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6E–62 ENGINE DRIVEABILITY AND EMISSIONS Diagnostic Thought Process As you follow a diagnostic plan, every box on the • Improves your understanding and definition of the customer complaint • Saves time by avoiding testing and/or replacing good parts • Allows you to look at the problem from different perspectives • Guides you to determine what level of understanding about system operation is needed: – Owner’s manual level – Service manual level – In-depth (engineering) level – Owner’s manual level – Service manual level – In-depth (engineering) level 1. Verify the Complaint What you should do To verify the customer complaint, you need to know the • WHAT the vehicle model/options are • WHAT aftermarket and dealer-installed accessories exist • WHAT related system(s) operate properly • WHEN the problem occurs • WHERE the problem occurs • HOW the problem occurs • HOW LONG the condition has existed (and if the system ever worked correctly) • HOW OFTEN the problem occurs • Whether the severity of the problem has increased, decreased or stayed the same What resources you should use Whenever possible, you should use the following • Service manual Theory or Circuit Description sections • Service manual “System Performance Check” • Owner manual operational description • Technician experience • Identical vehicle for comparison • Circuit testing tools • Vehicle road tests • Complaint check sheet • Contact with the customer 2. Perform Preliminary Checks NOTE: An estimated 10 percent of successful vehicle What you should do You perform preliminary checks for several reasons: • To detect if the cause of the complaint is VISUALLY OBVIOUS • To identify parts of the system that work correctly • To accumulate enough data to correctly and accurately search for a ISUZU Service Bulletin on The initial checks may vary depending on the • Operate the suspect system • Make a visual inspection of harness routing and accessible/visible power and ground circuits • Check for blown fuses • Make a visual inspection for separated connectors • Make a visual inspection of connectors (includes checking terminals for damage and tightness) • Check for any DTCs stored by the on-board computers • Sense unusual noises, smells, vibrations or movements • Investigate the vehicle service history (call other dealerships, if appropriate) What resources you should use Whenever appropriate, you should use the following • Tech II or other technical equipment for viewing DTCs • Service manual information: – Component locations – Harness routing – Wiring schematics – Procedures for viewing DTCs • Dealership service history file • Vehicle road test • Identical vehicle or system for comparison |