SMU - SECTION 6D4 - REVISED INFORMATION. Chevrolet Caprice 1995

SUBJECT: SERVICE MANUAL UPDATE - SECTION 6D4 - REVISED INFORMATION FOR "GENERAL DESCRIPTION - IGNITION SYSTEM"

MODELS: 1994-96 BUICK ROADMASTER 1994-96 CADILLAC FLEETWOOD 1994-96 CHEVROLET CAPRICE 1995-96 CHEVROLET IMPALA WITH 4.3L AND 5.7L ENGINE (VINS W, P - RPOS L99, LT1)

THIS BULLETIN IS BEING ISSUED TO REVISE INFORMATION FOR "GENERAL DESCRIPTION - IGNITION SYSTEM" IN SECTION 6D4 OF THE SERVICE MANUAL.

GENERAL DESCRIPTION

IGNITION SYSTEM

THE ENGINE IGNITION SYSTEM CONSISTS OF A DISTRIBUTOR, CONTROL CIRCUITS FOR IGNITION TIMING IN THE POWERTRAIN CONTROL MODULE (PCM), A SEPARATE IGNITION COIL/IGNITION CONTROL MODULE, A VACUUM HARNESS, PRIMARY AND SECONDARY WIRING, AND THE SPARK PLUGS. ALL IGNITION TIMING IS CONTROLLED BY THE PCM; THERE IS NO "BYPASS" MODE BUILT INTO THE IGNITION CONTROL MODULE AS IN OTHER IGNITION SYSTEMS.

THE DISTRIBUTOR IS MOUNTED DIRECTLY TO THE CAMSHAFT BEHIND THE WATER PUMP. IT DIRECTS THE SPARK FROM THE IGNITION COIL TO THE PROPER SPARK PLUG SECONDARY WIRE THROUGH A ROTOR. SPARK PLUG SECONDARY WIRE CONNECTORS IN THE CAP ARE ARRANGED FOR CONVENIENCE IN ROUTING SPARK PLUG WIRE HARNESS ASSEMBLIES. THE CORRESPONDING CYLINDER NUMBER IS MOLDED INTO THE DISTRIBUTOR CAP NEXT TO EACH SPARK PLUG SECONDARY WIRE CONNECTOR.

THE DISTRIBUTOR ALSO CONTAINS A SIGNAL DISK AND TWO OPTICAL CAMSHAFT POSITION (CMP) SENSORS THAT PROVIDE SIGNALS TO THE PCM FOR TIMING CONTROL. ONE SENSOR SENDS 360 PULSES PER CAMSHAFT REVOLUTION FOR HIGH-RESOLUTION TIMING CONTROL. THE OTHER SENSES EIGHT DISK SLOTS OF VARIABLE LENGTH, PROVIDING FOUR PULSES PER CRANKSHAFT REVOLUTION FOR A LOW-RESOLUTION SIGNAL THAT TELLS THE PCM WHICH SPARK PLUG IS BEING FIRED. THE PCM CAN DETERMINE IF ONE OF THE TIMING INPUTS IS NOT BEING RECEIVED BY COMPARING THE TWO INPUTS. IF THE PCM DETECTS ONE TIMING PULSE WITHOUT DETECTING THE OTHER TIMING PULSE, DIAGNOSTIC TROUBLE CODES (DTC'S) MAY SET. REFER TO SECTION 6E IN THE SERVICE MANUAL FOR MORE INFORMATION. POWER FOR THE OPTICAL CMP SENSORS, A SYSTEM GROUND, AND THE TWO CMP SENSOR SIGNALS IS TRANSFERRED BETWEEN THE PCM AND THE DISTRIBUTOR THROUGH A SHIELDED HARNESS CONNECTED TO THE FOUR-TERMINAL CONNECTOR ON THE DISTRIBUTOR. THE REFERENCE SIGNALS TOGGLE BETWEEN 0 AND 5 VOLTS AS THE CAMSHAFT TURNS. THEREFORE, AN OPEN CIRCUIT, A SHORT TO VOLTAGE, A SHORT TO GROUND, OR A DEFECTIVE SENSOR INSIDE THE DISTRIBUTOR CAN PREVENT THE VOLTAGE FROM PULSING AT THE PCM.

THE IGNITION COIL/IGNITION CONTROL MODULE PROVIDES SPARK TO THE DISTRIBUTOR, TIMED BY SIGNALS FROM THE PCM. POWER (B+) FOR THE IGNITION COIL PRIMARY CIRCUIT AND THE IGNITION CONTROL MODULE IS SUPPLIED THROUGH THE IGNITION SWITCH. THE PCM COMBINES THE CAMSHAFT POSITION INFORMATION SUPPLIED BY THE DISTRIBUTOR WITH OTHER SYSTEM PARAMETERS AND CALCULATES THE REQUIRED SPARK ADVANCE. THE PCM SIGNALS THE IGNITION CONTROL MODULE, WHICH TURNS ON THE PRIMARY CURRENT TO THE IGNITION COIL BY GROUNDING THE PRIMARY CIRCUIT, AND THEN TURNS IT OFF BY REMOVING THE GROUND. WHEN THE PRIMARY CURRENT FLOW STOPS, HIGH VOLTAGE INDUCED IN THE IGNITION COIL SECONDARY WINDING BECOMES THE SPARK VOLTAGE FOR THE SPARK PLUG. THE SPARK VOLTAGE IS DELIVERED TO THE DISTRIBUTOR THROUGH THE COIL OUTPUT (SECONDARY) WIRE, AND THEN DIRECTED TO THE PROPER SPARK PLUG CONNECTOR BY THE DISTRIBUTOR ROTOR. FOR MORE INFORMATION, REFER TO SECTION 6E3 IN THE SERVICE MANUAL.

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GENERAL MOTORS BULLETINS ARE INTENDED FOR USE BY PROFESSIONAL TECHNICIANS, NOT A "DO-IT-YOURSELFER". THEY ARE WRITTEN TO INFORM THOSE TECHNICIANS OF CONDITIONS THAT MAY OCCUR ON SOME VEHICLES, OR TO PROVIDE INFORMATION THAT COULD ASSIST IN THE PROPER SERVICE OF A VEHICLE. PROPERLY TRAINED TECHNICIANS HAVE THE EQUIPMENT, TOOLS, SAFETY INSTRUCTIONS AND KNOW-HOW TO DO A JOB PROPERLY AND SAFELY. IF A CONDITION IS DESCRIBED, DO NOT ASSUME THAT THE BULLETIN APPLIES TO YOUR VEHICLE, OR THAT YOUR VEHICLE WILL HAVE THAT CONDITION. SEE A GENERAL MOTORS DEALER SERVICING YOUR BRAND OF GENERAL MOTORS VEHICLE FOR INFORMATION ON WHETHER YOUR VEHICLE MAY BENEFIT FROM THE INFORMATION.