For car enthusiasts and professionals alike, understanding the intricate workings of a vehicle’s engine is crucial. Modern vehicles are equipped with sophisticated onboard diagnostic systems, primarily OBD2, which provides a wealth of data about engine performance and health. Among the many parameters, two key indicators often discussed are Calculated LOAD Value (PID 04) and Absolute Load Value (PID 43). While these might seem abstract, they are deeply connected to how your engine operates, and indirectly, to sensors like the Absolute Throttle Position Sensor (ABSLT TPS).
This article delves into the meaning of these OBD2 Parameter IDs (PIDs), particularly focusing on engine load and how they relate to overall engine performance. We’ll explore what Calculated LOAD and Absolute Load signify, how they are calculated, and why understanding them is vital for diagnostics and performance tuning. Although the formulas for these PIDs do not directly include TPS, the Abslt Tps Obd2 reading plays a crucial role in the factors that do influence these load values, making it an important indirect element in this analysis.
Understanding Engine Load: The Basics
Engine load, in simple terms, represents how hard your engine is working at any given moment. It’s not just about RPM; an engine running at high RPM with no load is working less hard than an engine at lower RPM pulling a heavy load or accelerating uphill. OBD2 systems provide standardized ways to quantify this “load,” helping diagnose issues and understand engine efficiency.
There are two primary ways OBD2 defines engine load, as highlighted by PID 04 and PID 43:
Calculated LOAD Value (PID 04)
This PID offers a percentage-based representation of engine load, calculated based on airflow in gasoline engines and fuel flow in diesel engines. The formula, while seemingly complex, aims to provide a standardized metric across different vehicle makes and models.
The Formula Breakdown:
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 Barometric Pressure)
- BARO: Barometric Pressure
- SQRT: Square root
- WOT: Wide Open Throttle
- AAT: Ambient Air Temperature (in Celsius)
Key Characteristics of PID 04:
- Reaches 1.0 (or 100%) at Wide Open Throttle (WOT): Regardless of altitude, temperature, or RPM, when the throttle is fully open, LOAD_PCT should hit 100%, indicating maximum engine load capacity.
- Indicates Percentage of Peak Torque: This value gives you an immediate sense of how much of the engine’s maximum torque is being utilized.
- Linearly Correlated with Engine Vacuum: Higher engine load typically means lower manifold vacuum, and PID 04 reflects this relationship.
- Used for Power Enrichment Scheduling: Engine control units (ECUs) often use LOAD_PCT to determine when to enrich the fuel mixture for optimal power, especially under heavy loads.
- Diesel Adaptation: Importantly, for diesel engines, this PID uses fuel flow instead of airflow in the calculation, adapting the concept of “load” to diesel engine operation.
In essence, Calculated LOAD Value (PID 04) provides a normalized percentage of engine load, primarily useful for understanding torque demand and engine operating conditions in relation to its maximum potential.
Absolute Load Value (PID 43)
PID 43 takes a different approach, focusing on the mass of air entering the engine per intake stroke, normalized against the engine’s displacement. This provides a more direct measure of air intake and engine “breathing.”
The Formula Breakdown:
LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]Derivation Explained:
- air mass (g / intake stroke) = [total engine air mass (g/sec)] / [rpm (revs/min) (1 min / 60 sec) (1/2 # of cylinders (strokes / rev))]
This part calculates the air mass entering each cylinder per intake stroke based on total airflow and engine speed.
- LOAD_ABS = [air mass (g)/intake stroke] / [maximum air mass (g)/intake stroke at WOT@STP at 100% volumetric efficiency] * 100%.
This then normalizes the air mass per stroke against the theoretical maximum air mass the engine could intake at Wide Open Throttle under Standard Temperature and Pressure, assuming 100% volumetric efficiency. The constant 1.184 g/litre³ is the density of air at STP.
Key Characteristics of PID 43:
- Ranges: Typically from 0 to approximately 0.95 for naturally aspirated engines, and can go up to 4 or higher for boosted (turbocharged or supercharged) engines. The higher values in boosted engines reflect the forced air induction beyond atmospheric pressure.
- Linearly Correlated with Torque (Indicated and Brake): LOAD_ABS directly reflects the torque the engine is producing, both indicated torque (internal cylinder pressure) and brake torque (torque delivered at the crankshaft).
- Used for Spark and EGR Control: ECUs use LOAD_ABS to optimize spark timing and Exhaust Gas Recirculation (EGR) rates for fuel efficiency and emissions control.
- Peak Value Correlates with Volumetric Efficiency: The maximum LOAD_ABS value achievable at WOT is a good indicator of the engine’s volumetric efficiency – how effectively it fills its cylinders with air.
- Indicates Pumping Efficiency: From a diagnostic standpoint, LOAD_ABS can help assess the engine’s “pumping” efficiency – how well it moves air in and out of the cylinders.
- Spark Ignition Focus: It’s important to note that PID 43 is required for spark ignition (gasoline) engines but not required for compression ignition (diesel) engines, highlighting its design focus on gasoline engine characteristics.
In essence, Absolute Load Value (PID 43) offers a more direct physical measure of engine load related to air intake and torque production, particularly valuable for gasoline engine diagnostics and performance analysis.
The Role of ABSLT TPS OBD2 in Engine Load Context
Now, where does the Absolute Throttle Position Sensor (ABSLT TPS) fit into this picture? While neither PID 04 nor PID 43 formulas directly use TPS values, the TPS is a fundamental sensor that directly influences the airflow into the engine, which is the cornerstone of both load calculations.
How TPS Connects to Engine Load:
- Throttle Position and Airflow: The throttle valve, controlled by the accelerator pedal and monitored by the TPS, regulates the amount of air entering the engine’s intake manifold. A wider throttle opening (higher TPS reading) allows more air to flow in.
- TPS as an Input for Airflow Measurement: Mass Air Flow (MAF) sensors, used in many vehicles to measure “current airflow” (used in PID 04) and “total engine air mass” (related to PID 43), are directly affected by the throttle position. The TPS reading is a critical input for the ECU to interpret MAF sensor data correctly.
- WOT Detection: The TPS is crucial for identifying Wide Open Throttle (WOT) conditions. Both PID 04 and PID 43 definitions reference WOT as a key reference point. The ECU relies on the ABSLT TPS OBD2 reading to know when the throttle is fully open to calculate peak load and other parameters accurately.
- Engine Load Control Strategies: The ECU uses TPS data, along with other sensor inputs, to determine the appropriate engine load for various driving conditions. This load management affects fuel delivery, ignition timing, and other engine control parameters, which in turn are reflected in PID 04 and PID 43 values.
Troubleshooting with ABSLT TPS OBD2 and Load PIDs:
Monitoring ABSLT TPS OBD2 readings alongside PID 04 and PID 43 can be highly valuable in diagnosing engine performance issues.
- Example Scenario: If you observe a low Calculated LOAD (PID 04) or Absolute Load (PID 43) when you expect high load (e.g., during acceleration or going uphill), and simultaneously notice an abnormal ABSLT TPS OBD2 reading (e.g., TPS not reaching high values even with a fully pressed accelerator pedal), it could indicate a problem with the TPS itself, the throttle linkage, or the accelerator pedal sensor.
- Correlation is Key: By comparing TPS readings with engine load PIDs, you can verify if the engine is responding as expected to throttle inputs. Discrepancies can pinpoint problems in the throttle control system, air intake system, or even engine mechanical issues affecting load capacity.
Conclusion: ABSLT TPS OBD2 and Engine Load – A Holistic View
Understanding ABSLT TPS OBD2 readings in conjunction with OBD2 engine load parameters like PID 04 and PID 43 provides a comprehensive view of engine operation. While the load PIDs do not directly calculate TPS values, the TPS is a fundamental sensor influencing airflow and, consequently, engine load.
For effective car diagnostics and performance tuning, monitoring these PIDs together is essential. By analyzing ABSLT TPS OBD2 alongside Calculated LOAD and Absolute Load, you gain deeper insights into engine performance, throttle response, and overall engine health, empowering you to diagnose issues more accurately and optimize your vehicle’s performance.
