Have you ever upgraded your Jeep JK tires and noticed your speedometer is no longer accurate? Recalibrating your tire size is crucial for accurate speedometer readings, proper gear shifting, and the overall performance of your vehicle. While commercial programmers are readily available, did you know you can adjust your tire size settings directly through your OBDII port? This guide will walk you through the process of programming your tire size in Jeep JK ECUs (Electronic Control Units) using an OBDII interface, without needing to rely on expensive, specialized tools.
This method, tested on a 2018 Jeep JK, utilizes the OBDII Data Link Connector (DLC) conveniently located near the driver’s left knee. By understanding the underlying communication protocols, you can take control of your vehicle’s settings.
Understanding OBDII Communication for Tire Size Adjustment
The communication framework for this process is built upon two primary protocols: ISO 15765-4 CAN, operating at 500 kbaud with an 11-bit ID, and a protocol layer akin to Keyword Protocol 2000, also known as ISO 14230-3. This may sound complex, but the key takeaway is that these are standard automotive communication languages that allow you to interact with your vehicle’s computer systems.
Within the JK’s architecture, the tire size and speedometer ratio are primarily managed within the TIPM (Totally Integrated Power Module). Specifically, the setting resides in “local identifier” 0x0c. Interestingly, this same value is also found within the TCM (Transmission Control Module) at “local identifier” 0xb1, along with other related parameters.
Step-by-Step Guide to OBDII Tire Size Programming
The following command sequence outlines the process to reprogram your tire size. It’s crucial to enter these commands precisely as shown, in hexadecimal format. Each line begins with the CID (Controller Identifier), followed by the SID (Service Identifier), and then the PID (Parameter Identifier). Data bytes follow the PID in write commands.
Here’s the command stream for tire size programming:
01C 3E 02 // to unkn2, tester present
7DF 3E 02 // to general, tester present
620 10 92 // to tipm, diagnostic session control
620 3B 0C 0E 92 03 E8 0F A0 p1 p2 FF FF 00 03 11 1B A0 01 00 01 02 // to tipm, write local ID 0x0c
01C 3E 02 // to unkn2, tester present
7DF 3E 02 // to general, tester present
620 10 92 // to tipm, diagnostic session control
7E1 10 92 // to tcm, diagnostic session control
7E1 3B B1 6A 64 34 72 05 01 00 0A 02 00 05 10 27 27 01 06 02 02 0C 00 00 01 00 02 00 02 00 00 4F 01 92 0E E8 03 A0 0F p2 p1 23 00 qq rr 41 56 43 2D // write tcm local ID 0xb1
784 11 82 // to unkn4, reset
620 11 82 // to tipm, reset
784 11 82 // to unkn4, reset
7E0 11 82 // to ecm, resetUnderstanding the Variable Parameters: p1, p2, qq, and rr
Within these commands, p1, p2, qq, and rr are the variable parameters that directly relate to your tire diameter. The values for p1 and p2 are derived from a calculated value we’ll call ‘pp’.
Calculating ‘pp’ for Your Tire Size
The formula to determine ‘pp’ is:
pp = diameter (in inches) * 80 – 8
For example, if you have 32.75-inch diameter tires:
pp = 32.75 * 80 – 8 = 2612
Converting 2612 to hexadecimal gives us 0x0a34. Here, p1 represents the most significant byte (0x0a), and p2 is the least significant byte (0x34).
Important Note on Tire Diameter: Always use the rolling diameter of your tire, not the unloaded diameter. A tire advertised as 34 inches might have a rolling diameter of around 32.75 inches once the vehicle’s weight is applied. Consult tire specifications or measure the rolling circumference for the most accurate value.
Understanding ‘qq’ and ‘rr’ Values
Currently, a precise formula for calculating qq and rr values remains undetermined. The ‘qq’ value might represent a signed analog value, while ‘rr’ could be flags. For practical purposes, you can utilize the following table of observed values for different tire diameters:
| Diameter (inches) | qq (hex) | rr (hex) |
|---|---|---|
| 31.2875 | 35 | 29 |
| 32.0 | 21 | 9d |
| 32.25 | e3 | ad |
| 32.5 | eb | 3d |
| 32.75 | ee | cd |
| 33.0 | f7 | 5d |
| 34.0 | cf | 9d |
| 35.0 | d5 | 5d |
Choosing the Right OBDII Interface
This programming method has been successfully performed using affordable, readily available OBDII interfaces such as the Bafx and the OBDLink SX USB. The Bafx utilizes a clone of the ELM327 chip, while the OBDLink SX uses the STN1110 chip.
It’s worth noting that both ELM327 and STN1110 chips have limitations in handling multi-frame writes correctly. The STN1110 generally performs slightly better. Workarounds, such as toggling Auto Format (caf0/caf1) and Response (r0/r1) modes, might be necessary to overcome these limitations.
For a more robust and seamless interface, a Raspberry Pi with a CAN bus interface could be implemented. However, for basic tire size programming, ELM327 or STN1110 based interfaces can be sufficient.
Conclusion: DIY Tire Size Programming via OBDII
By understanding the OBDII communication protocols and utilizing readily available interfaces, you can successfully program your Jeep JK’s tire size settings yourself. This DIY approach offers a cost-effective alternative to commercial programmers. While some technical understanding is required, this guide provides a solid foundation for taking control of your vehicle’s calibration and ensuring accurate performance after tire upgrades.
Happy trails and happy coding!
