Engine Controls Self-Diagnostics - Cherokee: Other Jeep Cherokee III

Malfunction Indicator Light (MIL)

Note. If MIL does not come on for 2 seconds and then go off, bulb may be defective, or problem may exist on Chrysler Collision Detection (CCD) bus circuit between Powertrain Control Module (PCM) and instrument cluster. Check instrument cluster for inoperative MIL. See ACTUATOR TEST under SYSTEM TESTS in INSTRUMENT PANELS - CHEROKEE article in ACCESSORIES & EQUIPMENT for diagnosis.

MIL may also be referred to as CHECK ENGINE light. MIL is located on right side of instrument cluster and is represented by an engine symbol. When ignition is first turned on, MIL should come on and remain on for 2 seconds to verify bulb and circuit operation, and then go off.

PCM monitors several different engine control system circuits. PCM is located in engine compartment on left front fender, near radiator support. If a malfunction occurs which affects vehicle emissions, a Diagnostic Trouble Code (DTC) will be stored in PCM memory and PCM will enter limp-in mode. In limp-in mode, PCM substitutes values for the failed component to continue engine operation, but a loss of good driveability may result. When DTC is stored in PCM memory, PCM will turn on MIL and MIL will remain on steady. If problem exists which does not affect vehicle emissions, a DTC will be stored in PCM but MIL will not be turned on.

PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the criteria for the DTC has not been met. Criteria may be a specific engine RPM range, engine temperature and/or voltage input to PCM. For example, assume criteria for DTC requires PCM to monitor a specified sensor output circuit only when engine operates at 750-2000 RPM. If sensor output circuit becomes shorted to ground when engine is operating at 2400 RPM, DTC will not be stored, as short to ground occurred at 2400 RPM which exceeds the limit when PCM monitors the circuit.

If PCM detects an active engine misfire severe enough to cause catalytic converter damage, MIL will immediately start flashing to warn driver of possible catalytic converter damage and DTC will be stored. PCM may turn off MIL if malfunction is not detected during 3 consecutive trips or 3 consecutive engine misfire monitor tests or fuel system monitor tests. or additional information on good trips, see TRIP INDICATOR . For additional information on engine misfire monitor and fuel system monitor, see MONITORED CIRCUITS . PCM performs engine misfire and fuel system monitor tests within predetermined engine speed (RPM) and load operating conditions.

DTCs may only be retrieved for system diagnosis by using a scan tool. See RETRIEVING DIAGNOSTIC TROUBLE CODES . System malfunctions are identified as either hard failures or intermittent failures. For additional information on hard failures or intermittent failures, see HARD FAILURES or INTERMITTENT FAILURES .

Monitored Circuits

The PCM contains electronic circuit monitors that monitor fuel, vehicle emissions, engine and ignition system performance. Monitors use information from various sensor circuits for system monitoring. Monitors do not indicate a component failure, but indicate an implied failure within a specified system, and that problem must be diagnosed. If any monitor detects a problem affecting vehicle emissions, a Diagnostic Trouble Code (DTC) will be stored in PCM memory. The following monitors are used.

1. Engine Misfire Monitor
2. Fuel System Monitor
3. Oxygen Sensor Monitor
4. Oxygen Sensor Heater Monitor
5. Catalyst Monitor
6. EVAP System Leak Detection Pump Monitor

After 3 good trips, PCM will turn off the MIL. At that time, the PCM will automatically switch the trip counter to a warm-up cycle counter. The PCM will erase DTCs after 40 warm-up cycles if the recent malfunction does not reoccur in that time. For additional information on good trips and warm-up cycles, see TRIP INDICATOR .

Non-Monitored Circuits

The PCM does not monitor all circuits, conditions and systems that could cause a malfunction or driveability problem. However, problems with these systems may cause PCM to store DTCs for other systems or components.

For example, a fuel pressure problem will not cause a DTC directly, but could cause a rich/lean condition or misfire. This could cause PCM to store an oxygen sensor DTC. The major non-monitored circuits are identified as follows.

1. Cylinder Compression
2. Excessive Oil Consumption
3. Exhaust System
4. Fuel Injector Mechanical Faults
5. Fuel Pressure
6. PCM Connector Engagement
7. PCM System Ground
8. Secondary Ignition System
9. Throttle Body Airflow
10. Vacuum Assist

Trip Indicator

The trip is essential for running monitors and turning off the Malfunction Indicator Light (MIL). A trip is defined as a set of vehicle operating conditions that must be met for a monitor to run. All trips begin with an ignition key cycle. Good trip counters are: good trip, fuel system good trip and misfire good trip.

1. Specific Good Trip The term good trip has different meanings depending on the circumstances. If the MIL is off, a good trip is defined as when the oxygen sensor monitor and the catalyst monitor have completed in the same drive cycle. If the MIL is on and a DTC was set by the fuel system monitor or misfire monitor, vehicle must be operated in the similar conditions window for a specified amount of time. If MIL is on and a DTC was set by a PCM task manager commanded once-per-trip monitor (oxygen sensor/heater monitor, catalyst monitor, purge flow monitor, leak detection monitor or EGR monitor), a good trip is when the monitor is passed on the next engine start-up. If the MIL is on and any other emissions DTC is set (not an OBD-II monitor), a good trip is when the oxygen sensor monitor and catalyst monitor have completed, or 2 minutes of engine run time has occurred (if the oxygen sensor monitor and catalyst monitor has stopped running). NOTE: For more information on similar conditions window, see «SIMILAR CONDITIONS WINDOW»(ref-11706-S31328209332000112700000) .
2. Fuel System Good Trip To count a good trip (3 required) and turn off the MIL, the following conditions must be met. Engine must be in closed loop, must be operating in similar conditions window and short term multiplied by long term must be less than threshold value.
3. Misfire Good Trip If operating in similar conditions window and 1000 engine revolutions have occurred with no misfires, the PCM will count one good trip (3 required) in order to turn off MIL.
4. Warm-Up Cycles Once the MIL has been turned off by the good trip counter, the PCM will automatically switch to a warm-up cycle counter that can be viewed by the DRBIII(R) scan tool. Warm-up cycles are used to clear DTCs and freeze frame data from PCM memory. Forty warm-up cycles are necessary to clear DTCS and freeze frame data. A warm-up cycle is defined as the engine is started, an increase of 40°F (4°C) in engine coolant temperature exists after engine is started and engine coolant temperature reaches at least 160°F (71°C).

Similar Conditions Window

Similar conditions window data can be viewed by using the DRBIII(R) scan tool. The similar conditions window displays information about the engine operation during a monitor. Engine load (absolute manifold pressure) and engine RPM are stored in this window when a failure occurs. There are 2 different similar conditions windows, fuel system and misfire. For more information on similar conditions windows, see SELF-DIAGNOSTIC SYSTEMS in THEORY & OPERATION article.

Hard Failures

Hard failures cause Malfunction Indicator Light (MIL) to illuminate and remain on until problem is repaired. If MIL comes on and remains on during vehicle operation, cause of malfunction must be determined by retrieving Diagnostic Trouble Codes (DTC). See RETRIEVING DIAGNOSTIC TROUBLE CODES . If a sensor fails, PCM will use substitute value in its calculations to continue engine operation. In this condition, commonly known as limp-in mode, vehicle runs but driveability will not be optimum.

Intermittent Failures

Intermittent failures may cause Malfunction Indicator Light (MIL) to flicker or illuminate and go out after intermittent failure goes away. However, the corresponding Diagnostic Trouble Code (DTC) will be retained in Powertrain Control Module (PCM) memory. If related failure does not reoccur within a certain time frame, related DTC will be erased from PCM memory. Most intermittent failures are caused by a sensor, electrical connector or wiring related problems. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES article.

SERVICE PRECAUTIONS

Before proceeding with system diagnosis, following precautions must be followed

1. Ensure fuel pressure is released before disconnecting any fuel line or fuel fitting, as fuel system is under pressure and may cause personal injury. See «FUEL SYSTEM PRESSURE RELEASE»(ref-11706-S07847350592000112700000) .
2. When using diagnostic tests for diagnosis, DO NOT skip any steps or incorrect diagnosis may result. When instructed to disconnect any connector, check that the connector is clean and in good condition after disconnecting the connector. Repair or clean connector as necessary. Always perform indicated verification procedure after repairs are made. Verification tests are listed at the end of the diagnostic tests.
3. When using a jumper wire, ensure either jumper wire or circuit is fuse-protected.
4. Ensure ignition is off before disconnecting connector from any control module.
5. When checking voltage or continuity at any control module, probe connector for control module from pin side. DO NOT probe wire through insulation or backprobe connector (unless instructed to do otherwise in test procedure).
6. DO NOT cause short circuits when performing electrical tests. This will set additional Diagnostic Trouble Codes (DTC), making diagnosis of original problem more difficult.
7. DO NOT prolong testing of fuel injectors or engine may hydrostatically lock.
8. If replacing PCM, correct vehicle mileage and Vehicle Identification Number (VIN) must be programmed into PCM to prevent DTCs from being set in Anti-Lock Brake System (ABS) module, Air Bag Control Module (ACM) and Sentry Key Immobilizer Module (SKIM), if equipped. See «PROGRAMMING»(ref-11706-S25146649042000112700000) .
9. If replacing PCM on models with a Sentry Key Immobilizer Module (Skim), secret key data must be updated to enable engine starting. To update secret key data, see «PROGRAMMING»(ref-11706-S25146649042000112700000) .

FUEL SYSTEM PRESSURE RELEASE

1. Remove fuel pump relay from Power Distribution Center (PDC). PDC is located in right side of engine compartment, next to battery. (Scheme 11) Start and run engine until it stalls. Attempt to start engine. Continue restarting engine until it will no longer run. Turn ignition off.
2. Disconnect any fuel injector connector. Connect a jumper wire between either fuel injector terminal and positive battery terminal. Connect another jumper wire to other fuel injector terminal. Momentarily touch other end of jumper wire to negative battery terminal. CAUTION: DO NOT supply power to fuel injector for more than 4 seconds, or fuel injector may be damaged.
3. Place a shop towel under fuel line quick-connector at fuel rail. Use care when disconnecting fuel lines, as some fuel pressure may still exist in fuel lines. Disconnect fuel line quick-connector at fuel rail. Reinstall fuel pump relay in PDC. Reconnect fuel line quick-connector NOTE: One or more Diagnostic Trouble Codes (DTC) may set when fuel pump relay is removed.
4. Erase any DTCs that may have set after fuel pressure release procedure. See «CLEARING DIAGNOSTIC TROUBLE CODES»(ref-11706-S01383277712000112700000).

Scheme 11

POWERTRAIN CONTROL MODULE

Note. If replacing Powertrain Control Module (PCM), the correct vehicle mileage and Vehicle Identification Number (VIN) must be programmed into PCM to prevent Diagnostic Trouble Codes (DTC) from being stored in Anti-Lock Brake System (ABS) module and Air Bag Control Module (ACM). If replacing PCM on models equipped with a Sentry Key Immobilizer Module (SKIM), the secret key data must also be updated to enable engine starting.

Programming Powertrain Control Module

Connect scan tool to Data Link Connector (DLC) located below instrument panel, near steering column. see scheme 1 Using scan tool, enter correct VIN and mileage into PCM. Using scan tool manufacturer's instructions, clear DTCs from ABS module and ACM.

Updating Secret Key Data

1. Connect scan tool to Data Link Connector (DLC) located below instrument panel, near steering column. see scheme 1
2. Ensure transmission is in Park (A/T) or Neutral (M/T). Turn ignition on. Using scan tool, select THEFT ALARM, SKIM and then MISCELLANEOUS. Select PCM REPLACED. Place the SKIM in SECURED ACCESS MODE by entering the appropriate Personal Identification Number (PIN) for this vehicle. PIN may be obtained from the owner, vehicle's invoice, or from the manufacturer. Press ENTER to transfer secret key data from Sentry Key Immobilizer Module (SKIM) to the PCM. This will enable the vehicle to start.
3. If 3 attempts are made to enter the SECURED ACCESS MODE by using an incorrect PIN, the SECURED ACCESS MODE will be locked out for one hour. To exit lock out mode, leave ignition on with all accessories turned off. After one hour, enter correct PIN. It may be necessary to monitor battery voltage and connect a battery charger.

SUMMARY

If no hard failure DTCs are present, driveability symptoms exist or intermittent DTCs exist, proceed to TROUBLE SHOOTING - NO CODES article for diagnosis by symptom.

VERIFICATION TESTS

Note. Perform the following verification test only when directed to. Only verification tests that are applicable to diagnostic tests in this article have been included.