the PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) in the PCM's memory. If the code applies to a non-emissions related component or system, and the problem is repaired or ceases to exist, the PCM cancels the code after 40 warm-up cycles. Diagnostic trouble codes that affect vehicle emissions illuminate the Malfunction Indicator Lamp (MIL).

Certain criteria must be met before the PCM stores a DTC in memory. The criteria may be a specific range of engine RPM, engine temperature, and/or input voltage to the PCM.

The PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the DTC criteria for the circuit has not been met. For example, assume the diagnostic trouble code criteria requires the PCM to monitor the circuit only when the engine operates between 750 and 2,000 RPM. Suppose the sensor's output circuit shorts to ground when engine operates above 2,400 RPM (resulting in 0 volt input to the PCM). Because the condition happens at an engine speed above the maximum threshold (2,000 rpm), the PCM will not store a DTC.

There are several operating conditions for which the PCM monitors and sets DTC's. Refer to Monitored Systems, Components, and Non-Monitored Circuits below.

NOTE: Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance, pulling a spark plug wire to perform a spark teat may set the misfire code. When a repair is completed and verified, use the DRB scan tool to erase all DTC's and extinguish the MIL.

MALFUNCTION INDICATOR LAMP (MIL)
As a functional test, the Malfunction Indicator Lamp (MIL) illuminates at key-on before engine cranking. Whenever the Powertrain Control Module (PCM) sets a Diagnostic Trouble Code (DTC) that affects vehicle emissions, it illuminates the MIL. If a problem is detected, the PCM sends a message over the CCD Bus to the instrument cluster to illuminate the lamp. The PCM illuminates the MIL only for DTC's that affect vehicle emissions. The MIL stays on continuously when the PCM has entered a Limp-In mode or identified a failed emission component or system. The MIL remains on until the DTC is erased. Refer to the Diagnostic Trouble Code charts in this group for emission related codes.

Also, the MIL either flashes or illuminates continuously when the PCM detects active engine misfire. Refer to Misfire Monitoring in this section.

Additionally, the PCM may reset (turn off) the MIL when one of the following occur:
- PCM does not detect the malfunction for 3 consecutive trips (except misfire and fuel system monitor).
- PCM does not detect a malfunction while performing three successive engine misfire or fuel system tests. The PCM performs these tests while the engine is operating within +/- 375 RPM of and within 10 % of the load of the operating condition at which the malfunction was first detected.

Displaying Trouble Codes
Technicians can display stored DTC's by two different methods. The first is to cycle the ignition switch ON - OFF - ON - OFF - ON within 5 seconds. Then count the number of times the malfunction indicator (check engine) lamp on the instrument panel flashes on and off. The number of flashes represents the DTC. There is a slight pause between the flashes representing the first and second digits of the code. Longer pauses separate individual trouble codes.

The second method of reading DTC's uses a scan tool.

CCD BUS
Various controllers and modules exchange information through a communications port called the CCD Bus. The PCM transmits the malfunction indicator (check engine) lamp On/Off signal, engine RPM and vehicle load information on the CCD Bus.

STATE DISPLAY TEST MODE
The switch inputs to the Powertrain Control Module (PCM) have two recognized states; HIGH and LOW. For this reason, the PCM cannot recognize the difference between a selected switch position versus an open circuit, a short circuit, or a defective switch. If the State Display screen shows the change from HIGH to LOW or LOW to HIGH, assume the entire switch circuit to the PCM functions properly. From the state display screen, access either State Display Inputs and Outputs or State Display Sensors.

STATE DISPLAY INPUTS AND OUTPUTS
Connect the DRB scan tool to the data link connector and access the State Display screen. Then access Inputs and Outputs. The following list contains the PCM system functions accessible through the Inputs and Outputs screen:

- Park/Neutral Switch
- Speed Control Resume
- Brake Switch
- Speed Control On/Off
- Speed Control Set
- S/C Vent Solenoid
- Actual S/C Vent Sol.
- S/C Vacuum Solenoid
- Actual S/C Vacuum Sol.
- S/C Cancel
- S/C Last Cutout
- S/C Working Status
- S/C Denied Status
- A/C Clutch Relay
- Actual A/C Clutch Relay
- EGR Solenoid
- Actual EGR Sol.
- Automatic Shutdown Relay
- Actual Automatic Shutdown Relay
- Automatic Shutdown Relay Sense
- Radiator Fan Control Module
- Actual Radiator Fan Control Module
- Duty Cycle EVAP Purge Solenoid
- Actual EVAP Purge Sol.
- Torque Converter Clutch Solenoid
- Power Steering Switch
- Closed Loop State
- Current CMP Edge
- Current CKP State
- Current Sync State
- Fuel Pump Relay
- Actual Fuel Pump Relay
- Ignition Sense (A21)
- Malfunction Lamp
- Limp-in Reason

STATE DISPLAY SENSORS
Connect the DRB scan tool to the vehicle and access the State Display screen. Then access Sensor Display. The following list contains the PCM system functions accessible through the Sensor Display screen.

- Battery Temperature
- Engine Coolant Temperature
- Engine Coolant Temp Sensor
- Throttle Position Volts
- Minimum Throttle
- Knock Sensor Volts
- Battery Voltage
- MAP Sensor Reading
- Idle Air Control Motor Position
- Adaptive Fuel Factor
- Barometric Pressure
- Engine Speed
- Module Spark Advance
- Speed Control Target
- Intake Air Temp Degrees
- Intake Air Temp Volts
- Charging System Goal
- Theft Alarm Status
- Map Sensor Voltage
- Vehicle Speed
- Throttle Opening (percentage)
- TPS Calculated
- Cam Timing Position
- Target Idle
- Time From Start To Run
- Run Time At Stall
- Injector Pulse-width
- Upstream O2S Volts
- Downstream O2S Volts
- Closed Loop Timer
- Short Term Adaptive
- Current Adaptive Cell
- Adaptive Memory Cell 0
- Adaptive Memory Cell 1
- Adaptive Memory Cell 2
- Adaptive Memory Cell 3
- Adaptive Memory Cell 4
- Adaptive Memory Cell 5
- Adaptive Memory Cell 6
- Adaptive Memory Cell 7
- Adaptive Memory Cell 8
- Adaptive Memory Cell 9
- Adaptive Memory Cell 10
- Adaptive Memory Cell 11
- Adaptive Memory Cell 12
- Adaptive Memory Cell 13
- Adaptive Memory Cell 14
- Adaptive Memory Cell 15
- Purge Free Idle Cell
- Purge Free Cell 2 (corresponds to memory cell 2)
- Target IAC Steps
- Retard Cylinder (1)
- Retard Cylinder (2)
- Retard Cylinder (3)
- Retard Cylinder (4)
- Retard Cylinder (5)
- Retard Cylinder (6)


CIRCUIT ACTUATION TEST MODE
The Circuit Actuation Test Mode checks for proper operation of output circuits or devices the Powertrain Control Module (PCM) may not internally recognize. The PCM attempts to activate these outputs and allow an observer to veri& proper operation. Most of the tests provide an audible or visual indication of device operation (click of relay contacts, fuel spray, etc.). Except for intermittent conditions, if a device functions properly during testing, assume the device, its associated wiring, and driver circuit work correctly.

OBTAINING CIRCUIT ACTUATION TEST
Connect the DRB scan tool to the vehicle and access the Actuators screen. The following is a list of the engine control system functions accessible through Actuators screens. Subordinate screens for each actuator test are also listed:

- Stop All Tests
- Ignition Coil #1
- Ignition Coil #2
- Ignition Coil #3
- Fuel Injector #1
- Fuel Injector #2
- Fuel Injector #3
- Fuel Injector #4
- Fuel Injector #5
- Fuel Injector #6
- Idle Air Control Motor Open/Close
- Radiator Fan Control Module
- A/C Clutch Relay
- Auto Shutdown Relay
- Duty Cycle EVAP Purge Solenoid
- S/C Servo Solenoids
- Generator Field
- Torque Converter Clutch Solenoid
- EGR Solenoid
- All Solenoids/Relays
- Fuel System Test
- Speed Control Vacuum Solenoid
- Speed Control Vent Solenoid
- Fuel Pump Relay
- Set RPM 900
- Set RPM 1,000
- Set RPM 1,100
- Set RPM 1,200
- Set RPM 1,300
- Set RPM 1,400
- Set RPM 1,500
- Set RPM 1,600
- Set RPM 1,700
- Set RPM 1,800
- Set RPM 1,900
- Set RPM 2,000

NOTE: Not all of the above actuation tests may apply to the vehicle being tested.

DIAGNOSTIC TROUBLE CODES
A Diagnostic Trouble Code (DTC) indicates the PCM has recognized an abnormal condition in the system.

Each diagnostic trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of trouble codes as well as no trouble code problems. It is not necessary to perform all of the tests to diagnose an individual code.

Always begin by reading the diagnostic trouble codes using a scan tool. This procedure begins in DTC TEST - Checking the System for Diagnostic Trouble Codes. This will direct you to the specific test(s) that must be performed.

Hard Code
A diagnostic trouble code that comes back within one cycle of the ignition key is a "hard" code. This means that the defect is there every time the powertrain control module checks that circuit or function. Diagnostic Chart test procedures verify if the trouble code is a hard code at the beginning of each test. When it is not a hard code, an "intermittent" test must be performed.

Intermittent Code
A diagnostic trouble code that is not there every time the powertrain control module checks the circuit is an "intermittent" code. Most intermittent codes are caused by wiring or connector problems. Defects that come and go like this are the most difficult to diagnose; they must be looked for under specific conditions that cause them.

Reset Counter
The reset counter counts the number of times the vehicle has been started since codes were last set, erased, or the battery was disconnected. The reset counter will count up to 255 start counts. The number of starts helps determine when the trouble code actually happened. This is recorded by the PCM and can be viewed on the DRB as the RESET COUNTER.

When there are no trouble codes stored in memory, the DRB will display "NO TROUBLE CODES FOUND" and the reset counter will show "RESET COUNT = XXX."

Displaying Codes
The technician can display DTC's in two ways. The first way is to cycle the ignition switch and count the number of times the malfunction indicator (Check Engine) lamp on the instrument panel flashes on and off. The DRB scan tool provides the second method of displaying DTC's. Diagnostic trouble codes are the results of a system or circuit failure, but do not directly identify the failed component or components.

NOTE: For a list of Diagnostic Trouble Code and their descriptions SEE Testing and Inspection/Procedures/Trouble Code Descriptions.
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