Programming your Jeep JK’s tire size after upgrading to larger tires is crucial for accurate speedometer readings, proper transmission shift points, and optimal vehicle performance. While commercial programmers exist, this guide explores a DIY method using an OBD2 interface to calibrate your tire size without specialized tools. This process has been successfully tested on a 2018 Jeep JK.
Understanding the Basics
This method utilizes the OBDII Data Link Connector (DLC) located near the driver’s left knee. The underlying communication protocol is ISO 15765-4 CAN, with an 11-bit ID and a 500 kbaud rate. A secondary protocol layer resembling Keyword Protocol 2000 (ISO 14230-3) is also employed.
The key component for tire size programming is the speedometer ratio, stored within the Totally Integrated Power Module (TIPM) under local identifier 0x0c. This value is mirrored in the Transmission Control Module (TCM) at local identifier 0xb1, along with other parameters.
Command Sequence for Tire Size Programming
The general command sequence for modifying the tire size involves a series of hexadecimal commands sent via the OBD2 interface:
- Tester Presence:
01C 3E 02(unknown module),7DF 3E 02(general broadcast) - TIPM Diagnostic Session:
620 10 92 - TIPM Write Command (Local ID 0x0c):
620 3B 0C 0E 92 03 E8 0F A0 p1 p2 FF FF 00 03 11 1B A0 01 00 01 02 - Tester Presence:
01C 3E 02,7DF 3E 02 - TIPM Diagnostic Session:
620 10 92 - TCM Diagnostic Session:
7E1 10 92 - TCM Write Command (Local ID 0xb1):
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 - Module Resets:
784 11 82(unknown module),620 11 82(TIPM),784 11 82(unknown module),7E0 11 82(ECM)
Image source: Wikimedia Commons
In these commands, the first three digits represent the Controller Identifier (CID), addressing specific modules like the TIPM or TCM. The next two digits are the Service Identifier (SID), and the final two are the Parameter Identifier (PID). Data write commands include the PID followed by the data bytes to be written.
Calculating Tire Size Parameters
Crucially, p1, p2, qq, and rr are variable parameters related to tire diameter. Calculating p1 and p2 is straightforward:
pp = diameter (inches) * 80 - 8p1= Most significant byte ofpp(hexadecimal)p2= Least significant byte ofpp(hexadecimal)
For instance, a 32.75-inch tire: pp = 32.75 * 80 – 8 = 2612 = 0x0a34 (hex). Therefore, p1 = 0x0a and p2 = 0x34.
Important: Use the tire’s rolling diameter (under load) for accurate calculations, not the unloaded diameter. A 34-inch tire might have a rolling diameter of around 32.75 inches.
Determining qq and rr is more complex, potentially representing a signed analog value and flags respectively. A table of observed values for different tire sizes is provided below:
| 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 |
OBD2 Interface Considerations
Affordable OBD2 interfaces like the Bafx (using an ELM327 chip variant) or the OBDLink SX USB (using the STN1110 chip) can be used. However, limitations with multiframe writes on these chips may necessitate workarounds, such as toggling Auto Format and Response modes. A more robust solution could involve a Raspberry Pi with a dedicated CAN bus interface.
Image Source: scantool.net
This DIY method allows Jeep JK owners to calibrate their tire size using readily available OBD2 interfaces and a bit of technical knowledge. Remember to double-check all calculations and commands before implementation. This information is for educational purposes and should be used responsibly. Consult your vehicle’s documentation for official guidance.
