MODEL IDENTIFICATION
| Series (1) | Model |
|---|---|
| "C" | 2WD Pickup, Sierra, Suburban, Tahoe & Yukon |
| "G" | Express, G-Van, RV Cutaway Savana, Savana Special & Savana Camper Special |
| "K" | 4WD Pickup, Sierra, Suburban, Tahoe, Yukon & AWD Escalade |
| "L" | AWD Astro & Safari |
| "M" | 2WD Astro & Safari |
| (1) Vehicle series is fifth character of VIN. | |
| (1) | Vehicle series is fifth character of VIN. |
MODEL IDENTIFICATION - TRUCKS
DESCRIPTION & OPERATION
Note. For repair procedures not covered in this article, see appropriate STEERING article.
The Electronic Variable Orifice (EVO) power steering system consists of the following
- Power steering pump, outlet and return hoses, steering box and linkage.
- Power steering fluid flow actuator, mounted on pump.
- Power Steering Control Module (PSCM), mounted under instrument panel (behind instrument panel, to right of radio in Astro and Safari).
- Steering Wheel Position (SWP) sensor, mounted in lower bearing of steering column.
- Vehicle Speed Sensor (VSS), mounted on transmission or transfer case (4X4).
- Powertrain Control Module (PCM) (diesel only), mounted in center of instrument panel (Express and Savana), or right side of instrument panel, above blower motor (Escalade, Sierra, Suburban, Tahoe, Yukon, and "C" & "K" Series Pickup).
- Vehicle Control Module (VCM) (gasoline only), mounted in left side of engine compartment.
The system increases or decreases the amount of fluid leaving the power steering pump. Outlet hose has a check valve to prevent kickback on some models. When vehicle is stopped or moving at low speeds, system allows full hydraulic fluid flow for maximum assist. As vehicle speed increases, a variable orifice closes at pump, reducing fluid flow to improve road feel and directional stability. Flow ranges from 3.5 gallons per minute at idle to.7 gallon per minute at speed.
Note. Steering wheel position sensor is also referred to as "handwheel speed sensor".
Steering Wheel Position (SWP) sensor detects rapid movement when defensive maneuvers are made. PSCM uses this sensor input to adjust the current to fluid flow actuator. If PSCM detects a fault, EVO system is disabled, allowing maximum power assist. When ignition is turned off and vehicle is restarted, PSCM resets and allows EVO operation until it detects another fault. PSCM has self-diagnostics and stores diagnostic trouble codes. See DIAGNOSIS & TESTING . Codes will clear after 100 ignition cycles without fault.
Note. When replacing PSCM, EEPROM inside must be calibrated using special equipment.
GENERAL PRECAUTIONS
| WARNING | To avoid injury from accidental air bag deployment, read and carefully follow all SERVICE PRECAUTIONS and also see DISABLING & ACTIVATING AIR BAG SYSTEM procedures in appropriate AIR BAG RESTRAINT SYSTEMS article in ACCESSORIES & EQUIPMENT. |
| 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 appropriate COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION before disconnecting battery. |
| CAUTION | Electronic components used in control systems are designed to carry very low voltages. As little as a 30-volt charge created by static electricity can cause a total or degrading failure in PSCM or other electronic components containing integrated circuits. Before servicing PSCM, technician must ground himself and work area to discharge static electricity. |
SYSTEM PRECHECK
Check fluid for correct type, level and contamination. Check PSCM, fluid flow actuator and wiring for damage or corrosion. Check power steering system for poor performance and leaks. Check speedometer operation to verify VSS signal.
SELF-DIAGNOSTICS
PSCM monitors system and can set Diagnostic Trouble Codes (DTCs). Codes are retrieved and cleared using scan tool. Disconnecting PSCM or battery will not clear codes.
VARIABLE EFFORT STEERING SYSTEM CHECK
Perform variable effort steering system check prior to testing system. (Scheme 1)and (Scheme 2). After each repair is performed, repeat system check to verify problem has been corrected. See EVO TEST INDEX.
Scheme 1
Scheme 2
| DTC | Fault Description |
|---|---|
| C0000 | Vehicle Speed Information Circuit Malfunction |
| C0472 | Steering Handwheel Sensor (HWSS) Circuit Low |
| C0473 | Steering Handwheel Sensor (HWSS) Circuit High |
| C0495 | EVO Tracking Error |
| C0498 | Steering Assist Control Solenoid Feed Circuit Low |
| C0499 | Steering Assist Control Solenoid Feed Circuit High |
| C0503 | Steering Assist Control Solenoid Return Circuit Low |
| C0504 | Steering Assist Control Solenoid Return Circuit High |
| C0559 | EEPROM Checksum Error |
EVO TEST INDEX
INTERMITTENTS
Faulty electrical connections and wiring cause most intermittent faults, although a damaged EVO/Passlock® module can occasionally be at fault. Also, steering assist control solenoid valve may be stuck and unable to calculate power steering assist.
Check for poor connecitons, a break in wire insulation exposing internal wire, or a wire that is broken inside insulation. Thoroughly inspect EVO/Passlock® module, VCM, and PCM for backed-out terminals, broken locks, improperly formed or damaged terminals, weak terminal retaining tension, and proper wire routing.
DIAGNOSTIC TESTS
Note. To identify circuits and wire colors referenced in testing, see WIRING DIAGRAMS . After each repair is performed, repeat system check to verify problem has been corrected. See VARIABLE EFFORT STEERING SYSTEM CHECK .
DTC C0000: VEHICLE SPEED INFORMATION CIRCUIT MALFUNCTION
Scheme 3
DTC C0472: STEERING HANDWHEEL SPEED SENSOR SIGNAL CIRCUIT LOW
(Scheme 4)and (Scheme 5).
Scheme 4
Scheme 5
DTC C0473: STEERING HANDWHEEL SPEED SENSOR SIGNAL CIRCUIT HIGH
(Scheme 6)and (Scheme 7).
Scheme 6
Scheme 7
DTC C0495: EVO TRACKING ERROR
Scheme 8
Diagnostic Aids
If DTC(s) C0498, C0499, C0502 or C0593 are present, perform those tests first. Steering assist control solenoid valve may be stuck and unable to calculate power steering assist.
Check for possible cause of intermittent problem. See INTERMITTENTS under DIAGNOSIS & TESTING.
DTC C0498: STEERING ASSIST CONTROL ACTUATOR FEED CIRCUIT LOW
Scheme 9
DTC C0499: STEERING ASSIST CONTROL ACTUATOR FEED CIRCUIT HIGH
Scheme 10
Steering assist control solenoid valve may be stuck and unable to calculate power steering assist.
Check for possible cause of intermittent problem. See INTERMITTENTS under DIAGNOSIS & TESTING.
DTC C0503: STEERING ASSIST CONTROL SOLENOID RETURN CIRCUIT LOW
Scheme 11
DTC C0504: STEERING ASSIST CONTROL SOLENOID RETURN CIRCUIT HIGH
Scheme 12
DTC C0559: EEPROM CHECKSUM ERROR
Scheme 13
Removal & Installation
Raise and support vehicle. Remove pump from vehicle. Remove hose from actuator. (Scheme 14) Note position of wiring connector and remove. Move cooler lines as needed and remove actuator from pump. Remove retaining clip from backside of the solenoid. To install, reverse removal procedure. Tighten fasteners to specification. See TORQUE SPECIFICATIONS table. Fill pump reservoir. Bleed system.
Scheme 14
POWER STEERING CONTROL MODULE (PSCM)
Disconnect negative battery cable. Disconnect wiring connectors. Remove instrument poanel extension, see Instrument Panel and Gauge Replacement. Remove PSCM by sliding module forward, then pulling down. To install, reverse removal procedure.
Note. When replacing PSCM, EEPROM inside must be calibrated using special equipment.
STEERING WHEEL POSITION SENSOR
Note. Steering wheel position sensor is also referred to as "handwheel speed sensor".
Astro, Express, G-Van, Safari & Savana
Steering column must be removed from vehicle to access steering wheel position sensor located at bottom of column. See appropriate STEERING COLUMNS article.
Escalade, Sierra, Suburban, Tahoe, Yukon, "C" & "K" Series Pickup
Turn front wheels straight ahead. Turn ignition to LOCK. Disconnect negative battery cable. Disable air bag. See appropriate AIR BAG RESTRAINT SYSTEMS article in ACCESSORIES & EQUIPMENT. Remove upper-to-lower steering shaft nut and bolt. Slide lower steering shaft away from upper steering shaft. Disconnect and remove SWP sensor and bearing from upper steering shaft. (Scheme 15) Remove sensor from bearing. To install, reverse removal procedure. Tighten fasteners to specification. See TORQUE SPECIFICATIONS table.
Scheme 15
TORQUE SPECIFICATIONS
| Application | Ft. Lbs. (N.m) |
|---|---|
| Fluid Flow Actuator-To Pump | 46 (62) |
| Power Steering Hose-To-Fluid Flow Actuator | 21 (26) |
| Upper-To-Lower Steering Shaft Bolt | 46 (62) |
TORQUE SPECIFICATIONS
Electronic Power Steering Wiring Diagram (Astro & Safari 1998). Scheme 16
Electronic Power Steering Wiring Diagram (Express, G-Van & Savana 1998). Scheme 17
Electronic Power Steering Wiring Diagram (Sierra, Suburban, Tahoe, Yukon, & "C" & "K" Series Pickup 1998). Scheme 18
Electronic Power Steering Wiring Diagram (Escalade, Suburban, Tahoe, Yukon, & "C" & "K" Series Pickup 1999). Scheme 19
See also:
• DIAGNOSIS & TESTING
• C0000
• C0472
• C0473
• C0495
• C0498
• C0499
• C0503
• C0504
• C0559
• VARIABLE EFFORT STEERING SYSTEM CHECK
• INTERMITTENTS