1994 GMC Forward Control 6.5 Y, StandardFACTORY OEM SERVICE PROCEDURE

Engine Controls - System/Component Tests - Gasoline: Introduction

Before testing separate components or systems, perform procedures in BASIC TESTING - GASOLINE article in the ENGINE PERFORMANCE section. Since many computer-controlled and monitored components set a trouble code if they malfunction, also perform procedures in appropriate TESTS W/CODES article in the ENGINE PERFORMANCE section below.

  • G - TESTS W/CODES - 4.3L
  • G - TESTS W/CODES - 5.0L
  • G - TESTS W/CODES - 5.7L
  • G - TESTS W/CODES - 7.4L
NOTE: Testing individual components does not isolate shorts or opens. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure. Use ohmmeter and refer to WIRING DIAGRAMS  to isolate wiring harness shorts or opens.

The following table provides the location of commonly used diagnostic information. These former "A" and "C" charts are now written in text and inserted into the new Engine Performance workflow. To familiarize yourself with the Engine Performance workflow, see the AA - USING THIS SECTION (GENERAL HELP INFORMATION) article in GENERAL INFORMATION section.

GENERAL MOTORS "A" & "C" CHART REFERENCE

System or Component Diagnostic Information Location
DIAGNOSTIC CIRCUIT CHECK (1) See DIAGNOSTIC CKT CHECK
A-1 & A-2, Malfunction Indicator Light (MIL) (1) See DIAGNOSTIC CKT CHECK
A-3, No-Start (1) See NO-START DIAGNOSIS
A-5, Fuel Pump Relay See RELAYS, SOLENOIDS, MOTORS & MODULES
A-6 & A-7, Fuel System Diagnosis (1) See BASIC FUEL SYSTEM CHECKS
C-1, MAP Sensor See ENGINE SENSORS & SWITCHES
C-1, Power Steering Pressure Switch See ENGINE SENSORS & SWITCHES
C-1, Park/Neutral Switch See ENGINE SENSORS & SWITCHES
C-2, IAC Valve See IDLE CONTROL SYSTEM
C-3, Canister Purge System See EMISSION SYSTEMS & SUB-SYSTEMS
C-4, EST Ignition Check (1) See BASIC IGNITION SYSTEM CHECKS
C-5, ESC Ignition Check See IGNITION SYSTEM
C-6, Air Injection System See EMISSION SYSTEMS & SUB-SYSTEMS
C-7, EGR System See EMISSION SYSTEMS & SUB-SYSTEMS
C-8, Torque Converter Clutch See MISCELLANEOUS ECM CONTROLS
C-8, Manual Transmission Shift Lights See MISCELLANEOUS ECM CONTROLS
C-10, A/C Clutch Control See MISCELLANEOUS ECM CONTROLS
C-12, Electric Cooling Fan Control (2) See MISCELLANEOUS ECM CONTROLS
Injector Balance Test (PFI) See FUEL CONTROL
(1) See appropriate F - BASIC TESTING - GASOLINE article in the ENGINE PERFORMANCE section.
(2) Covered in entirety in appropriate ENGINE COOLING article.
Factory Service Tools

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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