Initial Inspection and Diagnostic Overview

DIAGNOSTIC NOTES
Diagnosing problems in modern automobiles can sometimes be very frustrating and confusing, especially when it involves computer controlled systems. The fact is, for the most part automotive electronics have been proven quite reliable, and the greatest number of problems with new cars are the same kinds of problems that older cars without computer controls have.

Before suspecting a computer problem, perform a careful visual inspection. Check under the hood for the same kinds of problems you would look for on a non-computer controlled engine. These include fluid leaks, vacuum leaks, dirty filters, overheating, oil burning, poor connections or loose wires, bad spark plug wires and/or spark plugs, restricted mufflers and exhaust systems, worn mechanical parts, exhaust leaks, and other familiar kinds of problems. Be thorough! You may save a lot of time.

Begin all troubleshooting by Checking the Basics. Certain basic faults may be undetectable by the Powertrain Control Module (PCM) self-diagnostic system and can actually interfere with self-checking and fault memory operation. Low battery voltage, for example, can cause erroneous faults to set in PCM memory or can cause a system to go "Fail Safe" without setting a fault in memory. Because system fault memory is cleared whenever PCM or battery are disconnected, fault codes should be read prior to any vehicle power interruption or troubleshooting. Prior to any teardown, repair or component replacement, consider the following steps:

COMPLAINT VERIFICATION
Whenever possible, the technicians should personally verify the complaint. Having experienced the malfunction, technicians are less likely to attempt repairing non-existent faults.

MALFUNCTION VERIFICATION
Sophisticated automotive systems are easily misunderstood. This can lead to repairs that attempt to force a system to perform as it was never intended. The troubleshooting technician should compare actual system operation to nominal system operation as described in Description and Operation. The technician is also encouraged to compare problem vehicle system operation with a known good vehicle.

PREVIOUS REPAIRS
Vehicle repair history can provide explanations to unusual complaints which seem to elude normal troubleshooting attempts. Incorrect components or unapproved repairs can have subtle influences on seemingly unrelated systems.

BATTERY STATE OF CHARGE
Batteries in a state of partial discharge can have a dramatic effect on the PCM and related electrical components.

POSITIVE BATTERY CABLE INTEGRITY
All B+ connections must be in perfect condition for trouble-free electronic system operation.

FUSIBLE LINK INTEGRITY
Fusible links prevent possible damage to electrical components and wiring harnesses. These links and their connections must be without dynamic resistance. Dynamic resistance can only be checked using the voltage drop method of testing.

NEGATIVE GROUND CONNECTIONS
All B- connections must be checked and verified to be in perfect condition. A poor "common" ground point will cause seemingly unrelated systems to influence one another. High current systems which encounter a poor "common" ground can back feed through other electrical systems, causing unusual operation and inexplicable component failure. Electrical system ground side should be checked dynamically using the voltage drop technique.

POWERTRAIN CABLE ATTACHMENTS
Metal cables attached to engine or transmission which appear overheated and/or discolored indicate the need to thoroughly test all ground connections.

POWER SUPPLY RELAYS
Fuel pump and main relays, as well as their plug connections, can be a source of intermittent operation which will not set a code in PCM fault memory.

HIGH TENSION COIL AND SECONDARY COMPONENTS WIRING
Secondary ignition system should be checked visibly as well as with a suitable engine analyzer and scope. Check components for tight connections and freedom from carbon tracking, moisture and corrosion.

FUEL DELIVERY AND FUEL RAIL PRESSURE
Fuel delivery must begin at once when cranking and pressure in the fuel rails must be within specifications. Fuel pressure must be maintained between fuel pump outlet and pressure regulator after engine shutdown.

SPARK PLUG CLEARANCE
Spark plug gap, if out of specification, can significantly impair engine performance. Verify spark plug type, condition, and gap according to specification.

VACUUM AND VAPOR HOSE INTEGRITY
Vacuum and vapor hoses must be routed correctly and not leaking. Always use the underhood label for primary information.

AIR/FUEL RATIO
Adaptive engine management systems efficiently compensate for conditions which may affect the combustion process. However, when troubleshooting idle quality or driveability complaints, it is still necessary to consider the following:
- Injector spray pattern quality
- Presence of unmetered air leaks
- Evaporative purge system
- Excessive engine oil dilution
- Substandard fuel or unapproved additives
- Carbon build-up

TESTING EQUIPMENT
High quality testing equipment is essential for accurate results. Faulty equipment results in erroneous test results. Use only suitable test leads, terminals, and probe tips.