About PID Calculated Load Value

There are two types of LOAD specified in  SAE J1979


Calculated LOAD Value (PID 04)

The OBD regulations previously defined CLV as:

(current airflow / peak airflow @sea level) * (BARO @ sea level / BARO) * 100%

Various manufacturers have implemented this calculation in a variety of ways. The following definition, although a little more restrictive, will standardise and improve the accuracy the calculation

LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]


  • STP = Standard Temperature and Pressure = 25 °C, 29.92 in Hg BARO, SQRT = square root,
  • WOT = wide open throttle, AAT = Ambient Air Temperature and is in °C

Characteristics of LOAD_PCT are:

  • Reaches 1.0 at WOT at any altitude, temperature or rpm for both naturally aspirated and boosted engines.
  • Indicates percent of peak available torque.
  • Linearly correlated with engine vacuum
  • Often used to schedule power enrichment.
  • Compression ignition engines (diesels) shall support this PID using fuel flow in place of airflow for the above calculations.

NOTE Both spark ignition and compression ignition engines shall support PID $04. See PID $43 for an additional definition

of engine LOAD.

05 Engine Coolant Temperature A –40 °C +215 °C 1 °C with

–40 °C offset

ECT: xxx °C (xxx °F)

ECT shall display engine coolant temperature derived from an engine coolant temperature sensor or a cylinder head temperature

sensor. Many diesels do not use either sensor and may substitute Engine Oil Temperature instead.

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Absolute Load Value (PID 43)

The absolute load value has some different characteristics than the LOAD_PCT defined in PID 04 This definition, although restrictive, will standardise the calculation. LOAD_ABS is the normalised value of air mass per intake stroke displayed as a percent.

LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]


  • 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 (101.3 kPa) BARO, WOT = wide open throttle.

The quantity (maximum air mass (g)/intake stroke at WOT@STP at 100% volumetric efficiency) is a constant for a given cylinder swept volume. The constant is 1.184 (g/litre 3) * cylinder displacement (litre 3/intake stroke) based on air density at STP.

Characteristics of LOAD_ABS are:

  • Ranges from 0 to approximately 0.95 for naturally aspirated engines, 0 – 4 for boosted engines,
  • Linearly correlated with engine indicated and brake torque,
  • Often used to schedule spark and EGR rates,
  • Peak value of LOAD_ABS correlates with volumetric efficiency at WOT.,
  • Indicates the pumping efficiency of the engine for diagnostic purposes.

Spark ignition engine are required to support PID $43. Compression ignition (diesel) engines are not required to support this PID.

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Friday, June 11th, 2010 Gauges & Dashboards

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