Changing tire sizes on your Jeep JK is a common modification, enhancing both its look and off-road capability. However, upgrading to larger tires can throw off your speedometer, odometer, and even your vehicle’s shift points because the factory ECU (Electronic Control Unit) is calibrated for the original tire size. Typically, correcting this requires a commercial Obd2 Tire Size Programmer. But what if you could do it yourself without expensive tools?
This guide delves into how you can program your Jeep JK’s tire size settings directly through the OBD2 port using readily available, budget-friendly OBD2 interfaces. This method, successfully tested on a 2018 JK, bypasses the need for proprietary commercial programmers, offering a cost-effective and empowering DIY solution.
Understanding OBD2 and ECU Communication for Tire Size Adjustment
The On-Board Diagnostics II (OBD2) port, usually located under the dashboard on the driver’s side, is your gateway to your Jeep’s computer systems. Communication happens via standardized protocols, and for JK models, it primarily uses ISO 15765-4 CAN (Controller Area Network) protocol at 500 kbaud with 11-bit IDs. Think of this as the language your OBD2 adapter uses to talk to your Jeep’s ECUs.
Within your Jeep JK, several modules are involved in tire size calibration. Key among these are:
- TIPM (Totally Integrated Power Module): This module appears to store the speedometer ratio at “local identifier” 0x0c.
- TCM (Transmission Control Module): The TCM also holds the speedometer ratio, located at “local identifier” 0xb1, ensuring correct transmission behavior with the new tire size.
Programming tire size involves sending specific command sequences to these modules via the OBD2 port. Let’s break down the process.
Step-by-Step Guide to Programming Tire Size Using OBD2
The following command stream outlines how to adjust tire size settings in your Jeep JK. These commands are sent through your OBD2 interface using a compatible software application that allows you to send raw CAN messages.
Command Stream for Tire Size Adjustment:
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, reset)Understanding the Commands:
- Each line represents a command sent via CAN bus.
- The first value (e.g.,
01C,7DF,620,7E1,784,7E0) is the CID (Controller Identifier), specifying the target module (like TIPM, TCM, ECM). - The second value is the SID (Service Identifier), and the third is the PID (Parameter Identifier).
- For “write” commands, the PID is followed by data bytes.
p1,p2,qq, andrrare variable parameters related to tire diameter, which we’ll calculate next.
Calculating Tire Size Parameters (p1, p2)
The parameters p1 and p2 are derived from your tire’s rolling diameter. It’s crucial to use the rolling diameter, not the sidewall diameter, as the rolling diameter accounts for tire deflection under the vehicle’s weight and is the effective diameter the vehicle “sees” on the road. A 34-inch tire, for instance, might have a rolling diameter closer to 32.75 inches.
The formula to calculate the value ‘pp’ used to determine p1 and p2 is:
pp = diameter (in inches) * 80 – 8Let’s take an example of a 32.75-inch rolling diameter tire:
pp = 32.75 * 80 – 8 = 2612Convert pp to hexadecimal: 2612 in decimal is 0x0a34 in hexadecimal.
p1is the most significant byte:0x0ap2is the least significant byte:0x34
These p1 and p2 values are then inserted into the command stream in the positions indicated.
Decoding qq and rr Values
Unfortunately, the exact formulas for qq and rr parameters are not fully understood. However, through observation, a table of values for different tire diameters has been compiled. These values seem to correlate with tire size and possibly other factors.
Observed qq and rr Values (Hexadecimal):
| 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 |
For tire sizes not listed, you may need to extrapolate or experiment with values near your target diameter.
OBD2 Interfaces and Software
This DIY approach is achievable with affordable, commonly available OBD2 interfaces. Successfully tested interfaces include:
