Tech Stuff

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Honda Electric Power Steering

On the Honda, the slider influences the current flow through the coils, and the change in those two currents tells the control unit the magnitude and direction of torque.
<p>On the Honda, the slider influences the current flow through the coils, and the change in those two currents tells the control unit the magnitude and direction of torque.</p>

Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. His began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.

The first patent for power steering, granted in 1900, describes a mechanical system that never went into production. Hydraulic power steering was developed in the 1920s, and it was first used on big Army trucks in the 1940s. The 1951 Chrysler Imperial was the first car with power steering, and most of the other big luxury cars had it within a few years. In the 1980s, most of the auto industry switched over to front-wheel-drive, and power steering became standard equipment on almost every car and truck sold in the U.S.

Honda was the first to introduce electric power steering (EPS) in a production car, the mid-engine 1991 Acura NSX. This was a low-volume exotic-car platform that Honda used as a test bed to develop expertise with new materials and new technologies. By the time Honda introduced its first hybrid car, the 1999 Insight, EPS was far less expensive and far more versatile. Today, rack-and-pinion EPS is used on anything from the tiny Fiat 500 to the latest Ford F-150.

Torque sensor

Early power steering was so over-boosted that a driver could parallel park a 4,000-pound Coupe de Ville using only one finger to turn the wheel. There was absolutely no feeling to the steering at all, but that was OK because isolating the driver from the road was part of that car’s mission. However, over-boosted steering causes fatigue on the highway, and most drivers prefer at least some feedback from the front wheels. By the late 1980s, most power steering systems had electronic controls that reduce steering boost at higher speeds.

Controlling steering boost starts by measuring the force and direction of the driver’s input to the steering wheel. Modern steering systems use an electronic torque sensor. A traditional steering torque sensor has an input shaft and an output shaft. The shafts are connected by a torsion bar that twists when torque is applied, allowing the shafts to move relative to each other. The relative movement might be just a few degrees, sometimes only tenths of a degree. On pure hydraulic power steering systems, the output shaft is connected to a finely tuned control valve. On EPS systems, the twisting motion is translated into a signal that’s sent to a control unit.

Torque sensor coil signals mirror each other. The slider influences the current as it moves away from one and toward the other.
<p>Torque sensor coil signals mirror each other. The slider influences the current as it moves away from one and toward the other.</p>

There are several different ways to generate that steering torque signal, and the sensor developed by Honda uses an unusual approach. The input shaft has a collar called the core, sometimes referred to as a slider. It’s mounted so that it turns with the input shaft but also can slide up and down on the shaft. A pin on the output shaft engages an angled slot in the slider, and a spring keeps the slider positioned so the pin is centered in the slot. Twisting the shafts moves the pin in the angled slot, forcing the slider to move a few millimeters up or down on the input shaft.

Surrounding the slider are two coils of extremely thin wire, stacked one above the other. The control unit supplies current to each coil, creating two stacked magnetic fields that surround the slider. When the slider is stationary and centered, the current flow is the same through both coils. When the slider moves up, it alters the magnetic fields, which disturbs the current flow in the coils. When the slider moves down, the current disturbance is reversed. The farther the slider moves up or down, the greater the change in the currents. The control unit monitors the current in these coils to detect the direction and magnitude of the torque that was applied to the steering shaft.

Steering motor

Depending on the design, the EPS motor is mounted on the column to act on the steering input shaft, or on the rack housing to act on the output shaft, or it’s mounted directly on the steering rack itself. Honda uses both of the latter, and on some models the motor is actually built into the rack housing. However, on the Fit, Insight and Civic, the motor can be removed from the rack.

Power steering motors draw a lot of power; on some hybrid vehicles with big batteries, the steering motor is operated on 42 volts to keep current draw low. However, on 12-volt systems, current draw is sometimes very high. Honda’s steering motor circuit is protected with a 60- or 70-amp fuse.

In normal driving, high current draw is only a transient load, but repeated rapid movement of the steering wheel can keep the average current draw high. This makes the motor heat up. The control unit monitors that temperature, and it will temporarily reduce steering motor current in steps to prevent the motor from overheating. Full boost is restored automatically when the motor cools down.

This will not turn on the EPS warning light. Honda says the motor will only overheat when repeatedly turning the steering wheel side-to-side for 20 seconds with the car not moving. However, on some of the Honda forums, people who race the Honda S2000, which has EPS, have noticed the steering boost decreasing in the later laps of the race, indicating high motor temperatures.

EPS motor current and motor speed: notice motor speed is positive and negative, and current flows even when motor speed is zero. That’s because the car is being steered against the crown in the road.
<p>EPS motor current and motor speed: notice motor speed is positive and negative, and current flows even when motor speed is zero. That&rsquo;s because the car is being steered against the crown in the road.</p>


Honda and Acura EPS control units will communicate with a professional aftermarket scan tool. Depending on the model, there could be over 40 data PIDs (parameter IDs). With a bit of imagination, live data recorded during a test drive can be quite useful. For instance, steering motor torque should be the same when steering in either direction. If it is not, check it again with the front wheels off the ground to see if there is a mechanical problem in the rack or suspension.

It also can be easier to tell the difference between mechanical and electrical/electronic faults; just disconnect the motor to see if the steering wheel torque remains equal.

CAUTION: When unplugging a connector, it is extremely important to turn the ignition switch off first. The same goes for reconnecting. Unplugging or reconnecting a live circuit can create a voltage spike that will ruin an electronic control unit. This is especially important in hybrid vehicles that have 42-volt circuits.

The EPS control unit carries out two different self-diagnostic routines, which Honda calls “initial diagnosis” and “steering diagnosis.” The initial diagnosis occurs when the engine is first started. The EPS warning light illuminates as the control unit checks its internal circuits, then the light turns off if no problems are detected. In the steering diagnosis, the control unit monitors the system as the car is driven. If a problem is detected during either diagnosis, a code is set, the warning light stays on, and current supply to the motor is reduced or turned off, which noticeably reduces steering effort.

Codes are displayed numerically, low-to-high, but most faults will not set a hard code. If the fault is not there during the next initial diagnosis (the next time the ignition is turned on), the code clears itself and the light turns off. However, torque sensor faults will always set a hard code. These won’t clear by turning the ignition off or disconnecting the battery even if the fault is not present. They must be cleared with the scan tool.

The first step in troubleshooting EPS fault codes is the same: read the code with a scan tool, then clear it and drive the car to see if it returns. If there are powertrain codes in the PCM in addition to steering system codes, address the powertrain codes first.

On the Insight and Fit, the motor is geared to the steering column’s output shaft.
<p>On the Insight and Fit, the motor is geared to the steering column&rsquo;s output shaft.</p>

The fault codes are displayed as two pairs of digits, like this: 11-01. The first two digits define the circuit and are the same across all platforms, but the second pair describes details particular to that model. Here’s a list of Honda/Acura EPS fault codes:

11-XX: IG-1 (ignition) terminal voltage

12-XX: battery voltage

21-XX: vehicle speed or engine speed signal

22-XX: engine speed signal

31-01: torque sensor neutral position

32-XX: control unit internal circuit

35-XX: control unit internal circuit

36-XX: control unit internal circuit

37-XX: control unit internal circuit

33-XX: internal motor circuits

34-XX: internal control unit relays

50-XX: torque sensor

51-XX: torque sensor

61-XX: motor circuit

71-XX: motor angle sensor

In the list, notice code 31-01: “torque sensor neutral position.” This code is the same on all models, and it indicates the sensor does not know the straight-ahead position of the front wheels. It’s not a hard code, but it will turn on the EPS warning light in the initial diagnosis. It’s likely to appear after a wheel alignment or other steering/suspension work, and it simply means the sensor needs to relearn the “neutral” or center-steer position.

If that torque sensor code is set, steering effort will not be equal in both directions, or the car may wander or pull to one side while driving, making it feel as though the wheels are not aligned properly. Fortunately the relearn procedure is simple; just access the EPS menu with a scan tool and select “Torque Sensor Learn,” then follow the prompts. Honda says the torque sensor is temperature sensitive, and it must be stabilized at 48 to 88 degrees Fahrenheit (10 to 30 degrees Celsius) before resetting the neutral position. Some techs report that the relearn process doesn’t always run to completion on the first attempt.

Even if there is nothing wrong with the system, the EPS warning light may come on under a few specific conditions.

After a panic stop from more than 12 mph and engine speed remains above 2,000 rpm for five seconds.

The steering wheel turns for 20 seconds when vehicle speed is less than 1 mph and engine speed is more than 2,000 rpm.

Engine speed is less than 500 rpm, but vehicle speed is more than 6 mph.

This is a result of the system’s complex operating logic and does not indicate a problem that requires attention. Simply cycle the ignition key off and on to reset the system.

Repair issues

Compared to other manufacturers, Honda’s EPS seems to be relatively trouble-free, but there are a few known issues on the Fit, Civic and Insight. The steering racks are built lighter and are more compact than hydraulic units, so the rack fits into smaller spaces. Unfortunately, this makes it a bit more susceptible to collision damage. Since there is no hydraulic fluid, a cracked housing is not always obvious. In fact, Honda techs and collision repair shops have reported that steering rack damage can’t be seen with the rack mounted in the car. Also unfortunately, on most models, the subframe must be removed in order to remove the EPS steering rack.

Fortunately, though, the control unit is inside the vehicle, usually under the right side of the dashboard or behind the right kick panel.

It can be easily accessed once the trim panels are removed.

Mechanical wear or damage to the rack or motor will usually produce chattering noises or rough steering, often more pronounced when turning one direction. Boost in only one direction also indicates motor or rack problems. However, if these symptoms go away when the motor is disconnected, the rack is probably OK and it’s time to look for fault codes.

Last year Honda issued TSB 14-058 that extends the EPS warranty on the Fit and Civic to 10 years and 150,000 miles, but only for fault codes 61-04 or 32-09, which require replacement of the EPS control unit.

What’s next

Honda’s newer models have what they call “Motion Adaptive” EPS, which operates in tandem with the Vehicle Stability Assist system. It nudges the steering wheel to prompt the driver to add or reduce steering input. The Lane Keeping Assist System will nudge the wheel if the vehicle strays across the lines that mark the lane. Although not intended to actually steer the car, if the driver is not touching the wheel, it will steer itself through a mild turn. So while Honda’s EPS was originally developed as a way to have power steering in a mid-engine car without long hydraulic lines, it’s turning into a lot more than just power steering.   ●

A brushless motor has electromagnets on the stator, permanent magnets on the rotor and some method of reporting the rotor’s position to the control unit.
<p>A brushless motor has electromagnets on the stator, permanent magnets on the rotor and some method of reporting the rotor&rsquo;s position to the control unit.</p>

Brushless motor

Many automotive EPS systems use a brushless DC motor. A typical brush-type DC motor consists of two sets of magnets, one set rotating inside the other, and at least one set consists of electromagnets. It works by changing the alignment of the magnetic fields so they continuously repel each other as the shaft rotates. The electromagnets on the rotor are switched off and on in sequence by the brushes as they make-and-break contact with the rotating commutator bars. In effect, the brushes and bars serve as a rotating mechanical switch.

In a brushless motor, the permanent magnet rotates and the electromagnets are held stationary. This makes it possible to hardwire the windings to a control unit. Instead of turning magnets on and off with a rotating switch, the control unit can pulse the voltage to each magnet’s winding in the correct sequence and at any speed. It also can adjust the pulse width to control the strength of the electromagnets and therefore the motor’s torque.

A brushless motor needs a control unit that can sense the position of the rotor so it can pulse the current to each winding at the right time. Most brushless motors use a resolver, a type of Hall-effect sensor, to report the “rotor angle position” to the control unit, but small motors use something similar to a two-wire crankshaft sensor. When a magnet moves past a coil of wire, voltage is generated in the coil. The voltage rises and falls above and below zero in a sine wave pattern, and this tells the control unit where the permanent magnets are, relative to the electromagnet windings.

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