COSTS A TENTH OF THE AFTERMARKET SOLUTIONS, AND SAVES YOU HORSEPOWER AND FUEL! While hot rodders begrudge fuel economy regulations like a childhood curfew, the OEMs’ conquest for efficiency has left us with some excellent LEGO bricks for our projects. Namely, the abundance of electric power steering (EPS) systems that have found their way into factory cars over the last decade. Though Honda/Acura was first to introduce the voltage-assisted tech in the 1991 NSX, it wasn’t until the mid-2000s that the industry saw widespread adoption of EPS, when the Energy Independence and Security Act of 2007 bumped Corporate Average Fuel Economy (CAFE) requirements (or the average fuel economy of an automaker’s passenger cars) up to 35mpg by 2020. The byproduct of these fuel economy-minded changes? What makes an engine more fuel efficient often helps make more horsepower!
By eliminating the hydraulic pump and steering box, a significant – and always present – parasitic drag on the engine is removed. While the Toyota EPS system we’re about to dive into is rated for up to 60amps of current draw, most of the time it will pull a tenth of that as you go about typical driving, and the battery is able to compensate for much of that current draw before it passes the load to the alternator. This means that you can retain one-finger power steering while freeing up a little extra horsepower and maybe even a little fuel mileage. Plus, it removes a lot of plumbing, flammable fluids, and weight out of the engine bay. EPS isn’t necessarily new to the aftermarket, but with most kits starting around $1,000 for universal components, this $150 junkyard jewel makes for a perfect donor.
HOW DOES THIS EPS WIZARDRY WORK? EPS is an incredibly simple system that uses a torque sensor to measure driver input (steering position, torque, and velocity), an electronic control unit (ECU) to interpret that data, and a beer-can-sized electric motor on a worm-drive to directly assist the steering column (though some systems, like the aforementioned NSX, place the motor on the steering rack instead). While not necessary for this particular swap, the EPS ECU also takes data from the CAN-bus to interpret vehicle speed and stability control information into how much assist it produces – mostly to provide extra boost at low speed to ease parking lot maneuvers, while lessening the boost at higher speeds so that the steering is less sensitive and tracks straighter with less input.
The torque sensor is the real magician in this system. The steering shaft is magnetized with multiple North and South poles along its circumference, which act similar to the teeth of a Hall Effect crank or wheel sensor. They as an encoder to tell the EPS ECU what angle the steering wheel is and how quickly it’s turning.
When you steer the wheel and rotate the magnetized steering shaft, the internal magnetic field distorts with the input torque (twist), which is read by the magnetic field sensor and the EPS ECU, and calculated into steering torque number to measure how hard you’re steering and how much feedback is coming through the steering gear.
With this data, along with the optional VSS input, the EPS ECU decides what direction to spin the motor along with how much force.
A popular donor for this mod are the GM Delta platform cars: the Saturn Ion, Chevy Cobalt and HHR, and Pontiac G5. The cars were prolific, and with 15 years of production, abundant in junkyards today; but they’re not as brain-dead as the Toyota EPS when removed from the car. The EPS ECU found in the Delta cars still requires a vehicle speed sensor signal, which can be simulated with an extra little module found online for $75 to $100. For the Toyota system, however, it uses a fail safe mode (DTC U0073 and U0121) when it can’t see the CAN-bus or vehicle speed sensor, setting its assist like it would at 43 mph. This sounds like a bodge, but pioneers of this swap have reported thousands of miles of usage with no issues. This makes the Toyota EPS entirely stand-alone, simplifying the swap further while reducing costs by eliminating the need for extra parts. Depending on how your local yard charges, the entire column, motor, and ECU can be had for $50 to $150. We found ours in a second-gen 2004-2009 Prius, but these can also be found in most 2009+ Corollas and Yaris as well. We’ve read that Nissan, Hyundai, and Kia possibly use a similar stand-alone EPS, but can’t confirm it ourselves. INSTALLING INTO A PROJECT We’re going to be a little brief here, as each install is unique to the project it’s going into, but we’ll give you the skinny on what the Toyota EPS needs to operate.
The Toyota EPS only needs three wires to operate. An 8-gauge power feed with a 60 amp fuse supplies the main juice along with an 8-gauge ground (6-gauge or larger may be necessary for longer, trunk-mounted battery harnesses). Next, Pin 6 of Connector P8 is run to a fused, switched 12v source to trigger the system when the key is turned “on.”
First and foremost, the factory fuse is 60 amps, so we’d recommend 8-gauge or larger wiring for the primary 12v and ground wires that power the ECU and motor. While this current draw is one of the highest you’ll see in most automotive systems, EPS operates at about a tenth of that in typical operation. Think of it like a hydraulic system, it builds maximum pressure and stresses the system most when the wheel is against something, like a curb or the steering-stops themselves, and cannot steer further. Next, the only other hook-up needed is to a switched 12v source that turns on with the ignition. Once wired, you’ll hear the ECU click its main power relay, and within a few seconds, the motor will come alive. You may notice that without any connection to a steering system, that the motor does basically nothing. The torque sensor needs to see a load on the steering shaft to begin its assist, so be sure to lock down the steering box/rack end of the assembly when testing the unit’s function. Mounting the Toyota EPS column is project-dependent, but it comes with two very usable factory mounts for 12mm hardware. The lower mount is a simple, using a Delrin-isolated cross bolt that doubles as a pivot for the tilt; and the upper mount incorporates a sliding lock for tilt.
The lower mount can pivot with the tilt mechanism in the upper mount, and is retained by a long, 2.5- to 3-inch-long, 12mm bolt.
Depending on how much of the steering column tube and upper mount is retained when adapting your column, the factory’s upper mount incorporate the tilt mechanism.
What you’ve got to engineer is the adaption to your factory or aftermarket steering shaft and column to connect the Toyota EPS to your machine’s steering wheel and tie rods. We’ve seen a handful of solutions for this, but it’s similar to the skills necessary to shorten and install any other steering column. Some folks machine (or carefully grind) the Toyota steering shaft to accept a D-shaft coupler, like what’s found in ididit’s universal columns. Others have bored the Toyota shaft out while turning the diameter of the factory steering shafts down to fit inside, then press-fitting and welding the assembly together. We made sure to grab the steering shaft and its U-joint so that adapting it downstream would be easier without needing to source the Prius-specific pieces, saving a few more bucks – but CJ’s Pony Parts sells the appropriate 11/16-inch 36-spline weldable coupler if needed.
You’ll also have to “de-power” your hydraulic steering box or rack, or use a manual-steering unit to complete the swap, but this is required for any EPS conversion.
We’ll focus on install options in a future story, but for now, we wanted to plant the EPS seed in your brain as you chew on ideas for your next project.