gen 3 steering feel mods

Sigh, writing this a second time since the first one disappeared.

So I saw the post on pulling the power steering fuse and how much it improves steering feel. I tried it, and man, it is much nicer to be able to feel the wheels on the road and the resistance when turning. When going under 5mph It makes for a good workout, but really sucks if you need to make a quick move or do a 3 point turn. I found an article on tricking the sensors into decreasing the power to the steering. It's for the first generation prius, but looking at the manual, the third generation has the same 2 torque sensors varying form 0 to 5v, and the same voltage lines.

The power steering control module is under the dash, and quite a pain to get to. But after disconnecting the battery and 15 minutes of pulling off plastic bits, I finally reached the power steering ECU.
(disconnect the battery and wait 10 minutes before unplugging the yellow airbag wires)

I followed the article, using a 5k ohm pot between the positive and negative, and a 1.8k ohm pot between the output of the first and the input from the sensor. Output of this pot (the wiper) then goes into the power steering ECU. However, once I plugged everything in and turned the car on, it didn't work as expected. Once I turned the pot past 3% (so basically no resistance between the original input and the output), steering cuts out and it gives me the error C1513: too much deviation between the two torque sensors.

I measured the resistance on the 1.8k ohm pot, and it turns out there is a huge jump in resistance after turning it about 3%. I'm going to get a 10k ohm 10 turn pot, which should hopefully fix this. If not, both torque sensor values may need to be modified for this to work.

People on hot rodder forums have used gen 2 prius systems in classic cars, since they don't need a CAN connection, and default to slightly heavier steering. so I tried disconnecting the two CAN bus wires, but this just caused the power steering to turn off completely. The voltage on the 5v lines also dropped to zero, so clearly, this version needs CAN connection.


Sorry, I can't insert a link until I have 4 posts, just look up 'autospeed modify electric power steering'

1i2TZX1 by NoInstructions posted Jul 25, 2020 at 4:46 PM

Ub1B8hf by NoInstructions posted Jul 25, 2020 at 4:46 PM

GVgJMDN by NoInstructions posted Jul 25, 2020 at 4:46 PM

Rx35Vn3 by NoInstructions posted Jul 25, 2020 at 4:46 PM

3LoPhcs by NoInstructions posted Jul 25, 2020 at 4:46 PM

 

So I asked on an electronics discussion board and discovered that this is just an inherent effect of the signal into the ecu being part of a circuit, and eventually going to ground. To fix this, an amplifier needs to be used to isolate the output voltage from the circuit before it. I purchased an op-amp and set it up so there is no gain/amplification.
Also, Instead of using a potentiometer to try to produce 2.5 volts, I decided to purchase a DC voltage regulator designed to output 2.5v

I hooked this all up and ran it, and it works much better. I can turn the regulator pot 2 turns (out of 10) before the steering detects something is wrong and shuts off. This happens because I am only modifying one of the signal wires, not both of them. When the signal from one differs too much from the other it shuts off. Interestingly, this only happens when the wheel is turned all the way until it stops. I'm guessing the sudden spike in force when you hit the end stop makes it easier for the ECU to realize the two signals are too far apart.

I may just keep it with the slightly improved steering feel, as this lets me feel when the wheels are going over uneven patches or pulling to one side, but doesn't increase the effort to turn the wheels when stopped by much. It's also a bit of a safety feature to have that second sensor as stock, so the EPS can always compare the two.

If you pull the power steering fuse, you'll notice that after about 20mph the steering wheel isn't even that hard to turn. Definitely want to be careful not to fool it too much, say getting the power steering assist to act the opposite way you turn the wheel.

 

I doubt there's any wear, as this is literally just spoofing the sensor values the computer sees, resulting in the computer giving slightly less power to the motors.

I have attached a circuit diagram below in case anyone is interested. I'm going to solder it together and test it some more.