Powertrain Control Module Description Part 2

Typical OBD II Drive Cycle

Primary System Based Diagnostics
There are primary system based diagnostics which evaluate the system operation and their effect on vehicle emissions. The primary system based diagnostics are listed below with a brief description of the diagnostic functionality.

Oxygen Sensor Diagnosis
Diagnose the Fuel Control Oxygen Sensors (O2S) for the following conditions:
- Slow Response
- Response Time (time to switch R/L or L/R)
- Inactive Signal (output steady at bias voltage - approximately 450 mV)
- Signal Fixed High
- Signal Fixed Low

Diagnose the Catalyst Monitor Heated Oxygen Sensors (HO2S) for the following functions:
- Heater Performance (time to activity on cold start)
- Signal fixed low during steady state conditions or power enrichment (hard acceleration when a rich mixture should be indicated)
- Signal fixed high during steady state conditions or decel fuel mode (deceleration when a lean mixture should be indicated)
- Inactive Sensor (output steady at approximately 438 mV)

Fuel Control Oxygen Sensors (O2S)
The main function of the fuel control heated oxygen sensor is to provide the control module with exhaust stream information in order to allow proper fueling and maintain emissions within the mandated levels. After the sensor reaches the operating temperature, the sensor generates a voltage inversely proportional to the amount of oxygen present in the exhaust gases.
The control module uses the signal voltage from the fuel control heated oxygen sensors in a closed loop in order to adjust the fuel injector pulse width. While in a closed loop, the Control Module can adjust fuel delivery in order to maintain an air to fuel ratio which allows the best combination of emission control and driveability.
If the oxygen sensor pigtail wiring, connector or terminal are damaged, replace the entire oxygen sensor assembly. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, the sensor must have a clean air reference provided to it. This clean air reference is obtained by way of the oxygen sensor wires. Any attempt to repair the wires, connectors or terminals could result in the obstruction of the air reference. Any attempt to repair the wires, connectors or terminals could degrade oxygen sensor performance.

HO2S Heater
The oxygen sensor heaters are required by catalyst monitor sensors to maintain a sufficiently high temperature which allows accurate exhaust oxygen content readings further from the engine.

Catalyst Monitor Heated Oxygen Sensors (HO2S)
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the system uses a three~way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen.
The Control Module has the ability to monitor this process using the HO2S heated oxygen sensors. The HO2S sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the O2S signal will be far more active than that produced by the HO2S sensor.

Catalyst Monitor Diagnostic Operation
The OBD II catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, the heated sensors are installed before and after the Three-Way Catalyst (TWC). The voltage variations between the sensors allow the control module to determine the catalyst emission performance.
As a catalyst becomes less effective in promoting chemical reactions, the capacity of the catalyst to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on a correlation between conversion efficiency and oxygen storage capacity.
A good catalyst (e.g. 95% hydrocarbon conversion efficiency) shows a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S). A degraded catalyst (65% hydrocarbon conversion) shows a greatly increased activity in output voltage from the post catalyst HO2S.
The post-catalyst HO2S 2 is used to measure the oxygen storage and release capacity of the catalyst. A high oxygen storage capacity indicates a good catalyst; low oxygen storage capacity indicates a failing catalyst. The TWC, HO2S 2, and HO2S 3 must be at operating temperature in order to achieve correct oxygen sensor voltages like those shown in the Post-Catalyst HO2S 3 Outputs graphic.
The catalyst monitor diagnostic is sensitive to the following conditions:
- Exhaust leaks
- HO2S Contamination
- Alternate fuels
Exhaust system leaks may cause the following results:
- Prevent a degraded catalyst from failing the diagnostic
- Cause a false failure for a normally functioning catalyst
- Prevent the diagnostic from running Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of these contaminants prevents the TWC diagnostic from functioning properly.








Three-Way Catalyst Oxygen Storage Capacity
The control module must monitor the Three-Way catalyst system (TWC) for efficiency. In order to accomplish this, the control module monitors the pre-catalyst and post-catalyst oxygen sensors. When the TWC is operating properly, the post-catalyst (2) oxygen sensor will have significantly less activity than the pre-catalyst (1) oxygen sensor. The TWC stores the oxygen as needed during its normal reduction and oxidation process. The TWC releases oxygen as needed during its normal reduction and oxidation process. The control module calculates the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor voltage levels. Whenever the voltage levels of the post-catalyst (2) oxygen sensor nears the voltage levels that of the pre-catalyst (1) oxygen sensor, the efficiency of the catalyst is degraded.
Stepped or staged testing levels allow the control module to statistically filter the test information. This prevents falsely passing or falsely failing the oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. For this reason, do not use post catalyst oxygen sensor activity in order to determine the oxygen storage capacity unless directed by the electronic service information.
Three stages are used in order to monitor catalyst efficiency. Failure of the first stage indicates that the catalyst requires further testing in order to determine catalyst efficiency. Failure of the second stage indicates that the catalyst may be degraded. The third stage then looks at the inputs from the pre and post 02 sensors more closely before determining if the catalyst is indeed degraded. This further statistical processing is done in order to increase the accuracy of the oxygen storage capacity type monitoring. Failing the first (stage 0) or the second (stage 1) test Does Not indicate a failed catalyst. The catalyst may be marginal or the fuel sulfur content could be very high.
Aftermarket HO2S characteristics may be different from the original equipment manufacturer sensor. This may lead to a false pass or a false fail of the catalyst monitor diagnostic. Similarly, if an Aftermarket catalyst does not contain the same amount of cerium as the original part, the correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.





Evaporative Emission (EVAP) Purge System Vacuum Switch
The EVAP system uses a switch located in the closed, applying a 12 volt signal to the control purge line between the canister and the purge valve module as a NO PURGE signal. When the canister in order to detect when the purge is occurring. This purging occurs, the switch opens, interrupting OFF switch senses the flow from the engine through the the 12 volt signal to the control module. A scan tool purge valve. When no purge is present, the switch is display will indicate that the purge is occurring.
Clogging of the canister fresh air vent could allow the purge hose between the switch and the canister to trap the vacuum with the purge valve closed. This would result in a diagnostic indication of a purge valve stuck open or a vacuum switch failure. Similarly, leaks or blockages in the purge hoses may result in misdiagnosis of the purge valve or the vacuum switch.
When servicing a purge valve diagnostic trouble code, check the canister fresh air vent, the vacuum switch and the integrity of all of the purge hoses prior to servicing the valve.

Misfire Monitor Diagnostic Operation
The misfire monitor diagnostic is based on the crankshaft rotational velocity (reference period) variations. The control module determines the crankshaft rotational velocity using the crankshaft position sensor and the camshaft position sensor. When a cylinder misfires the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the control module can calculate when a misfire occurs.
For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 100~3200 engine revolutions.
For a catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions. Rough roads may cause a false misfire detection. A rough road applies torque to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity. The Control module detects this as a false misfire. On the automatic transmission equipped vehicles, the Torque Converter Clutch (TCC) will disable whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs on the crankshaft rotation.
When the TCC has disabled as a result of misfire detection, the TCC will re-enabled after approximately 3200 engine revolutions if no misfire is
detected. The TCC remains disabled whenever the misfire is detected, with or without a DTC set. This allows the misfire diagnostic to reevaluate the system.
During a transmission high temperature condition, the misfire diagnostic will disable and the TCC will operate normally. This avoids further increasing the temperature of the transmission.





Misfire Counters
Whenever a cylinder misfires, the misfire diagnostic counts the misfires.Then the misfire diagnostic notes the crankshaft position at the time the misfire occurred. These misfire counters are basically a file on each engine cylinder.
A current and a history misfire counter is maintained for each cylinder. The misfire current counters (Misfire Cur #1 - 6) indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counters displays real time data without a misfire DTC stored.
The misfire history counters (Misfire Hist #1 - 6) indicate the total number of cylinder firing events which were misfires. The misfire history counters displays 0 until the misfire diagnostic has failed and a DTC P0300 Is set.
Once the misfire DTC sets, the misfire history counters will be updated every 200 cylinder firing events.
The Misfire counters graphic illustrates how these misfire counters are maintained. If the misfire diagnostic reports a failure, the Diagnostic Executive reviews all of the misfire counters before reporting a DTC. This way, the Diagnostic Executive reports the most current information.
When crankshaft rotation is erratic, the control module detects a misfire condition. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring.
The Misfire Counters graphic shows there are misfires counted from more than one cylinder. Cylinder #1 has the majority of counted misfires. In this case, the Misfire Counters would identify cylinder #1 as the misfiring cylinder. The misfires in the other counters were just background noise caused by the erratic rotation of the crankshaft. If the number of accumulated is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300 - Misfire Detected.
Use Techline equipment to monitor misfire counter data on OBD II compliant vehicles. knowing which specific cylinders misfired can lead to the root cause, even when dealing with a multiple cylinder misfire. Using the information in the misfire counters, identify which cylinders are misfiring. If the counters indicate cylinders number 1 and 4 misfired, look for a circuit or component common to both cylinders number 1 and 4 such as an open ignition coil in an electronic ignition system.
Misfire counter information is located in the Specific Eng. menu, Misfire Data sub-menu of the of the data list.
The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:
- Contaminated fuel
- Running out of fuel
- Fuel fouled spark plugs
- Basic engine fault

Fuel Trim System Monitor Diagnostic Operation
This system monitors the averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If either value is outside their thresholds, a rich or lean DTC will set.
In order to meet OBD II requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if fuel trim counts in the weighted fuel trim cells exceed specifications. This means that the vehicle could have a fuel trim problem which is causing a concern under certain conditions (i.e. engine idle high due to a small vacuum leak or rough due to a large vacuum leak) while it operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or O2S DTC may set). Remember, use a scan tool in order to observe fuel trim counts while the problem is occurring. Remember, a fuel trim DTC may be triggered by a list of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition,etc.) when diagnosing a fuel trim fault.

Comprehensive Component Monitor Diagnostic
The comprehensive component monitoring diagnostics are required to monitor emissions-related input and output Powertrain components. The CARB OBD II Comprehensive Component Monitoring List of Components Intended To Illuminate The MIL is a list of components, features or functions that could fall under this requirement.

Input Components
The control module monitors the input components for circuit continuity and out-of-range values. This includes performance checking. Performance
checking refers to indicating a fault when the signal from a sensor does not seem reasonable (i.e. a Throttle Position (TP) sensor that indicates high throttle position at low engine loads or MAP voltage). The input components may include but are not limited to the following sensors:
- The Vehicle Speed Sensor (VSS)
- The Crankshaft Position (CKP) sensor
- The knock Sensor (KS)
- The Throttle Position (TP) sensor
- The Engine Coolant Temperature (ECT) sensor
- The Camshaft Position (CMP) sensor
- The Manifold Absolute Pressure (MAP) sensor
- The Mass Air Flow (MAF) sensor In addition to the circuit continuity and rationality check, the ECT sensor is monitored for its ability to achieve a steady state temperature in order to enable a closed loop fuel control.

Output Components
The Output Components respond to control module commands. Components where functional monitoring is not feasible will be monitored for circuit continuity and out-of-range values if applicable.
Output components to be monitored include, but are not limited to the following circuits:
- The Idle Air Control (IAC) Motor
- The Control module controlled EVAP Canister Purge Valve
- The Electronic transmission controls
- The A/C relay
- The Cooling fan relay
- The VSS output
- The MIL control The Cruise control inhibit





California Air Resources Board (CARB) OBD II Comprehensive Component Monitoring List of Components intended to Illuminate MEL

Important: Not all vehicles have these components.

Harness Service
The control module harness electrically connects the control module to the various solenoids, switches, and sensors in the vehicle engine room and passenger compartment.
Replace the wire harnesses with the proper part number replacement. When splicing signal wires into a harness, use the wiring that has high temperature insulation
Consider the low amperage and voltage levels utilized in the Powertrain control systems. Make the best possible bond at all splices. Use rosin~ore solder in these areas.
Molded-on connectors require complete replacement of the connector. Splice a new connector Into the harness. Replacement connectors and terminals are listed in Group 8.g85 in the Standard Parts Catalog. For wiring repair refer to Wiring Repairs.

Connectors and Terminals
In order to prevent shorting between opposite terminals, use care when probing a connector and when replacing terminals. Damage to the components could result.
Always use Jumper wires between connectors for circuit checking.
Never probe through the Weather-Pack seats. Use the tachometer adapter J 35512, or the equivalent, which provides a convenient connection to the tachometer lead. The connector test adapter kit J 35616, or the equivalent, contains an assortment of flexible connectors used In order to probe the terminals during the diagnosis. The fuse remover and the test tool BT-5616, or the equivalent, is used for removing a fuse and to adapt the fuse holder to a meter for diagnosis.
Open circuits are often difficult to locate by sight because oxidation or terminal misalignment are hidden by the connectors. Merely wiggling a connector on a sensor or In the wiring harness may temporarily correct the open circuit. Oxidized or loose connections may cause intermittent problems.
Be certain the type of connector and terminal before making any connector or terminal repair. Weather-Pack and Corn-Pack Ill terminals look similar, but are serviced differently.