Checking the Basics

CAUTION: NEVER USE A BULB TYPE TEST LIGHT TO CHECK COMPUTER CIRCUITS. A quality digital volt ohm meter, logic probe or a lab-type oscilloscope are the only instruments that can be used to check computer circuits without damaging digital circuits.

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"--also known as "Limp-In" Mode 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.

CHECKING THE BASICS
Complaint Verification - Verify the complaint when possible--to avoid attempts to repair non-existent faults and shorten diagnostic time considerably.

Malfunction Verification - Verify system malfunction by comparing system operation to expected system operation. SEE Description and Operation.
- Compare system operation with that of a "known good vehicle" when possible.

Previous Repairs - Check the vehicle's repair history. Incorrect components or improper repairs can influence seemingly unrelated systems.

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.

Powertrain 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 - Either of these items can significantly impair engine performance.
- Valve clearance adjustment procedures should be strictly observed.
- Spark plug type, condition and gap must verified according to specification.

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

VISUAL INSPECTION

NOTE: Inspect all related components to avoid overlooking an obvious problem.

Electrical Connections
Terminals in connectors must lock together. Look for connections that are not fully plugged into each other or terminals that are not fully plugged into the insulator. Also make sure the wiring between each connector and where it goes into the convolute of the harness is not pinched behind any screws, bolts, or is rubbing on metal which would result in a short to ground.

In addition to all electronic component connections the following electrical connections should also be checked for integrity:

Under Hood
- Battery cables
- Engine to firewall ground strap
- Spark plug wires
- Fuses and fusible links

Passenger Compartment
- ECM/MPI Control Module
- MPI control relay
- Fuse Block

Underbody Connection
- In-Tank fuel pump

Hose And Vacuum Line Connections
All hose and vacuum line connectors must be fully and firmly fitted at their connections. Also they cannot be pinched anywhere along their routing. Look for hoses and vacuum lines that are not fully plugged in or are pinched, melted, or cut. Also make sure hoses are not egg-shaped at their connection points causing a leak.