Modifry Products - Freebies

OK, now that we have that done with, I'll describe what I did to my car as an experiment only, as I have no intention of driving with this mod permanently installed on my car. This mod adjusts the amount of power steering assist provided by the EPS system from "full factory assist" to an estimated "10% of factory assist". It does this by reducing the signals sent from the steering torque sensor to the EPS computer, essentially telling the computer you are applying less torque to the steering wheel, and in turn the computer provides less power assist. Why would I want to do this? Partly just for the knowledge gained, and partly because a customer wrote to me asking if it could be done. He referenced an article in a European car magazine that described a similar project on a Toyota Prius, and after looking at the article I became intrigued with the idea and decided to do some experimenting. If you check out the article please note that I saw several flaws in their project, both in the suggested cost (under-estimated by 500%) but also in the design and implementation of the circuitry. Do NOT follow their instructions; all you will get is an EPS fault light. I'm not even sure their mod actually worked on the Prius, but it sure made for a "cool" article. The intent of this mod is to gain more "feedback" or "feel" for what the front wheels are doing, with the idea that more "feel" makes it easier to discern the limits of adhesion and thus improve the driver's ability to control the car. I don't know if that's true or not. My testing was limited to an empty parking lot at low speeds and for the most part both myself and my trusty assistant didn't notice any difference in "feel", but if adjusted throughout the range it was possible to make the steering really, really difficult to turn. Perhaps on a race course or more uneven surfaces a difference might be noted, what do I know?

Above is a wiring diagram of the factory S2000 EPS torque sensor and control unit. The pink and white/green wires carry the torque signals to the computer, and based on these signals the EPS unit tells the electric motor attached to the rack to apply power to "assist" the driver turning the steering wheel. For this mod I cut the sensor wiring and inserted a 100-ohm potentiometer between the 2 sensor signals and the EPS unit, as shown in the drawing to the right. In order to splice into the circuit I removed the EPS unit and cut the wiring harness just below the EPS connectors, running the wires through the firewall into the car. The pot is wired in series with the signal wires, and as the pot is turned the added resistance reduces the torque signal seen by the EPS computer, causing it to add less power assist.		In the above diagram, new wiring is shown in blue, potentiometer in red. Note that the pot is a "2-gang" unit, which means two pots on a single shaft. The dotted red line indicates both pots are physically linked so they turn in sync with each other. You must use a 2-gang pot because both signals must be attenuated equally, otherwise you will cause an EPS fault condition and the power steering will shut down.

Here is what I did for my silly "drive by knob" demonstration (video on youtube). I used the same pot as before but wired one side "backwards". The effect is that as the pot is turned, one torque signal is increased and the other is decreased, but they are still adjusted equally to prevent the EPS unit from generating a fault condition. Since the resistance of the 100-ohm pot reduces the torque signal too much I added a 22-ohm resistor in parallel with the pot, effectively giving me a 20-ohm pot. With the knob centered, each torque signal is reduced by the same resistance, approximately 14 ohms, and the EPS unit interprets this as "no pressure on the steering wheel". As the knob is turned to one side of center, one pot increases resistance and the other decreases, simulating pressure on the wheel and causing the EPS unit to provide assist. If enough assist is applied the steering wheel will move without the driver touching it. I don't know how much power assist the system can provide but with the setup I used here an average person can easily overpower the EPS system. I tested this with a helper and he was able to hold the wheel relatively still as I swung the pot from side to side. If you move it too quickly it will generate a fault light, I assume because the EPS computer knows that a human can't turn the wheel that fast so it thinks a fault has occurred and shuts down the power steering.

Scope trace below shows the signal from the EPS control unit to the torque sensor in blue and one of the sensor signals in red. The frequency of the waveform is 14.2 KHz and you can see lots of ringing in the signal coming from the EPS unit, I assume this is due to the inductance of the torque sensor.		Here we have both sensor signals, which are essentially identical and in-phase. When the driver applies pressure to the steering wheel the amplitude (height) of the signals changes. One signal gets larger and the other gets smaller; when pressure is applied in the other direction the amplitude changes are reversed.