Adjusting the odometer reading on a Toyota Prius requires specialized knowledge and tools, unlike many American cars with simpler, menu-driven correction tools. This guide delves into the intricacies of Prius odometer correction, offering a DIY approach for those comfortable with electronics and soldering.
Locating and Accessing the Odometer Data Chip
The Prius stores its odometer data in a 93C66 EEPROM chip located beneath the LCD screen on the combination meter. Accessing this chip requires careful desoldering of the LCD, which is attached to the board with numerous pins. While removing the LCD is tedious, it’s necessary to access the EEPROM chip’s pins.
After removing the LCD, you’ll need to remove the solder mask from the 93C66 chip’s pins to prepare for connecting an EEPROM programmer.
Deciphering the Odometer Data Structure
The odometer reading is stored as a four-digit hexadecimal code, multiplied by 17 to obtain the actual mileage. For instance, a hex code of 1673 (decimal 5747) translates to 97,699 miles (17 x 5747). This hex code is repeated 17 times within the EEPROM data. However, achieving single-mile increments requires a more complex approach. The Prius utilizes a combination of two hex codes and a “mask” to achieve precise mileage adjustments. The ratio of these two hex codes, along with corresponding changes in the mask, determines the final odometer reading.
This image showcases how two hex codes (1673 and 1672) are used in combination to represent a specific mileage. Understanding this system is crucial for accurate odometer correction.
Reading and Writing to the EEPROM
An EEPROM programmer is essential for reading and modifying the data on the chip. Affordable options like the XGecu programmer are available online, but may require some troubleshooting and familiarity with the software. More user-friendly, automotive-specific programmers are also available from companies like Andromeda Research Labs. To streamline the process, creating a custom adapter with pogo pins for connecting to the chip’s test points can significantly reduce setup time.
This image shows a custom adapter built for quicker connection to the necessary test points. Investing in such a setup can save considerable time on subsequent adjustments. Various adapters can be created for different chip types and programming needs.
Verifying the Corrected Mileage
Before reinstalling the combination meter, it’s crucial to verify the adjusted mileage. This can be done by applying 12V power to specific pins on the meter (pin 22 for positive and pin 14 for negative). This will activate the mileage display, allowing you to confirm the accuracy of the correction. While speedometer function testing is more complex, requiring simulation of network signals, confirming the odometer reading is a vital step.
Successfully adjusting a Prius odometer demands patience, attention to detail, and a solid understanding of hexadecimal code and electronics. While more involved than using an odometer correction tool designed for many American cars, this DIY method provides a viable solution for those willing to tackle the challenge.
