Basic Inspection

CAUTION: Never use a bulb type test light to check computer circuits.
- A Digital Volt Ohm Meter (DVOM)--with a minimum 10M ohm input impedance, a logic probe or a lab-type oscilloscope are the only instruments that can be used to check computer circuits without causing damage.

Purpose
Certain basic faults are undetectable by the self-diagnostic system, and in some cases can actually interfere with on-board diagnosis and trouble code memory operation.
- Low battery voltage, for example, can cause erroneous trouble codes to be set in memory, or cause the system to go into "Fail Safe" without setting a code in memory.
- On the other hand, trouble codes are cleared whenever the ECM or the battery is disconnected. Therefore, all trouble codes should be read prior to any vehicle power interruption or troubleshooting.

Check the Basics
Experience the Complaint
Whenever possible personally experience the complaint.
- If you know what the customer wants, you will have a better idea where to begin your diagnosis.

Verify problem is a Malfunction
Automotive systems are easily misunderstood, leading to repairs that attempt to force a system to perform in a way it was never intended to.
- Always compare system operation to normal system operation as described in the owners manual or in Description and Operation.
- If possible compare vehicle operation with that of a known good vehicle.

Check Previous Repairs
Check the vehicle's repair history. Incorrect components or improper repairs can influence seemingly unrelated systems.
- Always attempt to get the "whole story" from the customer.
- If the problem appeared after other repairs were made--verify that the problem is not related to the system or component which was last repaired.

Battery State of Charge
Batteries in a state of partial discharge can have a dramatic effect on Control Modules and related components.

Positive Battery Cable Integrity
Check dynamically using the "voltage drop technique".
- All B+ connections must be in perfect condition for trouble-free electronic system operation.

Negative Ground Connections
Check dynamically using the "voltage drop technique".
- As all electrical circuits are a circle, all ground connections must also 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 perhaps inexplicable component failure.

Power Train Cable Attachments
Metal cables attached to the engine or transmission which appear overheated and/or discolored indicate poor ground connections. See above.

Fusible Link Integrity
Fusible links prevent damage to electrical components and wiring harnesses.
- These links and their connections must be without dynamic resistance. Check using the "voltage drop technique".

Power Supply Relays
Fuel pump and overprotection relays and plug connections can be a source of intermittent operation which may not set a code in memory.

Crankshaft Position Sensor
Monolithic/optical sensors should be checked statically and dynamically. Physical mounting and condition of the sensor and segment wheel must be assured.

High Tension Coil And Secondary Components Wiring
All secondary ignition system components should be inspected for tight connections and freedom from carbon tracking, moisture and corrosion, and system operation should be checked using a suitable engine analyzer and scope.

Fuel Delivery and Pressure
Fuel delivery should begin when cranking is initiated, and the pressure in the fuel rails must be within specifications.
- Fuel pressure retention must be maintained between the fuel pump outlet and the pressure regulator after engine shutdown.

Valve Clearance and Spark Plugs
- Valve clearance adjustment procedures should be strictly observed.
- Spark plug type, condition and gap must verified according to specification.

NOTE: If either of the above is out of specification, engine performance can suffer significantly.

Air/Fuel Ratio
When troubleshooting idle quality or driveability complaints, 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