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Cracking a Toyota's code

This Prius brake-by-wire system may be new technology, but old-fashioned troubleshooting led to the fix.
Wednesday, June 27, 2012 - 06:29
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The ECU calculates the necessary hydraulic pressure braking force based on the necessary braking force and regenerative braking force. Necessary hydraulic pressure is supplied to each wheel by adjusting the brake accumulator (hydraulic pressure source) pressure with each solenoid valve. If there is a problem with braking function, the rest of the normal operating parts will maintain brake control as a failsafe.”

Now I must have had a very rare exception because my “failsafe” didn’t feel too safe with the rest of those hydraulic brake components giving me back up to the electronically controlled system. One thing was certain though: this was my first problem on a brake by wire system and it had to be fixed right.

Being an old GM guy at heart, I began to rephrase the Toyota components and theory in GM terminology the best I could. Looking at the schematic, I observed the wheel speed sensors connecting into the skid control ECU along with the typical Toyota Vehicle Stability Control (VSC) inputs such as steering angle, yaw and lateral acceleration. The main brain for the hybrid system (Hybrid ECU) also reported to the Skid Control Module via the CAN bus.

The wheel speed sensor input told me that the Skid Control Module was in charge of ABS. The hybrid ECU communication relationship told me the regenerative braking is performed by varying amounts of field activity to MG2 (Motor Generator 2 – connected to the final drive/wheels) to give progressing amounts of what I would call engine braking (electric motor braking actually), which takes care of 85 percent of the vehicle’s stopping force (front brakes usually do about that much on a FWD car). This is while the remaining 15 percent is accomplished with hydraulically applied friction brakes (the ones we all know and love) courtesy a commanded apply of the Brake Actuator Module.

Looking at that module, the GM terminology side of my brain said, “Oh, an ABS hydraulic modulator assembly.” Well we all know that today’s ABS-equipped vehicles can apply braking force either in an ABS stop or even in a less than locked up situation (dynamic rear proportioning) by simply running the modulator pump motor and activating the correct solenoids in the modulator assembly to apply hydraulic braking force to the necessary drive wheel(s) in the event of a traction control event. Thinking back to the original high-voltage vehicle, the GM EV1, I recalled that this was similar to how it accomplished ots hydraulic braking. Only the EV1 used a ball/screw motor-based (instead of solenoid and pump based) Traction Control System (TCS) like brake-by-wire system.

On this Toyota, aside from a failsafe mode of driver’s foot application actually getting the attention of the brake hydraulics, the electronics are completely in charge. Years ago I tested an early Prius system’s pad apply during a road test by rigging a test light to glow when a front brake caliper applied a pad to the rotor. Its front pads never touched the rotor until the car got below 5 mph. Now that’s regen braking!

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