Hot Film air mass Meter

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The air volume entering the engine is measured by the HFM ( Hot-film air mass meter) also referred to as an AMM or AFM. Previous air flow meters were mechanical in there design and measurement of the incoming air. As the air enters the intake, the air would move a mechanical valve or flap and produce a signal proportional to the air entering the engine.

The input signal to the DME is produced by the HFM, the HFM electronically heats a metal film in the intake tract to 180°C above the intake air temperature. As the air enters the HFM it flows across the metal film cooling it and changing the resistance thus affecting the amount of current that flows through the HFM. The HFM produces a 1-5volt signal, based on the current flow through the HFM as the air passes over the metal film, the DME uses this signal to adjust the amount of fuel injected.

The HFM cannot be adjusted or recalibrated. For instance, an HFM could send conflicting signals to the DME that are noticable in the adaptation values, no repair can be made to the HFM, only replacement.

It is recommended not to use any cleaning agents on the HFM, which could damage the metal film that operates at extremely high temperatures or cause incorrect signals that are sent to the DME.

The HFM is powered by the DME main relay or KL87. The intake air temperature sensor is also integrated into the HFM with a 5v reference voltage. The air temperature sensor is an NTC type sensor. The air temperature sensor also affects fuel injection, ignition timing and secondary air activation. If the intake air temperature sensor should fail, the DME will use the engine coolant temperature sensor signal as backup.

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