OBD II regulations require that the functionality of the Catalyst system be monitored. If the catalyst system deteriorates to a point where vehicle emissions increase by more than 1.5 times the Federal Test Procedure (FTP) standard, the MIL must be activated.
The oxygen content in a catalyst is important for efficient conversion of exhaust gases. When a lean air-fuel (A/F) ratio is present for an extended period of time, the oxygen content in the catalyst can reach a maximum. Conversely, when a rich A/F ratio is present for too long a period, the oxygen content in the catalyst can become totally depleted.
Catalyst operation is dependent on its ability to store and release oxygen needed to complete the emissions-reducing chemical reactions. As a catalyst deteriorates, its ability to store oxygen is reduced. Since the catalyst's ability to store oxygen is somewhat related to proper operation, oxygen storage can be used as an indicator of catalyst performance.Comprehensive Component Monitor
OBD II regulations require that all emission related circuits and components controlled by the PCM that could affect emissions are monitored for circuit continuity and out-of-range faults. The Comprehensive Component Monitor (CCM) consists of 4 different monitoring strategies: 2 for inputs and 2 for output signals.
One input strategy is used to check devices with analog inputs for opens, shorts, or out-of-range values. The CCM accomplishes this task by monitoring A/D converter input voltages of various Sensors.
A second input strategy is used to check devices with digital and frequency inputs by performing rationality checks. The PCM uses other Sensor readings and calculations to determine if a Sensor or switch reading is correct under existing conditions. Some tests of the CKP, CMP and VSS run continuously. Other tests run only after actuation.Output Strategies
An Output Device Monitor in the PCM checks for any open and shorted circuits by observing the control voltage level of a particular device.
The control voltage is low when the device is on and the voltage is high when the device is off. Monitored outputs include the AC relay, ASD relay SS1, SS2, SS3, EGR and EVAP and O2 heater control.Crankshaft Relearn
Select Crank Relearn on the scan tool to allow the PCM to enable a fast CMP and CKP Sensor relearn procedure. The PCM relearns the crank target window spacing during Decel Fuel Shutoff mode The PCM reset step requires 3 closed throttle decelerations from 55 mph in order to relearn the Crank Target Spacing.EGR System Monitor
Emissions of NOx increase proportionally with the temperature in the combustion chamber. The Exhaust Gas Recirculation (EGR) system is designed to circulate non-combustible exhaust gases into the manifold to dilute the A/F mixture. In this manner, the EGR system lowers the combustion temperature and reduces NOx emissions and pre-detonation (knocking).EVAP System Monitor
OBD II regulations require that all vehicles equipped with an EVAP system be monitored for component integrity, system functionality and loss of hydrocarbons. The Enhanced EVAP system test is required to detect leaks as small as 0.040" in diameter. The test must detect leaks of 0.040-0.080" in diameter on fuel tanks larger than 25 gallons.Front & Rear Heated Oxygen Sensors
There are 2 Oxygen (O2) Sensors used to monitor the catalyst storage capability. One Sensor is mounted in front of the catalyst (pre-catalyst O2) and another Sensor is mounted in back of the catalyst (post-catalyst O2). By utilizing an O2 Sensor in front of the catalytic converter, and a second Sensor located behind the catalyst, the oxygen storage capability of the catalyst can be determined by comparing the voltage signals from the 2 Sensors.Fuel System Monitor
To comply with clean air regulations, DaimlerChrysler vehicles are equipped with catalytic converters that reduce the amount of HC, CO and NOx emissions released from the vehicle. The catalyst works best when the A/F ratio is at or near an optimum value of 14.7:1. The PCM is programmed to maintain this optimum A/F ratio.
This is accomplished by making Short Term fuel trim corrections in injector pulse width based upon the O2 Sensor input. The programmed memory in the PCM acts as a self-calibration tool to compensate for variations in engine specifications, sensor tolerances and engine fatigue over the life span of the engine. As injector pulse width increases, the amount of fuel delivered by the fuel injector is increased. A shorter pulse width decreases the amount of fuel delivered. The PCM monitors engine load, RPM, TP Sensor, ECT Sensor signals in order to make changes to injector pulse width.Misfire Monitor
OBD II regulations require that the OBD II system monitor the engine for misfire conditions and identify specific cylinders that experience a misfire. The PCM should also identify and set a different code to indicate if a multiple misfire condition exists. It should also be able to identify a specific cylinder that misfires in a multiple misfire condition. This monitor detects misfires related to engine mechanical, ignition or fuel system faults under positive load conditions (idle, cruise, etc.).Oxygen Sensor Heater Monitor
JTEC Key "On" Heater Monitor
This test is for Jeep and Truck models. The operation of this Monitor is similar to the test used for Cars and Minivans except that it is run with the key on or at engine startup. In this particular type of heater test, the PCM sends a 5v bias signal to the Oxygen sensor to measure the amount of voltage decrease as the sensor warms up.Revolution Counter
The PCM contains a program called a Revolution Counter. In the logic portion of this counter, each 1000-revolution window contains five 200-revolution windows. The PCM counts the misfires for each 200-window and carries that value over to the 1000-revolution window.Secondary Air Monitor
This monitor is a PCM diagnostic that monitors the Secondary AIR system for component integrity, system functionality and faults that could cause vehicle tailpipe output levels to exceed 1.5 times the FTP Standard. The Secondary AIR System Monitor is run once per trip once the required enable criteria are met.
Systems & Terminology
In order to diagnose DaimlerChrysler vehicles equipped with an OBD II System, it is important that you understand the terms related to these test procedures. Some of these terms and their definitions are discussed in the next few articles.Cylinder Bank
The cylinder bank identifies the location of a specific component. For example, a specific group of engine cylinders may share a common control sensor and it would be identified as Bank 1, which is the location of cylinder No. 1 while Bank 2 identifies cylinders on the opposite cylinder head.Data Link Connector
DaimlerChrysler vehicles equipped with OBD II Systems use a standardized Data Link Connector (DLC). It is typically located between the left end of the instrument panel and 12 inches past vehicle centerline. The connector is mounted out of sight from the passengers, but should be easy to see from outside by a technician in a kneeling position (door open).DLC Features
The DLC is rectangular in design. It can accept up to 16 terminals. The DLC in the graphic to the right shows many common pin designations, but does not represent any specific vehicle.
OBD II DaimlerChrysler vehicle will have power at pin 16, and ground at pins 4 and 5. All other pins vary by year and model. All applications use the SCI circuits, but the pin assignments vary. All vehicles will utilize either the PCI bus, or the CCD bus circuits.
Both the DLC and Scan Tool have latching features that ensure that the Scan Tool will remain connected to the vehicle during operation.
Common uses of the Scan Tool while connected to the DLC include:
The term "pending code" is used to describe a fault that has been detected once and is stored in memory. This type of fault has not been detected on two consecutive trips (i.e., it has not matured into a hard code).
It is possible to access a "pending code" with a Generic Scan Tool (GST) on most DaimlerChrysler vehicles. Be aware that you may not be able to read a pending code with a Generic Scan Tool on some 1995 phase-in models.Sensor
If sensors are numbered (Bank 1 Sensor 1, or B1 S1), they follow the convention described above. If they are identified with letters ('A', 'B', 'C'), they are manufacturer defined. If only 1 sensor is used, the letter or number may be omitted.Similar Conditions
If a "pending code" is set because of a Fuel System or Misfire Monitor detected fault, the vehicle must meet similar conditions for two consecutive trips before the code matures and the PCM activates the MIL and stores the code in memory. The meaning of similar conditions is important when you attempt to diagnose a fault detected by the Fuel System Monitor or Misfire Detection Monitor.
To achieve similar conditions, the vehicle must reach the following engine running conditions simultaneously (for the first failure recorded that set the code):
In many cases, an emission related system or component must fail a Monitor test more than once before it activates the MIL. The first time an OBD II Monitor detects a fault during a related trip, it sets a "pending code" in PCM memory. These codes appear when the Memory or Continuous codes are read. For a "pending code" to mature into a hard code (and illuminate the MIL), the original fault must occur for two consecutive trips (two-trip detection). However, a "pending code" can remain in the PCM for a long time before the conditions that caused the code to set reappear. Fuel Trim and Misfire Detection trouble codes can cause the PCM to flash the MIL after one trip because faults in these systems can cause damage to the catalytic converter.
A warmup cycle is defined as vehicle operation (after a cool-down period) when the engine temperature increases by at least 40ºF and reaches at least 160ºF.
Most trouble codes are cleared from the PCM memory after 40 "warmup cycles" if the fault does not reappear.
Cherokee (XJ Body Code) & Grand Cherokee (ZJ Body Code)
Grand Cherokee (WK Body Code)