About the author: Leon serves as one of the leading technical editors for Mitchell 1. He is a graduate of Universal Technical Institute and has previously worked for Aamco Transmissions and as a mobile service technician. He holds 609 Certification and specializes in automotive diagnostics.
As gas prices seem to never drop to a comfortable price, it’s important for vehicles to be as fuel efficient as possible. That is where an electronic power steering (EPS) system comes in handy.
By eliminating the power steering pump which can use up to 10 hp under load, an EPS system provides up to a 2% increase in fuel economy over the conventional system. Another benefit of having an electric steering system is that it eliminates the use of hoses and fluid, therefore eliminating power steering leaks as well as a reduction in weight.
Electronic power steering systems are becoming widely popular among auto manufacturers due to the fact that they provide a more refined feel that can be adjusted as needed.
The EPS system consists of four major components: the EPS control module which collects data from the EPS components and sends out the required information; the EPS motor, its speed strength and direction controlled by the EPS Control Unit; the reduction gear, which inputs the power assist to the steering rack assembly; and torque sensor, which monitors the driver’s input and the EPS system’s mechanical output.
The EPS is powered by a permanent magnet alternating current motor and is not dependent on the engine for its power source so steering feel is not affected when the engine is shut off. The torque sensor itself has two independent coils of wire. One of the coils determines if a right hand turn is being made, the other coil determines if a left is being made. The signal is then sent from the EPS module to the appropriate coil, which will assist the vehicle in steering.
How electronic power steering works
A hybrid type of electronic power steering has been in place for some time now, but that included the use of an electric motor to drive a hydraulic pump.
The new version of EPS is all electronic. The system works by incorporating information with the EPS control unit, EPS motor, reduction gear and torque sensor.
An EPS system works by using a pinion gear assist that provides the power assist by rotating the pinion gear. The reduction gear is press fitted onto a set of splines on the pinion shaft and delivers the assist to the rack gear instead of pushing on the rack gear as in a hydraulic system.
The steering gear itself is a manual rack with an electric motor mounted on the steering column or the rack. When the driver turns the wheel, a steering sensor detects the position and rate of rotation of the steering wheel. This information along with input from a torque sensor mounted in the steering shaft is sent to the power steering control module. The system also uses other inputs from vehicle speed sensors and the traction control system which are factored in to determine how much steering assist is required. The control module then tells the motor to rotate the required amount.
Attached to the motor is the motor resolver sensor, which measures the rotation of the motor and sends the data to the EPS control module.
Different surfaces will require different amounts of steering assist. For example, a vehicle traveling on pavement will require much less steering assist than a vehicle traveling on sand or snow. With the EPS system working with other sensors, it can much more easily provide the required assistance for any kind of terrain and vehicle speed.
Electric power steering modes
• Normal mode — Left and right assist is provided in response to inputs and vehicle speed. During normal operation power assist levels will decrease as the vehicle speed increases.
• Assist limitation — Will occur if there is a problem with the information going back to the EPS control module, overheating of the EPS module or a malfunction with the controller area network.
• Assist off — System is turned off if there is a problem with any of the major EPS components.
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Diagnosis tips
The steering warning light will illuminate during the light check cycle and when the EPS is in assist off mode. The steering warning light will not illuminate during limited assist operation. By turning the steering wheel all the way to one side, it will cause the EPS system to provide the maximum amount of current to the EPS motor.
If the steering wheel is then held in this position for an extended period of time, the system will go into protection mode so the motor doesn’t overheat. If this occurs the EPS system will limit the amount of current to the motor and reduce the level of power assist. If it senses high system temperature the overload protection mode will enable. If a sensor or other another component in the EPS system fails, the self-diagnostics should detect the fault, set a code and disable power-assist. Though power steering hoses and fluid were eliminated, there are still going to be other parts that malfunction over time. Most system malfunctions present problems such as:
• Heavy steering: Check torque sensor, power steering motor, speed sensors, powers steering ECU, and ECU source voltage.
• Uneven steering between right and left: Check calibration of torque sensor (built into steering column), power steering motor, power steering ECU or wheel alignment.
• Steering effort doesn’t decrease while driving: Check torque sensor, power steering motor, power steering ECU.
• Knocking when turning steering wheel: Steering intermediate shaft, ball joint.
• Noise or vibration in steering wheel: Check power steering gear, steering column.•Squeaking – Power steering motor.
Torque sensor testing
1. Measure the resistance between torque sensor connector terminals 1 and 2, and 2 and 3.
2. If there is any malfunction, replace the steering gear and linkage.
• The torque sensor zero point should be calibrated whenever you remove and replace the steering column assembly (containing the torque sensor), power steering ECU assembly, steering wheel steering gear assembly, and if a difference is felt in steering effort between right and left.
Check steering wheel looseness, play
1. Move the steering wheel toward the shaft and in four right angle directions to inspect for looseness and play.
2. If there is any malfunction, inspect the following, and repair or replace the applicable part.
Steering wheel effort
Steering effort also varies per road conditions and type of surface the vehicle is on.
1. Verify that the equipped tire size and tire air pressure is as specified.
2. With the vehicle on a hard, level surface, put the wheels in the straight-ahead position.
3. (Check proper procedures before working on the air bag system). Remove the air bag module.
4. Start the engine and idle it.
5. Verify that the EPS warning light does not illuminate.
6. Inspect the steering wheel effort using a torque wrench. If it exceeds specifications, adjust the steering gear and, as needed, linkage.
Check steering shaft
1. Inspect the column bearing for excessive play and damage.
2. Verify that the measurement of the steering shaft is within specification. If not within the specification, replace the steering shaft component.
Specification: 508.5 mm {20.02 in}
3. Check that the tilt lever moves smoothly from the lock to the unlock position.
4. Verify that the steering shaft is fixed firmly when the tilt lever is locked.
5. If there is any malfunction, replace the steering shaft.
Check EPS motor power supply for open
1. Disconnect the battery positive terminal.
2. Inspect for continuity between EPS control module terminal and battery positive terminal.
3. Check for continuity.
Check if malfunction is caused by a bad connection of the EPS control module
1. Turn the ignition switch off.
2. Inspect connection of the EPS control module and wiring harness.
3. Disconnect the EPS control module connector.
4. Check if malfunction is caused by a bent or poorly connected EPS control module connector pin.
5. Is the connection and pin of connector and wiring harness normal?
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Vehicle-specific EPS example
Electronic power steering (EPS): 2007 Toyota Yaris. (Information provided by Toyota Motor Sales U.S.A. Inc.)
The 2007 Toyota Yaris features an electronic power steering system (EPS). This system generates steering torque (to assist power steering effort) through the operation of an electric motor and reduction gear installed on the steering column shaft.
The power steering ECU determines the direction and the amount of power-assist according to the vehicle speed signals and signals from the steering torque sensor (the torque sensor is built into the steering column assembly). As a result, steering effort is controlled to a light level during low speed driving and moderately high during high speed driving. This allows less driver effort during low speed driving, without overly sensitive power steering at higher speeds.
The power steering ECU calculates assisting power based on steering torque signals from the torque sensor and vehicle speed signals from the skid control ECU. For vehicles without ABS, the power steering ECU receives vehicle speed signals from the speedometer.
The torque sensor detects the steering effort generated when the steering wheel is turned and converts it to an electrical signal.
The EPS motor is activated by the current from the power steering ECU and generates torque to assist the steering effort.
The power steering ECU is located behind the dash, above the steering column assembly.
The steering motor assembly is located on the forward end of the steering column assembly.
The EPS fuse is located in the engine room relay box.
The main body ECU is located under the dash at the far lower left of the dashboard area. The DLC3 is located immediately under the ECU.
EPS handling precautions
Avoid any impact to ECUs and relays. If dropped or subjected to a severe impact, replace with new parts.
• Do not expose electronic parts to high heat or humidity.
• Do not touch the connector terminals, in order to prevent terminal deformation or static electricity damage.
• When the power steering ECU has been replaced with a new unit, perform the torque sensor zero point calibration.
• Avoid any impact to the steering column assembly, especially to the motor and torque sensor. If dropped or impacted, replace with new parts.
• Do not pull the wire harness when moving the steering column assembly.
• When the steering column assembly has been replaced, perform the torque sensor zero point calibration after initializing the torque sensor zero point.
NOTE: When disconnecting the connectors relating to the electronic power steering system, first center the steering wheel, then turn the ignition switch ON, then turn the ignition switch OFF, then disconnect the connectors. Do not turn the ignition switch ON when the steering wheel is not centered.
If the above operations are not performed properly, the steering center point (zero point) will deviate, which may lead to a difference in steering effort between turning right and left. If there is a difference in steering effort (left/right), perform the torque sensor zero point calibration.
Precautions for CAN communication
CAN communication lines are used to receive information from the skid control ECU (ABS ECU) and the ECM, and to transmit warnings to the combination meter. If a problem exists in the CAN communication lines, DTCs of the CAN communication line are output.
Perform troubleshooting of communication line problems when the CAN communication DTCs are output.
Be sure to start troubleshooting on the electronic power steering system after confirming that the CAN communication system is normal.
Torque sensor zero point calibration
Using the intelligent tester, perform the torque sensor zero point calibration whenever any of the following conditions occur:
• The steering column assembly (containing the torque sensor) has been replaced.
• The power steering ECU has been replaced.
• The steering wheel has been replaced.
• The steering gear assembly has been replaced.
• There is a difference in steering effort between right and left turning.
Center the steering wheel and align the front wheels straight ahead.
Connect the intelligent tester to the DLC3.
Turn the ignition switch ON and turn the tester ON. Initialize the torque sensor zero point calibration signal and perform the zero point calibration by following the prompts on the tester screen.
NOTE: Do not touch the steering wheel during the calibration process. Confirm that no DTCs are output after the zero point calibration has been completed.
If DTC C1515/15 is output, this indicates that the torque sensor zero point adjustment has not been initialized. If DTC 1516/16 is output, this indicates that torque sensor zero point adjustment is incomplete. If DTC C1534/34 is output, this indicates a malfunction of the EPS ECU.
NOTE: The torque sensor zero point calibration is performed automatically when the test mode inspection is conducted after the power steering ECU has been replaced with a new ECU. When using an intelligent tester, connect the tester to the DLC3. Turn the ignition switch ON and enter the test mode by following the prompts on the tester screen.
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Problem symptoms
Symptom: Heavy steering
Suspected area: front tires (improper inflation and/or unevenly worn), incorrect front wheel alignment, worn lower ball joint, steering gear assembly, power steering motor, battery and power source system, power source voltage of the power steering ECU or the power steering ECU.
Symptom: Steering effort differs between right and left turns
Suspected area: Steering center point (zero point) not recorded completely, front tires (improper inflation or unevenly worn), incorrect front wheel alignment, worn lower ball joint, steering gear assembly, torque sensor in steering column, steering column assembly, power steering motor, power steering ECU.
Symptom: While driving, steering effort does not change in accordance with vehicle speed or steering wheel does not return properly.
Suspected area: Lower ball joint, speed sensor (w/ABS), skid control ECU (ABS ECU), combination meter (w/o ABS), torque sensor in steering column, power steering motor, power steering ECU, controlling CAN communication system.
Symptom: Friction occurs when turning steering wheel during low speed driving.
Suspected area: Power steering motor, steering column assembly.
Symptom: High-pitched squeaking sound occurs when turning steering wheel slowly with vehicle stopped.
Suspected area: Power steering motor.
Symptom: Steering wheel vibrates and noise occurs when turning steering wheel with vehicle stopped.
Suspected area: Power steering motor, steering column assembly.
Symptom: Power steering warning (P/S warning) always indicated on the combination meter.
Suspected area: Power source voltage of power steering ECU, combination meter, power steering ECU. NOTE: See Diagnostic Trouble Codes on page 17.
Fail-safe operation
When a problem occurs in the EPS, the P/S warning light on the combination meter illuminates and, depending on the problem detected, the EPS ECU stops the power steering assist, maintains the amount of power assist constant, or reduces the amount of power assist in order to protect the system.
The amount of power assist may be decreased to prevent the EPS motor and ECU from overheating if the steering wheel is continuously turned when the vehicle is either stopped or driven at a low speed, or if the steering wheel is kept at either full-lock position for an extended time. In such cases, the amount of power steering assist returns to normal if the steering wheel is not turned for approximately 10 minutes with the engine idling.
Malfunction: Torque sensor (DTC C1511/11, C1512/12, C1513/13 and C1514/14)
Fail-safe: Assist force is restricted.
Malfunction: Torque sensor (DTC C1517/17)
Fail-safe: Power assist is stopped.
Malfunction: Motor (DTC C1524/24)
Fail-safe: Power assist is stopped.
Malfunction: Power steering ECU (DTC C1531/31, C1532/32)
Fail-safe: Power assist is stopped.
Malfunction: Temperature sensor in power steering ECU (DTC C1533/33)
Fail-safe: Assist force restricted.
Malfunction: EEPROM abnormal (DTC C1534/34)
Fail-safe: The assist force has been restricted.
Malfunction: Steering wheel location data error (DTC C1535/35)
Fail-safe: Power assist stopped.
Malfunction: Speed sensor (DTC C1541/41, C1542/42)
Fail-safe: The amount of power assist remains constant at a speed of 43 mph.
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Malfunction: IG power source voltage error (DTC C1551/51)
Fail-safe: Assist force restricted.
Malfunction: PIG power source voltage error (DTC C1552/52)
Fail-safe: Assist force restricted.
Malfunction: Over-voltage at IG and PIG terminals.
Fail-safe: Assist force restricted.
Malfunction: Power source relay (DTC C1554/54)
Fail-safe: Power assist stopped.
Malfunction: Motor relay (DTC C1555/55)
Fail-safe: Power assist stopped.
Malfunction: ECM communication error (DTC U0105)
Fail-safe: Amount of power assist remains constant at a speed of 43 mph.
Malfunction: ABS ECU communication error (DTC U0121)
Fail-safe: Amount of power assist remains constant at a speed of 43 mph.
Malfunction: High temperature in EPS ECU
Fail-safe: Assist force restricted until ECU temperature is normal.
Malfunction: Power source voltage drop
Fail-safe: Assist force suspended until voltage recovers. ●
Diagnostic trouble codes
DTC C1511/11
Detection item: Torque sensor 1 malfunction
Trouble areas: Steering column assembly (torque sensor), power steering ECU
DTC C1512/12
Detection item: Torque sensor circuit malfunction
Trouble areas: Steering column assembly (torque sensor), power steering ECU
DTC C1513/13
Detection item: Torque sensor circuit malfunction
Trouble areas: Steering column assembly (torque sensor), power steering ECU
DTC C1514/14
Detection item: Torque sensor power source circuit malfunction
Trouble areas: Steering column assembly (torque sensor), power steering ECU
DTC C1515/15
Detection item: Torque sensor zero point adjustment is not initialized
Trouble areas: Torque sensor zero point calibration not performed; steering column assembly.
DTC C1516/16
Detection item: Torque sensor zero point adjustment incomplete
Trouble areas: Torque sensor zero point calibration failure, steering column assembly
DTC C1517/17
Detection item: Torque sensor hold malfunction
Trouble areas: Steering column assembly (torque sensor), power steering ECU
DTC C1524/24
Detection item: Motor circuit malfunction
Trouble areas: Steering column assembly, power steering ECU
DTC C1531/31
Detection item: EPS ECU circuit malfunction
Trouble areas: Power steering ECU
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DTC C1532/32
Detection item: EPS ECU circuit malfunction
Trouble areas: Power steering ECU
DTC C1533/33
Detection item: EPS ECU circuit malfunction
Trouble areas: Power steering ECU
DTC C1534/34
Detection item: EPS ECU circuit malfunction
Trouble areas: Power steering ECU
DTC C1535/35
Detection item: Steering wheel location data error
Trouble areas: Power steering ECU
DTC C1541/41 (w/o ABS)
Detection item: Speed sensor malfunction
Trouble areas: Speed sensor, speed sensor circuit, combination meter, power steering ECU
DTC C1542/42 (w/o ABS)
Detection item: Speed sensor malfunction
Trouble areas: Speed sensor, speed sensor circuit, combination jeter, power steering ECU
DTC C1551/51
Detection item: IG power source circuit malfunction
Trouble areas: ECU-IG fuse, IG power source circuit, power steering ECU
DTC C1552/52
Detection item: PIG power source circuit
Trouble areas: EPS fuse, PIG power source circuit, power steering ECU
DTC C1553/53
Detection item: When resetting voltage, vehicle is being driven
Trouble areas: IG and PIG power source circuit, power steering ECU
DTC C1554/54
Detection item:EPS relay circuit
Trouble areas: EPS fuse, PIG power source circuit, power steering ECU
DTC C1555/55
Detection item: EPS motor relay circuit
Trouble areas: Power steering ECU
DTC C1571/71 (w/o ABS)
Detection item:Speed sensor malfunction (test mode DTC)
Trouble areas: Speed sensor, speed sensor circuit, combination meter, power steering ECU. In test mode, this indicates a vehicle speed check. The code may be cleared by driving the vehicle at a speed of 6 mph or more. Trouble areas can include the speed sensor, combination meter, power steering ECU or wire harness and connector
DTC C1581/81
Detection item: Assist map — un-writing
Trouble areas: Power steering ECU
DTC U0073
Detection item: Control module communication bus off
Trouble areas: CAN communication system
DTC U0105
Detection item: Lost communication with ECM
Trouble areas: CAN communication system, ECM
DTC U0121 (with ABS)
Detection item:Lost communication with anti-lock brake system (ABS) control module
Trouble areas: CAN communication system, ABS ECU
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Data active list
With the intelligent tester connected to the DLC3, turn the ignition switch ON and turn the tester ON. Operate the intelligent tester according to the screen prompts and select DATA LIST. This allows you to check active values against reference values.
Item: TRQ1 (Torque sensor 1 output value...min. 0 V, max. 5 V)
Inspection condition:
1. Steering wheel not turned (without load). Reference value 2.3 to 2.7 V
2. Steering wheel turned to the right with vehicle stopped. Reference value 2.5 to 4.7 V
3. Steering wheel turned to the left with vehicle stopped. Reference value 0.3 to 2.5 V
Item: TRQ2 (Torque sensor 2 output value...min. 0 V, max. 5 V)
Inspection condition:
1. Steering wheel not turned (without load). Reference value 2.3 to 2.7 V
2. Steering wheel turned to right with vehicle stopped. Reference value 2.5 to 4.7 V
3. Steering wheel turned to left with vehicle stopped. Reference value 0.3 to 2.5 V
Item: TRQ3 (Torque value for assist control....min. 0 V, max. 5 V)
Inspection condition:
1. Steering wheel not turned (without load). Reference value 2.3 to 2.7 V
2. Steering wheel turned to right with vehicle stopped. Reference value 2.5 to 4.7V
3. Steering wheel turned to left with vehicle stopped. Reference value 0.3 to 2.5 V
Item: SPD (Vehicle speed from meter...min. 0 mph, max. 158.8 mph)
Inspection condition:
1. Vehicle stopped. Reference value 0 mph
2. Vehicle driven at a constant speed. Reference value...no significant fluctuation
Item: MOTOR ACTUAL (Current to motor...min. – 128 A, max. 127 A)
Item: COMMAND VALUE (Requested current to motor min. – 128 A, max. 127 A)
Item: THERMISTOR TEMP (ECU substrate temperature min. – 122 deg F, max. 401 deg F)
Inspection condition: Ignition switch ON
Item: PIG SUPPLY (Power source voltage to activate motor...min. 0 V, max. 25.5 V)
Inspection condition: Power steering in operation. Reference value 11 to 14 V
Item: IG SUPPLY (ECU power source voltage...min. 0 V, max. 25.5 V)
Inspection condition: Reference value 11 to 14 V
Item: TRQ1 ZERO VAL (Zero point value of torque sensor 1...min. 0 V, max. 5 V)
Inspection condition: Steering wheel not turned (without load)
Reference value 2.3 to 2.7 V
Item: TRQ2 ZERO VAL (Zero point value of torque sensor 2...min. 0 V, max. 5 V)
Inspection condition: Steering wheel not turned (without load)
Reference value 2.3 to 2.7 V
Item: TRQ3 ZERO VAL (Zero point value of torque sensor for assist control....min. 0 V, max. 5 V)
Inspection condition: Steering wheel not turned (without load)
Reference value 2.3 to 2.7 V
Item: MOTOR TERMINAL (+) (Motor terminal M1 voltage...min. 0 V, max. 25.5 V)
Inspection condition:
1. Steering wheel turned to right. Reference value below 1 V
2. Steering wheel turned to left. Reference value 11 to 14 V
Item: MOTOR TERMINAL (-) (Motor terminal M2 voltage...min. 0 V, max. 25.5 V)
Inspection condition:
1. Steering wheel turned to right. Reference value 11 to 14 V
2. Steering wheel turned to left. Reference value below 1 V
Item: MTR OVERHEAT
Continuous overheat prevention control record: REC/UNREC
Item: MTR LOW POWER
PIG power source voltage drop record: REC/UNREC
Item: CONTROL MODE
Codes indicating DTC detection timing during ECU control displayed in hexadecimal
Item: IG ON/OFF TIMES
Number of times ignition switch turned on after DTC detection: min. 0 time, max. 255 times
Item: # CODE
Number of detected DTCs when freeze frame data stored: min. 0 time, max. 255 times
Item: ECU ID
ECU identification information
Item: TEST MODE STAT
Selected mode: Normal Mode/Test Mode
