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Here’s how to reprogram your digital odometer to read whatever mileage you want. It great for cluster swaps, as in my case I swapped a Lexus ES300 cluster into my Toyota Solara.
Many cars store digital odometer values on the instrument cluster itself. It’s stored on a little chip on the speedometer circuit board; it’s called an EEPROM chip, usually of the 93C46, 93C56 or 93C66 variety.
Once you get access to the chip by opening the cluster, it can be de-soldered and either swapped or reprogrammed.
I have gone through extensive testing and decoded the chip to present the following procedure. I’ve worked with Camry, Solara and ES300 clusters which are all interchangeable, but the procedure should be the same for many, many cars that use a similar chip setup, with only the programming differing.
Legal Stuff
It’s perfectly okay to change your odometer reading by yourself. However, it is illegal to roll back or misrepresent the mileage without disclosing to a potential purchaser of your vehicle that the odometer has been tampered with. Keep clean.
Tools and parts needed:
• Screwdrivers
• Soldering iron, solder and a de-soldering pump
• Computer with Windows XP and serial port
• 8 pin DIP socket
• Serial programmer
- Breadboard
- Hookup wire
- Female serial port header
- 5V from computer power supply
- 4.7K ohm resistors
- 5V Zener diodes
o Wire strippers
• Serial programming software (PonyProg freeware)
• A spare instrument cluster in case you screw up
Undo these tabs to pop off the front cover and bezel.
Turn it over and remove four screws holding the white cover and transformer circuitry at the back.
I attempted to disassemble the instrument cluster from the top down, i.e. remove needles and gauge face.
However, removing the needles is NOT NESSESARY.
I found this out the hard way. I broke the florescent tube that back-lights the cluster when putting everything back together.
Here’s what the gauges look like when the needles are removed. They just pry up.
Needles.
Gauge face removed.
If you remove the two black screws securing the speedometer circuit board from the diffuser panel, this is what you’ll see. Make sure the spiral cable that powers the lit needles doesn’t get messed up.
Instead, to get access to the odometer chip on the speedometer PCB from the back, you should flip over the instrument cluster and remove 24 screws. Keep track of where the screws go, some of them are different thread and lengths.
Then the needles, gauge face, speedometer / tachometer circuit board assemblies will drop out of the shell, and the florescent tube can be easily removed by sliding it up.
This is the speedometer board to which the odometer chip is attached to.
Turn it over, and you’ll see a little black 8 pin chip. That’s the EEPROM chip we are after.
Here’s what the chip looks like on the Camry / Solara PCB. It’s a 93C46 EEPROM chip.
De-solder the chip. Use the de-soldering pump to suck away excess solder on the joint, but be careful not to damage the solder pads, as you’ll need to re-solder to those.
Here’s the chip de-soldered from the board. A DIP socket was soldered in to provide quick swapping of chips when testing various programs.
Dip socket soldered on.
The next stage is to connect the EEPROM chip to the computer for reprogramming. A serial programmer can be bought off eBay, or you could build one yourself for a couple of cents.
Here’s the original schematic, which included a voltage regulator, which drew 5V directly from the serial port.
It didn’t work for us, so we opted to take 5V from the computer’s power supply. There shouldn’t be an issue with this, since the grounds are common.
Here’s my home-made serial programmer, as basic as it gets.
Here’s my hacked connection to the serial port, because I ordered the wrong serial port header.
Here’s a pinout of the serial port for the connections.
Once the chip is connected and powered on, open up your serial programming software. I used PonyProg, which is a freeware that can read information from the serial port. It can be downloaded here: http://www.lancos.com/prog.html
First start with a calibration under the options menu, and then go to setup.
Set the I/O to Serial, select the COM port, and leave everything else unchecked.
Then go to device, select, microwire eeprom and select your chip type. The Camry/ ES300/ Corolla of this vintage all use the 9346.
Click the top left hand icon to read data from the device. You’ll be presented with a dump of what’s in the EEPROM chip.
The odometer readings are stored in lines 00 and 10 for these cars. As indicated below, it’s coded in hexadecimal, in a cluster of four digits, repeated three times. These are the values we have to edit. There are sometimes little symbols on the right hand side of the dump that tend to point to the last digit/ data that was changed, which can give a clue when trying to decode these chips.
If you can obtain a copy of TachSoft software, it can vastly help with the decoding process, and determining what kind of chip you have.
Decoding
In the example above, the odometer read 629,209 km. The hex values in the addresses sited above read 6D FA FF 9D. As it turns out, each of those hex digits correspond to a digit on the odometer readout by means of an inverted hex. The inverted hex is essentially 0-9, then A-F, backward as noted in this lookup table.
The first digit “6” corresponds to the thousandths column, the second digit “D” corresponds to the hundredths column, the third digit “F” corresponds to the tenths column, the middle three digits are unknown, the seventh digit “9” corresponds to the one-hundred-thousandths column and the eighth digit corresponds to the ten-thousandths column.
I haven’t yet figured out what controls the ones column, but I don’t think it matters as much as changing the first 5 columns.
If we take the inverse of each hex digit using the lookup table, put these together, you can see how the code forms the odometer reading, 629,20x km.
I made an excel sheet to calculate all the inverted hex values when I type in my desired mileage.
In my case, the desired reading is 265,650km, which corresponds to A9 AF FF D9. I need to replace the hex values in each of the three spots it’s repeated in the EEPROM code. Click edit, then Edit Buffer Enable in PonyProg to enable editing. Good idea if you saved the original chip dump in case something gets messed up during editing.
Then click the second top left button to write the new edited file to the chip. Then click Edit, Verify to verify the write was successful.
Done with the programming! Disconnect the circuit and place the chip in the correct orientation on the DIP socket on the odometer circuit board.
Gauge coloring
If you’ve chosen to remove the diffusers from the gauge face, now’s a good time to grab a permanent marker to color your gauges. Color the top edge of the speedometer diffuser.
Also color the main diffuser that goes across the entire gauge. Do an even job across the top edge where it meets the florescent light tube, otherwise the color won’t be consistent across the gauge face.
Replace the gauge face and needles if they were removed. The needles are easy to align, just plug them into the two terminals that power them, no need for calibration.
Reinstall the gauge face assembly into the housing. Reinstall all 24 screws, making sure they go in their respective locations. A power screw driver really helps here, but don’t over tighten or you’ll crack the diffuser.
Replace the transformer board and its white cover and reinstall its four screws.
Reinstall the plastic bezel, clean off the gauge face from fingerprints, and reinstall the tinted front cover.
Reinstall the gauge cluster into the vehicle.
Turn on the vehicle and check to make sure all the gauges work. Note the new mileage reading, 265,654 km, which is correct for my vehicle. Also note the color distribution on from the marker on the diffuser. In person, the blue was quite hard to see in daylight.
Take a test drive and make sure all the gauges including the odometer works properly. Ultimately I decided to remove the blue tint on the diffuser with rubbing alcohol, because I liked the look of white gauges better.
Now you can sit back and enjoy your corrected mileage reading without being flagged come inspection time.
Mr.SpeedKar
How did you come up with the 4.7ohm and 5V Zener Diodes?
I was looking at a ponyprog adapter schematic and they used a 1k and 4v7 diode
Also do you think it matters on the type of resistor and diodes used as there is multiple variants with different percents and watts?
Thank You for your contribution.
The resistors control the current but is not so crucial. The diodes are more crucial for voltage regulation.
The chip has a large tolerance for logic level high, as long as the zener diodes are rated above the minimum logic level high, then it should work fine.
Have to modify it some to work with the chip in this era of Toyota/Lexus clusters. I spent a fair bit of time digging up the info, then found this video. Would have saved me a bunch of time.
05-02-14, 06:21 AM
DIY: Odometer Reprogramming
If anyone needs the required software send me a PM.
