DESCRIPTION
The Kelsey-Hayes 4-Wheel Anti-Lock (4WAL) brake system is used to prevent wheel lock-up during heavy braking. This allows driver to maintain steering control while stopping vehicle in shortest distance possible. The system consists of Electro-Hydraulic Control Unit (EHCU), 4 wheel sensors, warning lights, electrical wiring and hydraulic lines.
The EHCU consists of an Electronic Control Unit (ECU), hydraulic control unit, 2 accumulators, pump motor and a relay. (Scheme 34), (Scheme 35) or (Scheme 36). The hydraulic control unit consists of an Isolation Solenoid Valve (ISO), Pulse-Width Modulated (PVM) valve, a High Pressure Accumulator (HPA), Low Pressure Accumulator (LPA) and a reset switch on each channel. If overhaul or repair to any component on the EHCU is necessary, complete assembly must be replaced.
Scheme 34
Note. Figures 15 & 25 are Courtesy of General Motors Corp.
Scheme 35
Scheme 36
OPERATION
When ignition is turned on, EHCU performs a self-check of the 4WAL electrical system. The Yellow ANTILOCK and Red BRAKE warning lights will come on for 2 seconds. ANTILOCK light should turn off if no electrical 4WAL faults are detected by EHCU. The Red BRAKE light may come on or stay on if parking brake is applied or a mechanical brake problem is detected.
Once vehicle speed has exceeded 8 MPH and driver has fully released the brake pedal, EHCU performs a self-check on electro-hydraulic portion of the system. This includes pump, control valves and reset switches. If EHCU detects a problem during this self-check or initial self-test, an indicator light will come on and 4WAL system will be disabled. A related trouble code will be stored.
When the brake pedal is depressed, voltage to the EHCU drops from 12 volts to one volt. At this point, EHCU monitors wheel speed through an AC signal generated by speed sensors located at each wheel. If the deceleration rate of wheel speed reaches a preprogrammed rate, EHCU will activate various control valves to prevent wheel lock-up by increasing or decreasing hydraulic pressure to each channel: left front, right front or rear wheels.
On 4WD models, a front-axle mounted switch is used to inform EHCU when vehicle is in 2WD or 4WD mode. Switch is open during 2WD operation, thus sending a one volt or less signal. During 4WD operation, switch closes, sending a 12-volt signal to EHCU. This signal causes EHCU to modify the anti-lock braking program for 4WD operation.
ELECTRO-HYDRAULIC CONTROL UNIT (EHCU)
Note. It is not necessary to bleed EHCU during normal brake system bleeding. If EHCU is replaced, bleed EHCU first.
- Bleed EHCU after master cylinder and before wheel cylinders and calipers. Ensure ignition is off. Install 2 Combination Valve Depressors (J-39177) on High Pressure Accumulator (HPA) bleed valves. (Scheme 37) Install one combination valve depressor on combination valve bleed valve. Turn 2 EHCU internal bleed screws 1/4-1/2 turn counterclockwise. (Scheme 38) CAUTION: If internal bleed screws are left open, hydraulic fluid will by-pass accumulator and hydraulic pressure will be lost.
- Slowly apply brake pedal pressure and hold. Open left bleed screw on front of EHCU. (Scheme 37) Leave left bleed screw on front of EHCU open until all air is expelled or brake pedal is fully applied. Close left bleed screw on front of EHCU. Wait 15 seconds. Repeat this step until all air is purged from EHCU, then close left internal bleed screw on EHCU. (Scheme 38)
- Slowly apply brake pedal pressure and hold. Open right bleed screw on front of EHCU. (Scheme 37) Leave right bleed screw on front of EHCU open until all air is expelled or brake pedal is fully applied. Close right bleed screw on front of EHCU. Wait 15 seconds. Repeat this step until all air is purged from EHCU, then close right internal bleed screw on EHCU. (Scheme 38)
- Remove combination valve depressors from High Pressure Accumulator (HPA) bleed valves. Remove combination valve depressors from combination valve bleed valve. Ensure master cylinder fluid reservoir is full.
- Bleed wheel cylinders and calipers as necessary. Brake bleeding sequence is as follows: right rear, left rear, right front, left front. Use DOT 3 brake fluid only. Evaluate brake pedal feel and braking performance. Repeat bleeding procedure if necessary.
Scheme 37
Scheme 38
Note. Tech 1 scan tester is recommended when testing and diagnosing 4WAL system, but is not necessary.
The Electro-Hydraulic Control Unit (EHCU) contains a self-diagnostic capability to detect system failures. When a fault code is set, the EHCU may disable 4WAL system and illuminate ANTILOCK light for duration of ignition cycle. Fault codes stored by EHCU can be displayed using on-board diagnostics or Tech 1 with appropriate cartridge.
Before diagnosing 4WAL system, perform a comprehensive visual inspection of system by checking wiring harness connectors for looseness and harness routing (pay particular attention to wheel speed sensor wiring harness routing), applicable fuses in fuse block and ground connections. Ensure brake fluid level in master cylinder reservoir is full.
Start 4WAL system diagnosis with DIAGNOSTIC CIRCUIT CHECK chart under TROUBLE SHOOTING CHARTS. (Scheme 40) If failures are found during DIAGNOSTIC CIRCUIT CHECK, you will be directed to enter diagnostics to retrieve codes, perform symptom diagnosis or diagnose brake system warning light. DIAGNOSTIC CIRCUIT CHECK may indicate 4WAL system is functioning properly.
RETRIEVING CODES
Note. Perform DIAGNOSTIC CIRCUIT CHECK chart under TROUBLE SHOOTING CHARTS. (Scheme 40)
- If using Tech 1 for retrieving codes, go to step 3). If using ANTILOCK light for retrieving codes, turn ignition on. Using a jumper wire, connect terminal "H" to terminal "A" in ALDL connector. (Scheme 39) System will repeat trouble code(s) as long as ALDL connector terminals are jumpered.
- Trouble codes will begin displaying by a flashing ANTILOCK warning light. As an example, Code 25 would be displayed by ANTILOCK warning light flashing 2 times, followed by a pause, then flashing 5 more times. See 4WAL FAULT CODES table for diagnosis. See «CLEARING FAULT CODES»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel__clearing-fault-codes) to erase stored trouble codes. If no fault codes are present, see «4WAL SYMPTOM DIAGNOSIS»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) table.
- Turn ignition off. Connect Tech 1 to ALDL connector. Access 4WAL brake trouble codes with Tech 1. Tech 1 will display trouble code(s). Repair trouble codes in order they appear. See 4WAL FAULT CODES table for diagnosis. See «CLEARING FAULT CODES»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel__clearing-fault-codes) for erasing stored trouble codes. If no fault codes are present, see 4WAL SYMPTOM DIAGNOSIS table.
Scheme 39
| Symptom | Perform Chart |
|---|---|
| ANTILOCK Light On | ANTILOCK LIGHT ON (Fig. (Scheme 41) ) |
| ANTILOCK Light Off | ANTILOCK LIGHT OFF (Fig. (Scheme 42) ) |
| Brake Pedal Pulses (1) | BRAKE PEDAL PULSES (Fig. (Scheme 43) ) |
| Wheel Pull During Hard Braking (1) | WHEEL PULL DURING HARD BRAKING (Fig. (Scheme 44) ) |
| (1) These conditions must exist with no fault codes stored. | |
| (1) | These conditions must exist with no fault codes stored. |
4WAL SYMPTOM DIAGNOSIS
| Fault Code | Description |
|---|---|
| 21, 25, 31 or 35 (1) | Speed Sensor Failure Or Open Circuit |
| 22, 26, 32 Or 36 (2) | Missing Speed Signal |
| 23, 27, 33 Or 37 (3) | Erratic Speed Sensor |
| 28 | Erratic Speed Sensor Signal (2 Dropouts Above 20 MPH) |
| 29 | Simultaneous Dropout Of All 4 Speed Sensors Above 8 MPH |
| 35 (4) | Speed Sensor Failure Or Open Circuit |
| 36 (4) | Missing Speed Sensor Signal |
| 37 (4) | Erratic Speed Sensor Signal |
| 38 | Speed Sensor Error |
| 41-54 | Control Valves |
| 61, 62 Or 63 | Reset Switches |
| 65 & 66 | Motor Relay Open Or Shorted |
| 67 (5) | Pump Motor Circuit Open Or EHCU Output Shorted |
| 68 (5) | Pump Motor Locked Or Motor Circuit Shorted |
| 71-74 | Memory Errors |
| 81 | Brake Switch Circuit Open Or Shorted |
| 86 | ANTILOCK Light Shorted |
| 88 | BRAKE Light Shorted |
| (1) "C" and "K" series vehicles will only display Codes 21 and 25. (2) "C" and "K" series vehicles will only display Codes 22 and 26. (3) "C" and "K" series vehicles will only display Codes 23 and 27. (4) Applies only to "C" and "K" series vehicles. (5) Codes 67 and 68 use the same trouble shooting flow chart. | |
| (1) | "C" and "K" series vehicles will only display Codes 21 and 25. |
| (2) | "C" and "K" series vehicles will only display Codes 22 and 26. |
| (3) | "C" and "K" series vehicles will only display Codes 23 and 27. |
| (4) | Applies only to "C" and "K" series vehicles. |
| (5) | Codes 67 and 68 use the same trouble shooting flow chart. |
4WAL FAULT CODES
CLEARING FAULT CODES
Turn ignition on. Using a jumper wire, connect ALDL connector terminal "H" to terminal "A" for 2 seconds. Remove jumper wire for one second. Again, jumper ALDL connector terminal "H" to "A" for 2 seconds. Remove jumper wire. Fault codes are cleared when ANTILOCK and BRAKE lights illuminate, then turn off. Turn ignition off. Check to verify codes are cleared. See RETRIEVING CODES .
TROUBLE SHOOTING CHARTS
Note. See WIRING DIAGRAMS for circuit and terminal identification.
Scheme 40
Scheme 41
Scheme 42
Scheme 43
Scheme 44
Scheme 45
Scheme 46
Scheme 47
Scheme 48
Scheme 49
Scheme 50
Scheme 51
Code 36 - Missing Speed Sensor Signal ("C" & "K" Series)
Scheme 52
Scheme 53
Scheme 54
Scheme 55
Scheme 56
Scheme 57
Scheme 58
Scheme 59
Scheme 60
Scheme 61
Scheme 62
REMOVAL & INSTALLATION
| CAUTION | When battery is disconnected, vehicle computer and memory systems may lose memory data. Driveability problems may exist until computer systems have completed a relearn cycle. See COMPUTER RELEARN PROCEDURES article in the GENERAL INFORMATION section before disconnecting battery. |
Removal (Astro & Safari)
- Turn ignition off. Bleed vacuum and hydraulic pressure from system by depressing brake pedal several times. Position front wheels straight ahead. Index upper and lower "U" joints on intermediate steering shaft. Remove pinch bolt from upper and lower "U" joint. Index steering gear position. Remove steering gear mounting bolts. DO NOT disconnect pitman arm from pitman shaft. Separate steering gear from frame rail.
- Remove intermediate steering shaft. Disconnect hydraulic lines from bottom of combination valve. Disconnect electrical connector from combination valve. Remove master cylinder mounting bolts. Remove master cylinder and combination valve as an assembly.
- Disconnect hydraulic lines from EHCU. Disconnect electrical connectors from EHCU. Remove EHCU bracket-to-body bolts and nuts. Remove EHCU and bracket from vehicle as an assembly. Remove bracket from EHCU.
Installation
- To install, reverse removal procedure. Align all index marks for steering components. Ensure intermediate steering shaft pinch bolts pass through shaft undercut.
- Bleed brake system, including EHCU. See «BLEEDING BRAKE SYSTEM»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) . DO NOT overtighten EHCU-to-bracket bolts during installation. This may cause excessive noise transfer into vehicle. See «TORQUE SPECIFICATIONS»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) table.
Removal & Installation (Bravada & "S" & "T" Series)
- Disconnect hydraulic lines from bottom of combination valve. Disconnect electrical connector from combination valve. Disconnect hydraulic lines from EHCU. Disconnect electrical connectors from EHCU. Remove EHCU bracket-to-body bolts. Remove EHCU and bracket from vehicle as an assembly. Remove bracket from EHCU.
- To install, reverse removal procedure. Bleed brake system, including EHCU. See «BLEEDING BRAKE SYSTEM»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) . DO NOT overtighten EHCU-to-bracket bolts during installation. This may cause excessive noise transfer into vehicle. See «TORQUE SPECIFICATIONS»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) table.
Removal & Installation ("C" & "K" Series)
- Disconnect hydraulic lines from EHCU. Disconnect electrical connectors from EHCU. Remove EHCU bracket-to-body bolts. Remove EHCU and bracket from vehicle as an assembly. Remove bracket from EHCU.
- To install, reverse removal procedure. Bleed brake system, including EHCU. See «BLEEDING BRAKE SYSTEM»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) . DO NOT overtighten EHCU-to-bracket bolts during installation. This may cause excessive noise transfer into vehicle. See «TORQUE SPECIFICATIONS»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) table.
FRONT WHEEL SPEED SENSOR
Note. For installation purposes, note speed sensor wire routing before removing. Misrouted wiring may cause electromagnetic interference failures.
Removal & Installation (2WD Astro, Safari & "S" & "T" Series)
- Remove wheel and tire assembly. Remove brake caliper and wire aside. Remove hub and rotor assembly. Disconnect speed sensor electrical connector. Remove splash shield and speed sensor assembly. To install, reverse removal procedure. Tighten wheel bearing nut to 12 ft. lbs. (16 N.m), then back off until just loose.
- Hand tighten wheel bearing nut. Back-off wheel bearing nut no more than 1/2 of a flat until hole in spindle aligns with slot in wheel bearing nut. If wheel bearing nut is adjusted properly, hub end play is .001-.005" (.03-.13 mm). If hub end play is not within specification, repeat procedure. Speed sensor air gap is non-adjustable. See «TORQUE SPECIFICATIONS»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) table.
Removal & Installation (AWD Astro, Bravada & Safari; 4WD "S" & "T" Series)
1) Remove wheel and tire assembly. Remove brake caliper and wire aside. Remove drive axle nut. Disconnect speed sensor electrical connector. Remove speed sensor wire from clip on upper control arm. Remove bolts retaining hub and bearing assembly. Remove splash shield bolts. Using a puller, remove hub and bearing assembly.
2) Remove hub and bearing assembly, splash shield and speed sensor assembly. Remove speed sensor from hub and bearing assembly. Remove speed sensor wire harness from splash shield. To install, reverse removal procedure. Ensure drive axle nut is properly torqued.
3) DO NOT move drive axle nut more than 1/6 of a turn to align for cotter pin. If speed sensor is being replaced, drill out rivet in splash shield and replace rivet with a small bolt, nut and washer. Speed sensor air gap is non-adjustable. See TORQUE SPECIFICATIONS table.
Removal & Installation (2WD "C" & "K" Series)
- Remove wheel and tire assembly. Remove brake caliper and wire aside. Remove hub and rotor assembly. Disconnect speed sensor electrical connector. Remove speed sensor wire from clip on upper control arm. Remove splash shield and speed sensor assembly. To install, reverse removal procedure. Tighten wheel bearing nut to 12 ft. lbs. (16 N.m), then back off until just loose.
- Back-off wheel bearing nut again no more than 1/4 of a turn until hole in spindle aligns with slot in wheel bearing nut. If wheel bearing nut is adjusted properly, hub end play is .001-.008" (.03-.20 mm). If hub end play is not within specification, repeat procedure. Speed sensor air gap is non-adjustable. See «TORQUE SPECIFICATIONS»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) table.
Removal & Installation (4WD "C" & "K" Series)
- Remove wheel and tire assembly. Disconnect speed sensor electrical connector. Remove speed sensor wire from clip on upper control arm. Remove bolt attaching speed sensor wiring harness to vehicle frame.
- Remove 2 bolts attaching speed sensor to back of steering knuckle. Remove speed sensor. To install, reverse removal procedure. Speed sensor air gap is non-adjustable. See «TORQUE SPECIFICATIONS»(/gmc/typhoon/i-1991-1993/remont/anti-locktraction-control/#anti-lock-brake-system-4-wheel) table.
Remove wheel assembly. Remove brake drum. Remove primary brake shoe. Remove necessary clips and bolts. Remove speed sensor by pulling speed sensor wire through backing plate. To install, reverse removal procedure.
VEHICLE SPEED SENSOR (VSS) BUFFER
| CAUTION | VSS buffer is sensitive to Electrostatic Discharge (ESD). DO NOT touch VSS buffer terminals with hands or tools, or damage to VSS buffer may result from static electricity. |
Note. VSS buffer is an internal portion of Digital Ratio Adapter Controller (DRAC); however, manufacturer now refers to DRAC as the VSS buffer.
Ensure ignition is off. Disconnect negative battery cable. VSS buffer is located behind center of dashboard. Remove 4 screws mounting VSS buffer to dashboard. Disconnect 2 small electrical connectors from VSS buffer. Remove VSS buffer. To install, reverse removal procedure.
Note. Information for VSS buffer part number applicable to axle ratio and tire size combinations is not available from manufacturer. VSS buffer is an internal portion of Digital Ratio Adapter Controller (DRAC); however, manufacturer now refers to DRAC as the VSS buffer.
"C" & "K" Series
If axle ratio or tire size is changed, it is necessary to replace VSS buffer to ensure proper operation or 4WAL brakes, cruise control, speedometer and engine Electronic Control Module (ECM). Failure to replace VSS buffer as required may affect driveability of vehicle and 4WAL brake operation.
TORQUE SPECIFICATIONS
| Application | Ft. Lbs. (N.m) | |
|---|---|---|
| Brake Caliper Mounting Bolts | 37 (50) | |
| Brake Line Hydraulic Fittings | 16 (22) | |
| Drive Axle Nut | ||
| Astro & Safari (AWD) | 173 (234) | |
| Bravada (AWD) | 181 (245) | |
| EHCU Mounting Bracket-To-Body Bolt | 33 (45) | |
| Hub & Bearing Assembly Mounting Bolts | ||
| Astro & Safari (AWD) | 12 (16) | |
| Bravada (AWD) & "S" & "T" Series 4WD | 86 (116) | |
| Intermediate Steering Shaft | ||
| "U" Joint Pinch Bolt | 30 (41) | |
| Speed Sensor Mounting Bolts | ||
| Front (1) | 11 (15) | |
| Rear | 26 (35) | |
| Splash Shield Bolts | 11 (15) | |
| Steering Gear-To-Frame Bolts | 55 (75) | |
| Wheel Bearing Nut (2WD) (2) | 12 (16) | |
| Wheel Lug Nuts | 90 (122) | |
| INCH Lbs. (N.m) | ||
| EHCU Internal Bleeder Screw | 60 (7) | |
| EHCU-To-Mounting Bracket Bolt | 60 (7) | |
| (1) Not all models have speed sensor mounting bolts, some models have a speed sensor which is mounted onto a splash shield. (2) Refer to appropriate procedure in FRONT SPEED SENSOR under REMOVAL & INSTALLATION to ensure proper adjustment. | ||
| (1) | Not all models have speed sensor mounting bolts, some models have a speed sensor which is mounted onto a splash shield. |
| (2) | Refer to appropriate procedure in FRONT SPEED SENSOR under REMOVAL & INSTALLATION to ensure proper adjustment. |
TORQUE SPECIFICATIONS
