Navigating the world of OBD2 data can sometimes feel like deciphering a new language. Recently, a discussion sparked on the ScannerDanner forum, initiated by user jcascel1, who shared a data capture from their Mazda 3. This post highlighted something intriguing: the Absolute Load PID. While Calculated Load is a familiar parameter for many technicians, Absolute Load often flies under the radar, especially for those primarily working with OEM data.
OBD2 Data Capture of Mazda 3 Load Values
So, what exactly differentiates Calculated Load Value from Absolute Load Value within the OBD2 protocol? A deep dive into an SAE document reveals the technical distinctions. While the detailed mathematics can be complex, the core characteristics are crucial for diagnostic purposes.
According to SAE specifications, Calculated Load Value (LOAD_PCT) is designed to:
Reach 1.0 at Wide Open Throttle (WOT) regardless of altitude, temperature, or engine RPM for both naturally aspirated and boosted engines.
Indicate the percentage of peak available torque.
Correlate linearly with engine vacuum.
Be frequently used for power enrichment scheduling.
In contrast, Absolute Load Value (LOAD_ABS) is defined by these characteristics:
Range from 0 to approximately 0.95 for naturally aspirated engines, and 0 to 4 for boosted engines.
Correlate linearly with engine indicated and brake torque.
Be frequently used for spark and EGR rate scheduling.
Have a peak value that correlates with volumetric efficiency at WOT.
Indicate the engine’s pumping efficiency for diagnostic purposes.
The last point about Absolute Load indicating pumping and volumetric efficiency is particularly noteworthy for engine diagnostics. Engine breathing is fundamental to performance, and having a PID that directly reflects these efficiencies could be incredibly valuable. This raises a critical question: Should Absolute Load Value Obd2 data become a primary focus in diagnostics, potentially even superseding Calculated Load, or is there an advantage to utilizing both? Furthermore, in what diagnostic scenarios would employing both PIDs offer a more comprehensive understanding of engine performance?
To explore this further, practical data is essential. Input from the community is highly encouraged. If you have access to a vehicle that reports both Calculated Load and Absolute Load PIDs, capturing WOT data would be immensely helpful. Ideally, data from vehicles with known engine breathing issues, such as intake restrictions, would provide even deeper insights into how these load values respond to real-world problems. Sharing this data could significantly enhance our collective understanding of load value OBD2 and its diagnostic potential.
