On Christmas Eve 2009, a new PriusChat community user on Yahoo pointed to the article “NHTSA Tracking Braking Loss on Prius Hybrids,” which first raised the issue of a problem with the Prius braking system. It stated there were at least “33 complaints” out of 100,000 new model, 2010 Prius cars. Owners were wondering what was going on.
The reaction within the Prius community ranged from “What brake problem?” to “It’s a scary safety issue.” So taking the classic engineering approach, this engineer along with associates set out to gather more data, starting with a survey of Prius owners to find out the extent to which they had experienced the problem. The survey revealed that owners of both the 2009 and 2010 models had experienced intermittent braking problems:
PRIUS SURVEY CHART
Continuing the investigation, questions were asked about details and conditions under which Prius owners had experienced a braking problem. Soon, there were dozens of photos that clearly implicated potholes. The photo montage also revealed the truly awful state of roads across North America.
Through the results, four common elements were discovered of every situation in which a braking problem occurred: a road imperfection or significant bump; (2) winter or wet conditions; (3) slow speed, and; (4) gentle or moderate braking. Most of the reports were from Northern latitudes (where winter brings a bumper crop of potholes), which also might explain why a resident of Northern Alabama, had not experienced the braking problem…
The PriusChat community proposed that the combined elements of slow speed and gentle braking pointed to a transition between regenerative and mechanical braking. The Prius uses the electric motor as a generator when stopping to charge the traction battery. As the car slows down the brake computer has to transition seamlessly to mechanical braking. Reviewing the brake section of “2010 Toyota Repair Manual, Volume 2” (RM1291U2), references pointed to a “30 km/h (19 mph) to 0 km/h” (pp BC-85, BC-87) critical speed range. Unfortunately, feedback from the users was qualitative and lacked precise engineering units, which was the precise data we needed for our investigation.
Realizing some true field testing was needed, the engineering team ordered two Gulf Coast Data Concepts‘ accelerometers (one for an owner in Michigan who frequently experienced the problem) and the other for local testing on a Prius in Alabama.
The first morning of testing with the accelerometer in Alabama was perfect–wet and drizzly. Approaching two rows of speed bumps at a badge check gate at a work facility, it happened: The car ‘slipped’ just after the first speed bump. Knowing 19 mph was a critical speed, the Prius was slowed to just under 20 mph before hitting the first bump. As shown in the chart below, the accelerometer captured a momentary ‘slip and slide’ sensation at that instant.
Clearly the braking force, Ax, went to zero after the speed bump. The integrated velocity curve, V, shows an 800 milliseconds flat line before the braking force resumed. We finally had the brake anomaly, a pause, documented with engineering units. Meanwhile, about this time Toyota had a new release of the brake control software added to the Prius production line and announced a recall program, SSC-A0B. Regardless, we could use this protocol to validate the fix.
Ten days later after the fix and under similar conditions the accelerometer showed the problem gone:
The key steps in our investigation were to:
- Solicit user reports looking for patterns – intermittent problems often seem to hover around a few and are missed by many. We needed to capture as many descriptions with as much detail as possible to find a way to reproduce the problem at will.
- Understand the systems – we found boundary conditions in the braking system. Knowing 19 mph was a threshold speed, we focused our efforts in this range.
- Convert intermittent to reproducible problem – as soon as the problem is reproducible at will, experiments can map the boundary conditions.
- Quantify the problem – add instrumentation to convert imprecise text into engineering units
- Validate the fix – using the problem protocol, test to validate the fix.
Toyota’s fix, SSC-A0B, solves the vast majority of these intermittent, slow-speed, momentary, brake pauses. But often one intermittent problem can mask a less frequent problem. After eliminating incompletely applied patches, there remain scattered reports of another braking anomaly, and investigations continue.