Electric Power Steering

Are There Other Electric Power Steering Setups?

The NSX is was the only fully electronic rack in production until it was also put in the S2000, another sports car.  Some other manufacturers are looking into it. Some other cars (MR2, EV1, Subaru, Fieros) have electro-hydraulic racks which simply replace the traditional pump with an electric one. Honda was the first to develop a totally electronic rack that didn’t consume an unreasonable amount of power or cost an unreasonable amount of money. TRW’s version in the mid-80s could literally drain your battery during parallel parking…

Why Did Honda Use EPS?

Honda went with an electric power steering system  mainly because (1) It’s compact and light and (2) it’s not parasitic (robbing power from the engine) since it only draws electric power on demand. But it also provides precise control and is easy to vary the amount of assistance with an electric unit. Plus it’s simpler to install (especially in a mid- or rear-engined car), there are no hydraulic lines to break, and it’s quieter.

Are All NSX EPS Systems The Same?

Honda reduced the size of their original power steering unit in ’95 to make it about 4 pounds lighter than the original ones in the ’91-’94 automatics.

Are there other differences between EPS and non-EPS NSXs?

The main reason the EPS-equipped cars feel different is that they simply use different steering racks with different ratios.

  • '91-'94 5-speed cars have an 18.6:1 ratio
  • '95+ manual cars have a variable ratio of 18.2:1 to 20.8:1. All but the Zanardi have EPS.
  • All automatics to date have EPS with an 18.6:1 ratio regardless of year.
  • The Zanardi has the same variable rack as the '95+ EPS cars and they simply deleted the power assist.

How does the EPS sytem work?

The NSX system consists of a rack-and-pinion steering gear with an electric motor installed concentrically around the rack. The motor transmits its power through a recirculating ball drive mechanism to push the rack right or left.

A "steering sensor" is located on the input shaft where it enters the gearbox housing. The steering sensor is actually two sensors in one: a "torque sensor" that converts steering torque input and its direction into voltage signals, and a "rotation sensor" that converts the rotation speed and direction into voltage signals. An "interface" circuit that shares the same housing converts the signals from the torque sensor and rotation sensor into signals the control electronics can process.

Inputs from the steering sensor are digested by a microprocessor control unit that also monitors input from the vehicle’s speed sensor. The sensor inputs are then compared to determine how much power assist is required according to a preprogrammed "force map" in the control unit’s memory. The control unit then sends out the appropriate command to the "power unit" which then supplies the electric motor with current. The motor pushes the rack to the right or left depending on which way the voltage flows (reversing the current reverses the direction the motor spins). Increasing the current to the motor increases the amount of power assist.

The system has three operating modes: a "normal" control mode in which left or right power assist is provided in response to input from the steering torque and rotation sensor’s inputs; a "return" control mode which is used to assist steering return after completing a turn; and a "damper" control mode that changes with vehicle speed to improve road feel and dampen kickback.

If the steering wheel is turned and held in the full-lock position and steering assist reaches a maximum, the control unit reduces current to the electric motor to prevent an overload situation that might damage the motor. The control unit is also designed to protect the motor against voltage surges from a faulty alternator or charging problem.

The NSX electronic steering control unit is capable of self-diagnosing faults by monitoring the system’s inputs and outputs, and the driving current of the electric motor. If a problem occurs, the control unit turns the system off by actuating a fail-safe relay in the power unit. This eliminates all power assist, causing the system to revert back to manual steering. A dash EPS warning light is also illuminated to alert the driver. To diagnose the problem, a technician jumps the terminals on the service check connector and reads out the trouble codes.

Technical Information