It’s a common scenario in online car enthusiast forums: someone tries to use an OBD2 adapter, like a BAFX OBD2, with an app like Torque to monitor their vehicle’s wheel speed sensors, only to find that the data is missing or unreliable. If you’ve been wrestling with this issue, wondering why you can’t get consistent wheel speed readings using your BAFX OBD2 adapter, you’re not alone. Let’s delve into the technical reasons behind this frustration, particularly for owners of older GM vehicles.
Understanding the OBD2 Communication Barrier
The core of the problem lies in how different vehicle systems and diagnostic tools communicate. In many older vehicles, particularly those from GM, the wheel speed sensors are connected to the Electronic Brake Control Module (EBCM). The crucial detail here is that these EBCMs often do not support “service 22” parameter data requests via PID (Parameter IDs).
Service 22: The Common OBD2 Request Mode
Apps like Torque, and many generic OBD2 scanners, primarily use service 22. This service is designed for requesting a single parameter at a time. The app sends a request for a specific PID (like a wheel speed sensor PID) and expects a single response containing that data. This works well for many engine and transmission parameters monitored by the Engine Control Module (ECM) or Transmission Control Module (TCM).
EBCM and Service 22 Incompatibility
However, the EBCM in many older GM vehicles is not set up to respond to service 22 requests for wheel speed data. Even if you’ve added extended GM PIDs to your Torque app, and you believe you have the correct wheel speed sensor PIDs, Torque simply cannot retrieve the data from the EBCM using service 22 if the EBCM doesn’t support it in that way. This explains why you might see no data, or consistently incorrect data, when trying to monitor wheel speed with your BAFX OBD2 and Torque on these vehicles.
The Tech 2 Advantage: Service 2A and Data Packets
So, how do professional diagnostic tools like the Tech 2 scanner manage to read wheel speed data from these EBCMs? The answer is they utilize a different OBD2 service mode: “service 2A,” also known as “Request Diagnostic Data Packets.”
Service 2A: Efficient Data Retrieval
Service 2A is a more advanced and efficient way to retrieve diagnostic data. Instead of requesting parameters one by one, it can request pre-configured or dynamically defined “data packets.” In the case of the EBCM, it often has predefined data packets that contain multiple related parameters, such as wheel speeds for all four wheels and the rear axle speed.
Data Packets: Streamlined Information
These data packets can return multiple bytes of data at a selectable data rate. This allows a scanner to stream a collection of data points in a single response, rather than sending individual requests and receiving individual responses for each parameter, as is the case with service 22. This is significantly faster and more efficient, especially when monitoring multiple sensors in real-time.
Why Apps Like Torque Don’t Use Service 2A
The reason why Torque and similar OBD2 apps typically don’t use service 2A is likely due to the complexity of implementation and the focus of these apps on broader, more universally accessible OBD2 functions. Service 22 is simpler to implement and works across a wider range of vehicles and modules for basic diagnostics. Service 2A, while more powerful, requires more sophisticated programming and a deeper understanding of vehicle-specific data packet structures.
While there have been attempts to “trick” apps like Torque into using service 2A or similar services, these methods are often unreliable, can lead to app crashes, and are generally not recommended for consistent or safe diagnostic use.
Vehicle Year and System Architecture Matter
The year of your vehicle plays a significant role in OBD2 communication protocols and module architecture.
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Older Vehicles (Pre-CANBUS EBCM): Vehicles with EBCMs that predate the widespread adoption of CANBUS (Controller Area Network) for brake systems are more likely to have these service 22 limitations for wheel speed data. The 2002 Trailblazer example in the original forum post is a prime example.
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Newer Vehicles (CANBUS Systems): Even in newer vehicles, while CANBUS improves communication speed and flexibility, issues can still arise. For example, the 2005 Yukon example mentioned sporadic and inaccurate wheel speed data even when some data is received. This could be due to various factors, including how the EBCM on these vehicles reports data via CANBUS and how the app interprets it.
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Post-2008 GM CANBUS Expansion: The original forum post correctly points out that CANBUS integration in GM platforms expanded significantly around 2008, potentially affecting how ECMs and TCMs (and potentially EBCMs in later models) communicate and share data.
A Word of Caution: Tinkering with Braking Systems
It’s crucial to remember that the braking system, controlled by the EBCM, is arguably the most safety-critical system in your vehicle. While exploring OBD2 diagnostics is valuable for understanding your vehicle’s health, “tinkering” with EBCM communication protocols, especially in attempts to force data retrieval methods that are not officially supported, carries inherent risks.
Interfering with the EBCM’s communication or data flow could potentially have unintended consequences for braking performance, ABS functionality, and traction control systems. Therefore, while investigating OBD2 wheel speed readings is a legitimate diagnostic pursuit, proceed with caution and prioritize safety.
Conclusion: Managing Expectations with BAFX OBD2 and Wheel Speed
In summary, if you’re struggling to get reliable wheel speed sensor data using a BAFX OBD2 adapter and apps like Torque, especially on older GM vehicles, it’s likely due to the limitations of service 22 communication with the EBCM. These modules often require more advanced service modes like 2A, which are typically only found in professional-grade diagnostic tools.
While generic OBD2 adapters and apps are excellent for many basic diagnostic tasks, understanding their limitations, particularly when it comes to complex systems like ABS and braking, is essential. For in-depth wheel speed sensor diagnostics, especially on older vehicles, professional scan tools that support service 2A and EBCM-specific protocols may be necessary.
