Strategy-based troubleshooting (Chevrolet Captiva 1)

Strategy-Based Diagnosis is a unified approach to repairing all electrical/electronic (E/E) systems. The diagnostic process can always be used to solve E/E problems and is the starting point for repairs. The following steps guide the technician through the diagnostic process:

• Ensure that the customer's complaint is confirmed. To ensure that the customer's complaint is confirmed, the technician must be familiar with the normal operation of the system.
• Conduct preliminary checks as follows:
• Conduct a thorough visual inspection.
• View the archive of completed repairs.
• Identify unusual sounds or smells.
• Collect diagnostic trouble code information to ensure effective repairs.
• Check bulletins and other service information. This includes videos, newsletters, etc.
• View service information (instructions) to check the system.
• See service diagnostics.

No faults found

This condition occurs when the vehicle is considered to be operating normally. The condition described by the customer may be normal. Verify the customer's complaint based on another vehicle that operates normally. The condition may be unstable. Verify the complaint under the conditions described by the customer before releasing the vehicle.



Double-check the complaint.

If the complaint cannot be successfully found or localized, a re-evaluation is necessary. The complaint should be re-examined and may be intermittent as defined in the Intermittent Faults section or may be a normal condition.

Once the cause has been localized, repairs should be made. Ensure normal operation and that the symptom has been corrected. This may include a road test or other methods necessary to confirm that the problem has been corrected under the following conditions:

• Terms and conditions specified by the client.
• If a fault code has been diagnosed, check the repair under the same conditions in which the diagnostic fault code was set, as recorded in the fault protocols and in the status record data.

Check car repairs.

The vehicle repair check will be more complete on vehicles with a diagnostic system. When performing repairs, the technician must perform the following steps:

Important: Follow the steps below when checking for On-Board Diagnostic System (OBD) repairs. Failure to follow these steps may result in unnecessary repairs.

• View and record Failure Records and Freeze Frame data for diagnosed fault codes (status record data is stored only for type A or E diagnostics and only if the MIL has been requested).
• Clear diagnostic trouble codes.
• Perform the trip under the conditions recorded in the malfunction log and condition record data.
• Check the DTC status information for the trouble code that was diagnosed while performing the diagnostic test for that DTC.

Ease of maintenance of the on-board diagnostic system

Based on the on-board diagnostic (OBD) system experience of 1994 and 1995 model year vehicles, a list of non-automotive faults has been compiled that may affect the performance of the OBD system. These non-automotive faults are dependent on environmental conditions and the quality of the fuel used. With the introduction of OBD diagnostics for the passenger car and light truck market in 1996, the MIL illuminated by non-automotive faults may result in vehicle misdiagnosis, increased warranty costs, and customer dissatisfaction. The following list of non-automotive faults is not inclusive of all possible faults and does not apply equally to all model lines.

Fuel quality

Fuel quality is not a new topic in the automotive industry, but its impact on the illumination of the malfunction indicator lamp with OBD systems is new.

Fuel additives such as "dry gas" or "octane correctors" can affect fuel performance. If this results in incomplete or partial combustion, DTC P0300 will set. Vapor pressure can cause problems in the fuel system, especially in the fall and spring with large changes in ambient temperature. High vapor pressure can appear as a fuel trim DTC due to excessive loading of the carbon canister. High vapor pressure in the fuel tank can also affect the fuel vapor emission diagnostic.

Using fuel with the wrong octane rating can cause drivability problems. Many major fuel companies advertise premium gasoline as a way to improve your vehicle's performance. Most premium grades use alcohol to increase the octane rating. Although alcohol additives increase octane, they reduce the ability to evaporate in cold temperatures. This reduces cold engine starting and performance.



Low fuel level can result in fuel starvation, lean mixture and possibly misfires.

Non-original units

All OBD diagnostics have been configured to work with original (OEM) components. A common situation is that a powerful exhaust system, acting on back pressure, can affect the operation of the EGR valve and thus turn on the malfunction indicator lamp. Small leaks in the exhaust system near the oxygen sensor of the catalytic converter can also lead to the malfunction indicator lamp being turned on.

Additional electronic equipment such as cell phones, stereos, and anti-theft alarms can cause electromagnetic interference if improperly installed. This can cause false sensor readings and turn on the malfunction indicator lamp.

Environment

Temporary environmental conditions such as local flooding affect the operation of the vehicle's ignition system. If the ignition system is exposed to rain, it may cause misfires and the malfunction indicator lamp to come on.

Car delivery

Transportation of new vehicles from assembly plants to dealers involves at least 60 ignition cycles over 2 to 3 miles of travel. This type of driving contributes to contamination of the spark plugs and leads to the activation of the fault indicator lamp and the installation of DTC P0300.

Poor maintenance

The sensitivity of the OBD diagnostic system will cause the malfunction indicator lamp to illuminate if the vehicle has been improperly repaired. Clogged air and fuel filters, deposits in the crankcase due to poor oil circulation or improper oil viscosity can cause faults that were not detected before the OBD test. Poor maintenance cannot be classified as a "non-vehicle fault", but due to the sensitivity of OBD diagnostics, the maintenance schedule should be followed as closely as possible.

Strong vibration

Misfire diagnostics measure small changes in crankshaft speed. Severe driveshaft vibration caused by excessive wheel dirt can have the same effect on crankshaft speed as a misfire and can therefore set DTC P0300.

Side system failures

Many OBD diagnostics may not work if the ECM detects a fault in a dependent system or component. For example, if the ECM detects a misfire, the catalytic converter diagnostic will be suspended until the misfire fault is corrected. If the misfire fault is severe enough, the catalytic converter may be damaged due to overheating and will not set a catalytic converter diagnostic trouble code until the misfire fault is corrected and the converter diagnostic is complete. In this case, the customer will have to visit the shop twice to have the vehicle repaired.

Serial data transfer interface

GMLAN serial data interface

The General Motors Local Area Network (GMLAN) of a vehicle is a family of serial communication buses (subnets) that allow electronic control devices (ECU or nodes) maintain communication with each other or with the diagnostic tester.

GMLAN supports three buses, a high-speed two-wire bus, a medium-speed two-wire bus, and a single-wire low-speed bus.

• High speed tire (500 kbps) - usually used to share real-time data such as driver-specified torque, actual engine torque, steering angle, etc.
• Medium speed tire (approximately 95.2 kbps) - usually used for information support (display, navigation, etc.), where system response time requires that a large amount of data be transmitted in a relatively short time, such as updating the display of graphical information.
• Low speed tire (33.33 kbps) - typically used for driver-controlled devices where system response time requirements are in the order of 100-200 ms. This bus also supports high-speed operation at 83.33 kbps, used only when reprogramming the ECU.

The decision to use a particular bus on a given vehicle depends on what functions are shared among the various ECUs on that vehicle.



GMLAN buses use the Controller Area Network (CAN) communications protocol. Data is packaged into CAN messages, which are segmented into CAN "frames." Each CAN frame includes header data (also known as CAN identifier, or CANId) and a maximum of eight (8) bytes of data. A message may consist of a single frame or multiple frames, depending on the number of data bytes that define the complete message. Channel arbitration occurs only on the header, or CANId, portion of the frame.

On-Board Diagnostic (OBD) Checks

Diagnostics is a sequence of steps that result in a report from the executive program about the success or failure of diagnostics. If the diagnostic check is passed, the diagnostic executive program records the following data:

• The diagnostic check after the last ignition cycle is complete.
• The diagnostic check passed during the current ignition cycle.
• The fault identified by the diagnostic test is not currently active.

If the diagnostic check fails, the diagnostic executive program records the following data:

• The diagnostic check after the last ignition cycle is complete.
• The fault identified by the diagnostic test is now active.
• The fault was active during this ignition cycle.
• Operating parameters at the time the fault occurred.

Remember that a fuel trim diagnostic trouble code can be caused by a list of vehicle faults. Use all available information (other stored diagnostic trouble codes, lean or rich) when diagnosing a fuel injection adjustment malfunction.

General diagnostics of vehicle systems

General vehicle system diagnostics are required to monitor the input and output signals of emission-related transmission components.

Input components

Input components are monitored for continuity and out-of-range signals. This includes a plausibility check. A plausibility check determines if the signal received from a sensor is correct, i.e., a throttle position sensor indicating a high throttle position at low engine load or a low MAP sensor signal. Input components may include, but are not limited to, the following sensors:

• Vehicle Speed Sensor (VSS)
• Crankshaft Position Sensor (CKP)
• Throttle Position Sensor (TP)
• Coolant temperature sensor (ECT).
• Camshaft position sensor (CMP).
• Manifold Absolute Pressure (MAP) sensor.

In addition to circuit integrity and plausibility testing, the coolant temperature sensor is monitored for its ability to achieve a constant temperature for closed loop fuel control.

Output components

Output components are tested for correct responses to control module commands. Components that cannot be functionally tested are monitored for circuit integrity and out-of-range signals. Output components may include, but are not limited to, the following sensors:

• Idle speed control motor.
• Electromagnetic valve for purging the SUPB adsorber, controlled by the control module.
• Air conditioner relay.
• Cooling system fan.
• VSS output.
• Control of the malfunction indicator lamp.

Cm. "ECU Controller" and sensors in this section.

Passive and active diagnostic tests

A passive diagnostic test simply checks the vehicle's systems and components. In contrast, an active test takes some action when performing diagnostic functions, often in response to a passive test that failed. For example, an active EGR diagnostic test causes the EGR valve to open under braking with the valve closed and/or to close under steady travel. Either action should result in a change in manifold pressure.

Diagnostic tests with change of operating modes

These are any on-board diagnostic checks by the vehicle's control system that may affect the vehicle's performance or emissions levels.

Heating cycle

The warm-up cycle means the engine temperature must reach a minimum of 160°F (70°C) and drop at least 72°F (22°C) during the trip.

Recording the state

Recording the state (Freeze Frame) - is a diagnostic control system component that stores various information about the vehicle at the time an emission control fault is stored and the malfunction indicator lamp is illuminated. This data can help determine the cause of the fault.

Fault log

Failure History is an advanced feature of the OBD Record Status. Failure History stores the same information as the Record Status, but it stores information for any fault in the on-board memory, while the Failure History only stores information for emission control faults that activate the Malfunction Indicator Lamp.