Hot Wire Air Flow Meter Operation & Self Cleaning Information

Hot Wire Air Flow Meter Operation Simple in design, the new air flow meter provides rapid and precise air mass measurement without atmospheric or mechanically caused errors.

The heated wire method of measuring has a positive influence on fuel efficiency and emissions.




Air mass is determined by temperature changes and the resulting current flow changes in the heated platinum wire, The heated wire is kept at a temperature of 155~C above the ambient air inlet temperature. As the air mass increases, the wire cools and its resistance value is reduced. The ohmic change allows an increase in current flow and thus there is an exact relationship between incoming air mass and the current required to maintain the nominal heated wire temperature.

The heated wire, measuring resistor and inlet air compensating resistor as well as voltage divider resistances are connected to form a bridge circuit. Changes in air mass volume and ambient inlet temperature cause corresponding ohmic value changes in the hot wire and the inlet air resistor.

The resulting change in current flow is accurately monitored as a voltage drop at the measuring resistor. Further, this same "ebb and flow" of voltage causes the voltage regulating control amplifier to adjust the applied voltage which keeps the heated wire at the nominal temperature regardless of air flow or inlet temperature.

The voltage drop across the measuring resistor supplies the M1.2 Motronic control unit with an accurate and reliable load signal that does not require any additional correction.

Hot Wire Self-Cleaning

Deposits of any type could have a negative effect on the dynamic ohmic value of the heated load sensing wire. Following each period of engine operation, a pre-set self-cleaning sequence occurs. 2.5 seconds after engine shutdown the heated wire is superheated to 1,OOO~C for a 5-second period. Unnecessary cleaning is prevented as no self-cleaning takes place if the coolant temperature is below 60~C or a threshold of 2,000 RPM has not been exceeded during the previous period of operation.

WARNING: Cold start sprays must never be used on any engine equipped with a hot wire air flow meter.