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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 - 07:29
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Under pressure doesn’t begin to describe the challenge on this car. Not only is this car overly complicated, but Teresa, the lady who drives this little Prius, is absolutely the most important person in the world to me — she’s my wife! We’ve all heard the saying “a mechanic’s car is usually the last to get fixed.” Not true with this car. We had owned it only a couple of months, and with 78,000 miles on the odometer, I wasn’t even expecting brake pad wear out (thanks to regenerative braking) much less a total braking system loss.

The DTCs I found were in the Skid Control Module (SCM). These active codes were C1300, (ABS ECU malfunction), C1310 (high voltage system malfunction) and C1313 (open circuit in main relay). I also found codes stored in history. The first was U0129 (no ABS communications), found in both the hybrid control ECU and cruise control module, and the second was U0121 (lost communication with the ABS module) in the Electric Motor Powering Steering (EMPS). Not wanting to dive in too deep too quickly, I swapped relays (known good relay for the ABS main relay) with no change although the pulsing pedal would go away with the ABS main relay removed.

Because the factory scan tool could communicate with the ABS and SCM,even displaying wheel speed PIDs changing with road speed and pedal stroke sensor changing with pedal presses, it at least appeared that the ABS controller was not completely brain dead. Just to be on the safe side, I decided to connect my AES Wave LineSPI Diagnostic Link Connector (DLC) break out box and scope out the high-speed Controller Area Network (CAN) data bus.

As expected, it looked as good as any CAN bus pattern I’ve scoped. The Toyota factory manual says if C1300 is cleared and comes back again, the Skid control module must be replaced. This little feat requires a total instrument panel removal and $1,280 (wholesale) for the module. Nearly $1,300 is a lot of money so if I suspect the SCM is the root cause (by sheer nature of an ECU malfunction DTC) I had better be 100 percent sure before proceeding.

Back to the Books
Looking at the factory theory of operation, I see the system works in the following manner:

“ECB (Electronically Controlled Brake) — The skid control ECU receives signals from the pedal stroke sensor, master cylinder sensor and wheel cylinder pressure sensor. Based on these signals, the skid control ECU calculates necessary braking force for each wheel. The necessary hydraulic pressure braking force signal is sent to the Hybrid Control ECU via CAN communications. The skid control ECU receives braking force (regenerative braking force) signal from the Hybrid Control module via CAN communications.

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