Understanding OBD2 parameters is crucial for effective automotive diagnostics. Among these, “Load Value” often appears, but did you know there are actually two types: Calculated Load Value and Absolute Load Value? While Calculated Load is commonly used, Absolute Load offers a deeper insight into engine performance and can be a valuable tool for experienced technicians. Let’s dive into the differences and explore how to utilize both for superior diagnostics.
Data capture from Mazda 3 showing Calculated and Absolute Load at idle
Calculated Load Value: The Familiar Metric
Calculated Load Value (LOAD_PCT) is a PID (Parameter ID) that’s been a staple in OBD2 diagnostics. According to SAE documentation, it’s designed with specific characteristics:
- Reaches 1.0 (or 100%) at Wide Open Throttle (WOT): This is consistent across different altitudes, temperatures, and engine RPMs for both naturally aspirated and boosted engines.
- Indicates Percentage of Peak Available Torque: It gives a sense of how much of the engine’s maximum torque is being utilized.
- Linearly Correlated with Engine Vacuum: This relationship makes it useful for understanding engine load based on vacuum readings.
- Used for Power Enrichment Scheduling: Engine control units (ECUs) often use Calculated Load to determine when to enrich the fuel mixture for optimal power.
- Supported by Diesel Engines: Even diesel engines, which operate differently, support this PID, using fuel flow in place of airflow for calculations.
Essentially, Calculated Load provides a normalized percentage representing engine load, making it easy to understand load variations across different driving conditions and engine types.
Absolute Load Value: Unveiling Engine Efficiency
Absolute Load Value (LOAD_ABS) is a less commonly observed PID in standard OBD2 scanning, particularly when focusing on OEM-specific data. However, it offers unique diagnostic advantages:
- Ranges Differently for Engine Types: For naturally aspirated engines, it typically ranges from 0 to around 0.95. For boosted engines, it can go much higher, from 0 to 4 or even more.
- Linearly Correlated with Torque: Critically, it’s linearly correlated with both indicated torque (torque generated inside the cylinders) and brake torque (torque delivered at the crankshaft).
- Used for Spark and EGR Control: ECUs utilize Absolute Load to manage spark timing and Exhaust Gas Recirculation (EGR) rates for optimal combustion and emissions.
- Peak Value Relates to Volumetric Efficiency at WOT: The highest Absolute Load value achieved at WOT is a strong indicator of the engine’s volumetric efficiency – how well it fills its cylinders with air.
- Indicates Pumping Efficiency: Perhaps most diagnostically significant, Absolute Load reflects the engine’s pumping efficiency. This makes it a powerful tool for assessing engine breathing and mechanical health.
Absolute Load, therefore, goes beyond a simple percentage and delves into the engine’s operational physics, providing insights into its breathing capability and overall efficiency.
Calculated Load vs. Absolute Load: Which to Use for Diagnostics?
The question isn’t necessarily which one to use, but rather how to leverage both Calculated Load and Absolute Load for comprehensive diagnostics.
- Calculated Load is excellent for general load assessment and understanding the engine’s demand in various driving scenarios. It’s readily available and provides a quick, normalized view of engine stress.
- Absolute Load shines when diagnosing engine breathing issues, volumetric efficiency problems, and potential mechanical inefficiencies. Its direct correlation with pumping efficiency makes it particularly sensitive to restrictions in the intake or exhaust system, valve timing issues, or even cylinder compression problems.
Using both in tandem can be advantageous. For instance, if you observe a high Calculated Load but a comparatively low Absolute Load at WOT, it might indicate an issue preventing the engine from achieving its full volumetric efficiency, such as an intake restriction. Conversely, discrepancies between the expected and actual Absolute Load values under various conditions can point towards mechanical problems affecting engine pumping efficiency.
Call to Action: Share Your OBD2 Load Value Data
To further explore the diagnostic potential of Absolute Load, especially in comparison to Calculated Load, community data is invaluable. If you have access to a scan tool that reads both Calculated and Absolute Load PIDs on a vehicle, consider capturing some data, particularly:
- Wide Open Throttle (WOT) Data: Record data logs during WOT runs to observe peak Load values and their relationship.
- Data with Induced Engine Breathing Problems: If possible and safe, intentionally create a minor intake restriction and observe how both Load values respond. This kind of data would be incredibly helpful in understanding the practical diagnostic applications of Absolute Load.
Sharing your findings and data captures will contribute to a deeper understanding of OBD2 Load values and enhance our collective diagnostic expertise. Let’s collaborate to unlock the full potential of these valuable parameters!
