KELSEY HAYES 4WAL BRAKE SYSTEM OPERATION INFORMATION. Oldsmobile Bravada 1991

KELSEY HAYES 4WAL IS THE STANDARD EQUIPMENT ANTILOCK BRAKE SYSTEM USED ON 1991 BRAVADAS. THIS SYSTEM USES CONVENTIONAL HYDRAULIC BRAKES WITH ADDED-ON ELECTRO-HYDRAULIC COMPONENTS. 4WAL PROVIDES INDEPENDENT ELECTRONIC CONTROL OF THE LEFT FRONT, RIGHT FRONT, AND REAR HYDRAULIC CHANNELS (THREE-CHANNEL SYSTEM) DURING ANTILOCK BRAKING. AN ELECTRONIC CONTROL UNIT (ECU) MONITORS FOUR WHEEL SPEED SENSORS AND THE BRAKE PEDAL SWITCH TO DETERMINE WHEN WHEEL LOCK-UP CONTROL IS NEEDED. THE ANTILOCK HYDRAULIC CONTROL CIRCUIT IS INTEGRATED WITH THE EBCM. AS WITH ALL ANTILOCK BRAKE SYSTEMS, 4WAL ASSISTS THE DRIVER IN MAINTAINING VEHICLE CONTROL DURING ADVERSE BRAKING CONDITIONS.

4WAL COMPONENTS

THE BRAVADA 4WAL BRAKING SYSTEM CONSISTS OF THREE GROUPS OF PARTS: - BASE HYDRAULIC BRAKES - MASTER CYLINDER - VACUUM POWER BOOSTER - CALIPER AND WHEEL CYLINDERS - COMBINATION VALVE

ELECTRONIC BRAKE CONTROL MODULE (EBCM), FORMERLY LABELED ELECTRO-HYDRAULIC CONTROL UNIT (EHCU) - ELECTRONIC CONTROL UNIT (ECU) - HYDRAULIC CONTROL UNIT (HCU) - PUMP MOTOR AND ITS RELAY-CONTROLLED CIRCUIT

EXTERNAL EBCM ELECTRICAL CIRCUITS. - FOUR WHEEL SPEED SENSORS - BRAKE SWITCH - POWER CIRCUITS - ALDL TERMINAL "H" - "BRAKE" LIGHT CIRCUIT - "ANTILOCK" LIGHT CIRCUIT

BASE HYDRAULIC BRAKE FUNCTIONS ARE THE SAME AS NON-ANTILOCK SYSTEMS. DRIVER BRAKE PEDAL FORCE GENERATES HYDRAULIC PRESSURE AT THE FRONT AND REAR MASTER CYLINDER PORTS. DURING NON-ANTILOCK BRAKING, THIS PRESSURIZED FLUID FLOWS THROUGH THE COMBINATION VALVE AND EBCM TO THE FRONT AND REAR HYDRAULIC CHANNELS.

ON BRAVADA APPLICATIONS, THE EBCM IS MOUNTED TO THE INNER FENDER. LOCATED WITHIN THE EBCM, THE ELECTRONIC CONTROL UNIT IS A MICROPROCESSOR THAT MONITORS 4WAL INPUTS AND USES A STORED PROGRAM TO OPERATE THE HYDRAULIC COMPONENTS OF THE EBCM. THE ECU WILL OPERATE TWO VALVES AND A PUMP TO OBTAIN THREE MODES OF HYDRAULIC CONTROL ACTION IN ANY ONE CHANNEL. THESE MODES ARE ISOLATE PRESSURE, DECREASE PRESSURE, AND INCREASE PRESSURE.

THE HYDRAULIC CONTROL UNIT (HCU) OF THE EBCM CONSISTS OF THE FOLLOWING PARTS (SEE FIGURE 1);

ISOLATION SOLENOID-OPERATED (ISO) VALVE EACH CHANNEL (RIGHT FRONT, LEFT FRONT, AND REAR) CONTAINS ONE ISOLATION SOLENOID-OPERATED (ISO) VALVE. THE ISOLATION VALVES ARE NORMALLY IN THE OPEN POSITION ALLOWING UNRESTRICTED FLUID TO THE WHEEL CIRCUITS. WHEN THE ECU DETERMINES ANTILOCK INTERVENTION IS NEEDED, THE VALVE CLOSES TO ISOLATE THE HYDRAULIC CIRCUIT. HYDRAULIC FLUID IS TRAPPED IN THE ISOLATED CIRCUIT. THIS IS THE PRESSURE ISOLATE MODE.

PULSE-WIDTH MODULATION (PWM) VALVE EACH CHANNEL CONTAINS ONE PWM VALVE. THE PWM VALVE IS COMPUTER MODULATED ON AND OFF BY THE ECU. IT WILL CYCLE ONLY IF THE ISOLATION VALVE IS IN THE PRESSURE MAINTAIN MODE. WHEN THE VALVE IS ON, IT DIRECTS FLUID TO THE LOW PRESSURE ACCUMULATOR FOR TEMPORARY STORAGE AND BLOCKS HIGH PRESSURE PUMP OUTLET FLUID. THIS CAUSES THE HYDRAULIC CHANNEL PRESSURE TO DECREASE; THIS IS THE PRESSURE DECREASE MODE. WHEN THE VALVE IS MODULATED "OFF", FLUID IS DIRECTED TO THE WHEEL CIRCUIT.

LOW PRESSURE ACCUMULATOR (LPA) EACH FRONT AND REAR CHANNEL IS CONNECTED TO AN LPA. THE LPA IS A SPRING-LOADED ACCUMULATOR USED TO TEMPORARILY STORE FLUID PURGED FROM A HYDRAULIC CIRCUIT DURING THE PRESSURE DECREASE MODE. IT ALSO RECEIVES PUMP OUTLET PRESSURE THAT HAS ENTERED THE HIGH PRESSURE ACCUMULATOR (HPA) AND MOVED PAST THE BYPASS VALVE. WHEN THE SYSTEM IS NOT IN AN ANTILOCK STATE, THE ACCUMULATORS ARE EMPTY.

HIGH PRESSURE ACCUMULATOR (HPA) EACH FRONT AND REAR CHANNEL IS CONNECTED TO AN HPA. THE HPA IS A SPRING-LOADED ACCUMULATOR THAT STORES HYDRAULIC FLUID UNDER PRESSURE TO BE USED DURING A PRESSURE INCREASE MODE. EACH PISTON HAS A BYPASS PASSAGE AND VALVE THAT LIMITS HPA PRESSURE FROM PUMP PRESSURE. AS ANTILOCK BRAKING OCCURS, EACH CHANNEL PUMP MOVES FLUID INTO ITS RELATED HPA CIRCUIT. AS THIS PRESSURIZED FLUID MOVES THE HPA PISTON, THE STEM OF THE BYPASS VALVE BOTTOMS IN ITS BORE. FURTHER PISTON TRAVEL OPENS THE BYPASS PASSAGE TO THE LPA AND PUMP INLET.

CHECK VALVE A CHECK VALVE IS LOCATED IN THE PASSAGE BETWEEN EACH ISO AND PWM VALVE. THIS SPRING ASSISTED CHECK VALVE, ALONG WITH ISOLATED FLUID PRESSURE, KEEP THE HPA/PUMP OUTLET CIRCUIT FROM FEEDING INTO THE WHEEL CYLINDER/CALIPER CIRCUIT DURING THE DECREASE PRESSURE AND ISOLATE PRESSURE MODES.

RESET SWITCH THE RESET SWITCH MONITORS THE HYDRAULIC CIRCUITS FOR AN IMBALANCE IN SYSTEM PRESSURE. THE VALVE IS CENTERED IN ITS BORE BY EQUALIZED PRESSURES AND A RETURN SPRING. WHEN THE VALVE HAS A DIFFERENCE IN PRESSURES, IT WILL BE FORCED TO SLIDE IN ITS BORE. WHEN THE VALVE MAKES CONTACT WITH THE BORE, IT OPENS THE ELECTRICAL CONTACTS AND ALERTS THE ECU OF A PRESSURE IMBALANCE. WHEN THE VALVE RETURNS TO ITS BALANCED POSITION (CONTACTS CLOSED), THE ECU RESETS THE SYSTEM AND PREPARES IT FOR THE NEXT HYDRAULIC CIRCUIT PRESSURE MODIFICATION. IN THE NON-ANTILOCK MODE, THE RESET SWITCH ALERTS THE ECU TO ANY SYSTEM PRESSURE IMBALANCE, AS IN THE CASE OF A HYDRAULIC FAILURE IN THE BASE BRAKE SYSTEM. AT THIS POINT, IT WILL DISABLE ABS FUNCTION AND ILLUMINATE THE "ANTILOCK" LIGHT.

HYDRAULIC PUMPS DURING ANTILOCK BRAKING CONTROL, THE ECU OPERATES THE ELECTRIC MOTOR THAT DRIVES THE PUMPS FOR THE FRONT AND REAR CHANNELS. A RELAY WITHIN THE EHCU IS USED TO COMPLETE THE PUMP MOTOR CIRCUIT. A FUSIBLE LINK-PROTECTED FEED WIRE AND GROUND WIRE THAT IS EXTERNAL OF THE ECU, PROVIDE THE POWER-SUPPLY CIRCUIT FOR THE PUMP MOTOR. THE PUMPS PROVIDE FLUID MOVEMENT UNDER HIGH PRESSURE FOR THE TWO HPA CIRCUITS. THEY ALSO CREATE LOW PRESSURE (PUMP INLET PRESSURE) NEEDED FOR LPA CIRCUIT OPERATION.

THE EBCM ALSO CONTAINS INTERNAL AND EXTERNAL ELECTRICAL CONNECTIONS FOR THE ECU AS WELL AS SERVICE AIR BLEED SCREWS FOR INTERNAL PASSAGES AND AIR BLEED VALVES FOR THE ACCUMULATORS.

HYDRAULIC CONTROL UNIT OPERATION

NON-ANTILOCK BRAKING (SEE FIGURE 2) FOR EACH CHANNEL, FLUID UNDER PRESSURE FROM THE MASTER CYLINDER MOVES THROUGH THE ISO VALVE (SOLENOID DE-ENERGIZED) TO THE PWM VALVE. EACH PWM VALVE (SOLENOID DE-ENERGIZED) ALLOWS THE FLUID FROM THE ISO VALVE TO MOVE TO THE OUTLET PORT FOR ITS RELATED CHANNEL UNDER MASTER CYLINDER PRESSURE. THE RESET VALVE FOR EACH CHANNEL DOES NOT MOVE BECAUSE THE DIFFERENCE IN PRESSURE IS NOT GREAT ENOUGH. THE PUMP MOTOR CIRCUIT DOES NOT OPERATE BECAUSE THE ECU HAS DETECTED NO NEED FOR ANTILOCK BRAKING. LPA AND HPA CIRCUITS (FRONT AND REAR) HAVE RESIDUAL PRESSURE.

PRESSURE ISOLATE (SEE FIGURE 3) THE ECU HAS DETECTED A NEED FOR ANTILOCK CONTROL. FLUID UNDER PRESSURE FROM THE MASTER CYLINDER IS BLOCKED AT THE ISO VALVE (SOLENOID ENERGIZED) FOR THE CHANNEL BEING CONTROLLED. THE PWM VALVE FOR THAT CHANNEL REMAINS DE-ENERGIZED WHICH ALLOWS THAT WHEEL CIRCUIT TO MAINTAIN PRESSURE. BECAUSE THE DIFFERENCE IN PRESSURES IS NOT GREAT ENOUGH, THE RESET VALVE HAS NOT MOVED. THE ECU ACTIVATES THE RELAY FOR THE PUMP MOTOR CIRCUIT AND THE MOTOR ARMATURE OPERATES THE CHANNEL PUMPS. THE CHANNEL PUMPS PRESSURIZE THE FRONT AND REAR HPA CIRCUITS (BYPASS VALVES MAY BE OPEN). THESE PRESSURIZED ACCUMULATORS ARE READY TO BE USED IN A PRESSURE INCREASE BRAKING MODE. THE CHECK VALVE BETWEEN THE ISO AND PWM VALVES, ALONG WITH ISOLATED CHANNEL PRESSURE, KEEP THE HPA CIRCUIT FROM PUSHING FLUID INTO THE ISOLATED CHANNEL.

PRESSURE DECREASE (SEE FIGURE 4) FOR THE CHANNEL BEING CONTROLLED, MASTER CYLINDER FLUID PRESSURE STOPS AT THE ISO VALVE (SOLENOID ENERGIZED). THE ECU COMPLETES THE PWM VALVE SOLENOID CIRCUIT (MODULATED ON) AND CONNECTS THE LPA/PUMP INLET HYDRAULIC CIRCUIT TO THE WHEEL CYLINDER/CALIPER SIDE OF THE CHANNEL. THIS RESULTS IN A CHANNEL PRESSURE DECREASE ACTION. THE RESET VALVE MAY MOVE IF THE PRESSURE DIFFERENCE IS GREAT ENOUGH. THE PWM VALVE ALSO BLOCKS THE HPA CIRCUIT FROM PUSHING HIGH PRESSURE FLUID THROUGH THE CHECK VALVE AND INTO THAT CHANNEL. THE ECU CONTINUES PUMP MOTOR OPERATION AND THE CHANNEL PUMPS KEEP HIGH PRESSURE FLUID IN THE HPA CIRCUITS (BYPASS VALVES MAY BE OPEN). FRONT AND REAR LPA CIRCUITS HAVE LOW PRESSURE AND PROVIDE PUMP INLET PRESSURE (THE LPA CONNECTED TO THE CHANNEL BEING CONTROLLED RECEIVES LOW PRESSURE FLUID FROM THE WHEEL CYLINDER/CALIPER CIRCUIT).

PRESSURE INCREASE (SEE FIGURE 5) FOR THE CHANNEL BEING CONTROLLED, FLUID UNDER PRESSURE FROM THE MASTER CYLINDER STOPS AT THE ISO VALVE. THE ECU OPENS THE PWM VALVE SOLENOID CIRCUIT (MODULATED OFF), AND THE PUMP OUTLET/HPA PRESSURIZED FLUID MOVES THROUGH THE CHECK VALVE TO THE WHEEL CYLINDER/CALIPER SIDE OF THE CHANNEL. THE CHECK VALVE WILL CLOSE AFTER WHEEL CYLINDER/CALIPER PRESSURE IS STABLE. THIS ACTION RESULTS IN A CHANNEL PRESSURE INCREASE. THE RESET VALVE MOVES TO ITS ORIGINAL POSITION AS PRESSURES BALANCE. THE ECU CONTINUES TO OPERATE THE PUMP MOTOR AND THE CHANNEL PUMPS KEEP HIGH PRESSURE FLUID IN THE HPA CIRCUITS (THE HPA FOR THE CHANNEL BEING CONTROLLED WILL HAVE A MOMENTARY DROP IN PRESSURE DURING THE PRESSURE INCREASE MODE). LPA CIRCUITS HAVE LOW PRESSURE AND PROVIDE PUMP INLET FLUID.

ELECTRICAL PARTS

LOCATED IN THE EBCM, THE 4WAL ELECTRONIC CONTROL UNIT IS A MICROCOMPUTER WHICH MONITORS THE BRAKE SYSTEM. THROUGH THE BRAKE INPUT SWITCH, THE ECU DETERMINES WHETHER THE BRAKES ARE BEING APPLIED. IF THE ECU SENSES IMPENDING WHEEL LOCKUP, IT PREPARES THE SYSTEM FOR POSSIBLE ANTILOCK BRAKING. BY COMPARING WHEEL SPEED SENSOR SIGNALS TO A PROGRAM STORED IN MEMORY, THE ECU CONTROLS THE HYDRAULIC CONTROL UNIT (HCU) PORTION OF THE EBCM.

THE ECU WILL DISABLE ANTILOCK CONTROL, COMPLETE THE "ANTILOCK" LIGHT CIRCUIT , AND STORE TROUBLE CODES IF IT DETECTS ONE OF THE FOLLOWING CONDITIONS WITH ITSELF: - RANDOM-ACCESS MEMORY (RAM) ERROR (CODE 71) - READ-ONLY MEMORY (ROM) ERROR (CODE 72) - INTERNAL CIRCUIT ERROR (CODE 73) - INTERNAL CIRCUIT ERROR CAUSING EXCESSIVE ISOLATION TIME (CODE 74)

IT ALSO HAS THE ABILITY TO DETECT 4WAL COMPONENT PROBLEMS AND TO TAKE DIAGNOSTIC ACTION.

WHEEL SPEED SENSORS THE 4WAL SYSTEM USES WHEEL SPEED SENSORS AT EACH WHEEL TO SEND INPUT SIGNALS TO THE ECU. EACH SENSOR HAS A METAL RING, CALLED A TONE RING OR SENSOR RING, WITH TEETH ON ITS OUTSIDE DIAMETER. ON THE FRONT WHEELS, THE RING IS ATTACHED TO THE HUB/BEARING ASSEMBLY. IN THE REAR, THE METAL RINGS ARE FASTENED TO THE AXLE SHAFTS. A MAGNETIC PICKUP MOUNTS IN FRONT TO THE FRONT STEERING KNUCKLE SPLASH SHIELD AND IN THE REAR TO THE AXLE HOUSING FLANGE BACKING PLATE. A NONADJUSTABLE AIR GAP OF APPROXIMATELY 1.27 MM (0.50 IN.) EXISTS BETWEEN THE METAL RING AND MAGNET/COIL. AS THE TEETH OF THE TONE RING MOVE PAST THE MAGNETIC FIELD OF THE SENSOR, AN AC VOLTAGE SIGNAL IS GENERATED. THIS AC VOLTAGE INCREASES OR DECREASES PROPORTIONALLY TO THE SPEED OF THE WHEEL. THE ECU MONITORS THIS SIGNAL TO CHECK FOR A SUDDEN CHANGE IN SINGLE OR MULTIPLE WHEEL DECELERATION RATE(S). IF THE DECELERATION(S) IS (ARE) NOT WITHIN A PREDETERMINED AMOUNT, ANTILOCK INTERVENTION IS NECESSARY.

THE ECU WILL DISABLE ANTILOCK CONTROL, COMPLETE THE "ANTILOCK" LIGHT CIRCUIT, AND STORE TROUBLE CODES IF IT DETECTS THE FOLLOWING CONDITIONS WITH ANY OR ALL OF THE WHEEL SPEED SENSORS: - INCORRECT CIRCUIT RESISTANCE WHEN CHECKED WITH NO VEHICLE MOVEMENT (THE ECU WILL "LATCH" THESE CODES AFTER THE FIRST BRAKE APPLICATION ABOVE 8 MPH). - INCORRECT SENSOR VOLTAGE OUTPUT DURING VEHICLE MOVEMENT (THE ECU DETECTS NO SIGNAL WHEN VEHICLE SPEED IS ABOVE 8 MPH). - ERRATIC SENSOR VOLTAGE OUTPUT DURING VEHICLE MOVEMENT.

BRAKE PEDAL SWITCH THE BRAKE PEDAL SWITCH IS A SIGNAL CIRCUIT TO THE ECU. WHEN THE DRIVER APPLIES THE BRAKE PEDAL, THE CIRCUIT VOLTAGE AT ECU TERMINAL C IS ZERO VOLTS. A RELEASED BRAKE PEDAL WILL CLOSE THE SWITCH CONTACTS, PROVIDING 12 VOLTS AT TERMINAL C. THESE SIGNALS TELL THE ECU: - NOT TO PREPARE FOR POSSIBLE ANTILOCK BRAKING (12 VOLTS) - TO GET READY FOR HCU OPERATION DURING ANTILOCK BRAKING (LOW OR 0 VOLTS)

THE ECU WILL DISABLE ANTILOCK CONTROL, COMPLETE THE "ANTILOCK" LIGHT CIRCUIT, AND STORE A TROUBLE CODE IF IT DETECTS A CONTINUOUS LOW-VOLTAGE BRAKE SWITCH INPUT (BRAKES APPLIED SIGNAL) WHEN THE IGNITION SWITCH HAS BEEN TURNED TO "RUN" AND THE VEHICLE IS MOVING AT A SPEED ABOVE 35 MPH FOR 10 SECONDS. THIS INDICATES THAT THE BRAKE SWITCH IS OPEN CONTINUOUSLY.

"ANTILOCK" LIGHT AN AMBER-COLORED "ANTILOCK" WARNING LAMP IS USED TO ALERT THE DRIVER OF A 4WAL SYSTEM MALFUNCTION. THE ECU COMPLETES THE "ANTILOCK" LIGHT CIRCUIT WHEN IT DETECTS A MALFUNCTION. THE ECU WILL OPERATE THE LIGHT IN ONE OF TWO MODES: 1. LIGHT STEADY: - DURING AN INITIAL TEST AT THE BEGINNING OF AN IGNITION CYCLE. - WHEN A SYSTEM MALFUNCTION EXISTS. 2. FLASH ON AND OFF (TO PRESENT TROUBLE CODE INFORMATION) WHEN A TECHNICIAN GROUNDS THE ALDL DIAGNOSTIC TERMINAL "H".

AN INPUT MONITOR CIRCUIT WITHIN THE ECU ALLOWS IT TO DETECT AN OPEN OR SHORTED CIRCUIT FOR THE "ANTILOCK" LIGHT DURING THE INITIAL TEST FOR EACH IGNITION CYCLE. IF THE ECU DETECTS A MALFUNCTION IT WILL TAKE DIAGNOSTIC ACTION BY: - COMPLETING THE "BRAKE" LIGHT TO ALERT THE DRIVER. - STORING A CODE 86.

"BRAKE" LIGHT A RED "BRAKE" LIGHT IS USED TO ALERT THE DRIVER TO THE FOLLOWING CONDITIONS: - PARKING BRAKE APPLIED (PARKING BRAKE SWITCH IS GROUNDED) - A HYDRAULIC FAILURE IN EITHER THE FRONT OR REAR CHANNEL (WARNING SWITCH IN COMBINATION VALVE IS GROUNDED) - AN ANTILOCK LIGHT CIRCUIT PROBLEM EXISTS (ECU CONTROLLED GROUND) THE ECU COMPLETES THE "BRAKE" LIGHT CIRCUIT FOR THREE REASONS: - TO CONDUCT A "BRAKE" LIGHT CIRCUIT CHECK AT THE BEGINNING OF EACH IGNITION CYCLE. - TO ALERT THE DRIVER OF A 4WAL MALFUNCTION RELATED TO THE FOUNDATION BRAKES. - AN ANTILOCK LIGHT CIRCUIT PROBLEM EXISTS. THE ECU ALSO MONITORS THE "BRAKE" LIGHT CIRCUIT AT THE BEGINNING OF EACH IGNITION CYCLE. IF THE ECU DETECTS A SHORT IN THE "BRAKE" LIGHT CIRCUIT, IT WILL STORE A CODE 88.

DIAGNOSIS

4WAL OVERALL DIAGNOSTIC SEQUENCE INVOLVES THESE STEPS (FOR COMPLETE BRAKE SYSTEM DIAGNOSTICS USE THE 1991 BRAVADA SERVICE MANUAL SECTIONS 5A THROUGH 5A4) 1. PERFORM THE DIAGNOSTIC CIRCUIT CHECK. 2. FOLLOW THE CHART RELATED TO A STORED CODE. IF NO CODES ARE STORED, REFER TO SYMPTOM DIAGNOSIS (PAGE 5A4- 28). 3. MAKE THE REPAIR TO A RELATED CODE OR SYMPTOM. 4. CLEAR THE CODES FROM THE ECUS MEMORY. 5. REPEAT THE DIAGNOSTIC CIRCUIT CHECK.

A TECHNICIAN CAN FOLLOW THE CHARTS OF THE DIAGNOSTIC CIRCUIT CHECK BASED ON OBSERVING THE FLASHING "ANTILOCK" LIGHT OR BY READING THE CODES WITH A TECH 1 SCAN TOOL. TO USE THE FLASHING "ANTILOCK" LIGHT METHOD, DO THE FOLLOWING: - TURN THE IGNITION SWITCH TO THE "RUN" POSITION. - INSTALL A JUMPER WIRE BETWEEN ALDL H AND A (OR CHASSIS GROUND). - WAIT TWO SECONDS. - WATCH THE "ANTILOCK" LIGHT FLASH ON AND OFF IN A MANNER SIMILAR TO THE "SERVICE ENGINE SOON" LIGHT FOR THE FUEL AND EMISSIONS SYSTEMS.

CODES CAN BE CLEARED USING A TECH 1 SCAN TOOL OR BY DOING THE FOLLOWING: 1. TURN THE IGNITION SWITCH TO THE "ON" POSITION. 2. USE A JUMPER WIRE TO GROUND ALDL TERMINAL H TO A FOR TWO SECONDS. 3. REMOVE WIRE FOR ONE SECOND. 4. REPEAT THE GROUNDING FOR TWO SECONDS. 5. AN INDICATION OF SUCCESS, IS WHEN THE "ANTILOCK" AND "BRAKE" LIGHT CYCLE ON AND OFF AFTER THE JUMPER WIRE IS REMOVED FOR THE SECOND TIME.

A TECHNICIAN MAY ALSO REQUEST THE ECU TO PERFORM A FUNCTION TEST BY GROUNDING ALDL TERMINAL H TO A ONCE FOR 2 SECONDS. THIS WILL ACTIVATE THE PUMP MOTOR AND VALVES. THE IGNITION MUST BE CYCLED AFTER EACH ATTEMPT TO PREVENT A FALSE CODE FROM BEING SET. THE FUNCTION TEST IS USED IN DIAGNOSING AND BLEEDING AIR FROM THE 4WAL SYSTEM.

A COMPLETE AND THOROUGH METHOD OF DIAGNOSING THE 4WAL SYSTEM SHOULD INCLUDE THE USE OF A T-100 (CAMS) OR TECH 1 SCAN TOOL. WITH THE T100/ TECH 1 A TECHNICIAN: - CHECKS THE ALDL CONNECTOR TERMINAL H VOLTAGE. - VIEWS DIAGNOSTIC DATA: - TROUBLE CODES - LF, RF, LR, RR WHEEL SPEED (MPH) - LF, RF, REAR CHANNEL RESET SWITCH STATUS (OPEN/CLOSED) - BRAKE SWITCH STATUS (ON/OFF) - "ANTILOCK" LIGHT STATUS (ON/OFF) - "BRAKE" LIGHT STATUS (ON/OFF) - SOFT FAULT STATUS (ON/OFF) - PERFORMS AN ELECTRO-HYDRAULIC FUNCTION TEST. - CHECKS TROUBLE CODES CURRENTLY STORED BY THE 4WAL EBCM IN BLOCK 0. - CLEARS THE EBCM TROUBLE CODE MEMORY. - PERFORMS SNAPSHOT SAMPLING OF THE DIAGNOSTIC DATA LISTED ABOVE. - VIEWS MISCELLANEOUS DATA: - 4WAL EBCM PROM IDENTIFICATION - HISTORY CODE INFORMATION WHEN DIAGNOSING WITH THE TECH 1 SCAN TOOL, BE SURE TO USE A TECH 1 WITH A SERIAL NUMBER HIGHER THAN XX002000. ALSO, REFERENCE DEALER TECHNICAL BULLETIN 91-T-134 REGARDING INCORRECT DATA DISPLAY WHEN USING THE TK02640A RWAL/4WAL CARTRIDGE.

BRAKE SYSTEM BLEEDING PROCEDURE WHEN BRAKE SYSTEM BLEEDING IS REQUIRED, REFERENCE DEALER TECHNICAL BULLETIN 91-T-09. THE INFORMATION IN THIS BULLETIN UPDATES THE BRAKE SYSTEM BLEEDING PROCEDURE IN THE 1991 BRAVADA SERVICE MANUAL.

NORMAL CONDITIONS THE FOLLOWING ARE NORMAL OPERATING FEATURES OF THE 4WAL SYSTEM AND SHOULD NOT BE CONFUSED WITH "PROBLEM" CONDITIONS: - A PUMP NOISE DURING THE 8 MPH TEST (ONCE PER DRIVE CYCLE). - PEDAL MOVEMENT (GREATER THAN NORMAL) DURING ANTILOCK BRAKING ON MIXED TRACTION ROAD SURFACES (DRY PAVEMENT TO ICE, FOR EXAMPLE). - PUMP NOISE AND PEDAL MOVEMENT (VIBRATION) DURING ANY ANTILOCK STOP.

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.