For automotive technicians, understanding OBDII Parameter IDs (PIDs) is crucial for effective diagnostics and repair. These PIDs provide real-time data from the vehicle’s engine control unit (ECU), offering insights into various engine parameters. Among these, PIDs related to engine load are particularly important for assessing engine performance and identifying potential issues. While the prompt mentions Obdii Pid 18, it’s important to clarify that standard OBDII PID specifications, as commonly referenced in resources like SAE J1979, do not directly list a PID 18 with a widely recognized specific function related to engine load. However, the concept of engine load is central to several key PIDs that are essential for diagnostics, specifically PID 04 (Calculated LOAD Value) and PID 43 (Absolute Load Value), which we will explore based on the provided technical information. These PIDs, although not explicitly “PID 18”, are fundamental for understanding engine operation and are often considered when discussing engine load parameters within the broader context of OBDII diagnostics and tools like Autel scanners.
Calculated LOAD Value (PID 04)
The Calculated LOAD Value (PID 04) is a critical parameter that represents the percentage of peak engine torque currently being used. It’s not a direct measurement of pressure or airflow, but rather a calculated value derived from airflow measurements and atmospheric conditions. The formula for calculating LOAD_PCT is defined as follows:
LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]Where:
- STP: Standard Temperature and Pressure (25°C, 29.92 inches of Mercury).
- BARO: Barometric pressure.
- SQRT: Square root.
- WOT: Wide Open Throttle.
- AAT: Ambient Air Temperature (in Celsius).
Key characteristics of PID 04 include:
- It reaches 1.0 (or 100%) at Wide Open Throttle (WOT) regardless of altitude, temperature, or engine RPM for both naturally aspirated and turbocharged engines.
- It effectively indicates the percentage of peak torque available from the engine at any given moment.
- It exhibits a linear correlation with engine vacuum, making it a useful indicator of engine efficiency.
- Engine control systems frequently use this PID to manage power enrichment strategies.
- Notably, for compression ignition engines (diesel), the calculation adapts by using fuel flow measurements in place of airflow, ensuring its applicability across different engine types. Both spark and compression ignition engines are required to support PID 04.
Absolute Load Value (PID 43)
Another vital PID for understanding engine load is the Absolute Load Value (PID 43). This PID offers a different perspective on engine load, representing the normalized value of air mass per intake stroke, expressed as a percentage. The calculation for LOAD_ABS is:
LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]This can be further broken down from its derivation:
- air mass (g / intake stroke) = [total engine air mass (g/sec)] / [rpm (revs/min) * (1 min / 60 sec) * (1/2 # of cylinders (strokes / rev))]
- LOAD_ABS = [air mass (g)/intake stroke] / [maximum air mass (g)/intake stroke at WOT@STP at 100% volumetric efficiency] * 100%.
Where:
- STP: Standard Temperature and Pressure (25°C, 29.92 in Hg or 101.3 kPa BARO).
- WOT: Wide Open Throttle.
Key characteristics of PID 43 are:
- For naturally aspirated engines, it typically ranges from 0 to approximately 0.95 (or 95%). For boosted engines, it can range from 0 to 4 (or 400%), reflecting the increased air mass.
- It shows a linear correlation with both indicated and brake torque of the engine, providing a direct measure of engine output.
- Engine management systems often use LOAD_ABS to control spark timing and Exhaust Gas Recirculation (EGR) rates.
- The peak value of LOAD_ABS is directly related to the volumetric efficiency of the engine at Wide Open Throttle.
- From a diagnostic standpoint, it effectively indicates the pumping efficiency of the engine.
- It’s important to note that while spark ignition engines are required to support PID 43, compression ignition (diesel) engines are not mandated to do so.
Diagnostic Applications and Key Differences
Understanding both PID 04 and PID 43 is essential for comprehensive engine diagnostics. While PID 04 gives a percentage of peak torque based on airflow relative to standard conditions, PID 43 provides a more direct measure of air mass per intake stroke relative to the engine’s maximum capacity.
For technicians using tools like Autel diagnostic scanners, monitoring these PIDs in real-time can help:
- Assess engine performance under various conditions: By observing LOAD_PCT and LOAD_ABS during idle, acceleration, and cruise, technicians can evaluate if the engine is performing as expected.
- Diagnose performance issues: Abnormal readings can indicate problems with airflow, fuel delivery, or other engine mechanical issues. For instance, a lower than expected LOAD_ABS at WOT could suggest restrictions in the intake system.
- Evaluate engine efficiency: Both PIDs are linked to engine efficiency. LOAD_PCT is related to vacuum efficiency, while LOAD_ABS reflects pumping efficiency.
- Verify sensor readings: Correlating load values with other sensor data (like MAF sensor readings, MAP sensor readings, and throttle position) can help validate the accuracy of these sensors.
In conclusion, while OBDII PID 18 might not be a direct engine load parameter in standard OBDII implementations, focusing on PIDs like 04 and 43 provides invaluable insights into engine load, performance, and potential issues. For automotive professionals at autelfrance.com and beyond, mastering the interpretation of these PIDs is key to efficient and accurate vehicle diagnostics, enhancing repair capabilities and ensuring optimal vehicle performance. Using advanced diagnostic tools to monitor these parameters in real-time will significantly improve the troubleshooting process for modern vehicles.
