![]() ![]() A-Box/ B-Box cars (MY2000) came with Bosch MAFs. Each has totally different signal characteristics and are not compatible, even if properly rewired. In general, the higher the IAT is above approximately 50☌ (~120☏), the more timing will be pulled and the more fuel will be added.ī5 S4s and RS4s came with several different types of MAF sensorsĪll are film-type MAFs. Unfortunately, those adjustments are done before any correction factor (ignition retard) that comes from the knock control function, so IAT related timing adjustment values are not visible to tools like VAGCOM or ECUx. However, IAT does play a role in adjusting the base timing and fueling maps. By holding the overtemperature constant, the "cooling capacity" is linear with the air mass passing over the film, regardless of IAT. Note that since MAF readings are also used during closed loop operation, long term fuel trims (or LTFTs) may be affected by faulty or incorrectly tuned MAF setups, including the use of blow off valves (or BOVs).īecause of their design, both the Bosch and Hitachi MAF correct for ambient temperature, so the ECU does not use the IAT sensor in its load calculations.īoth types of thin film MAF keep the heating element at a constant overtemperature above ambient, so the ΔT between the upstream and the downstream sensor will always give a "cooling capacity" of the mass of air passing over the film. The O2 sensors (along with MAF readings) are used to determine fueling only during closed loop operation (idle and partial throttle). ![]() Unlike wideband O2 sensor equipped cars, the ME7.1's narrow band O2 readings are ignored during open loop operation. Thus, for proper functioning, MAF readings are critical for proper open loop behavior there is no way for ME7.1 to detect if the fueling is off. Generally, open loop fuel ratios are not stoichiometric, so narrow band O2 sensors can not be used for this. During open loop (for example, during engine warm-up and wide open throttle) operation, the MAF tells the ECU how much fuel to deliver to keep the air fuel ratio at the desired level. The B5 S4's Bosch Motronic ME7.1 ECU uses a mass air flow based fuel injection system. Reverse flow pulses from the pistion generate waves in the air upstream of the throttle body, and should not be allowed to affect the meter's readings. A thin film MAF can also detect a reverse flow of air, which may happen during high load factors. Thus, the difference in temperature (ΔT) between the upstream and downstream temperature sensor is proportional to the amount (and direction) of air mass flowing over the membrane. The more air that blows past the upstream sensor, the cooler it is when compared to the downstream sensor. A control circuit maintains the heating zone at a constant overtemperature above ambient such that the downstream sensor reads a lower (but constant) temperature. Therefore, at any given time, the amount of air flowing through the MAF is proportional to the amount of current flowing through the wire in the sensor.Ī hot film MAF is comprised of a diaphragm with a heating zone and two symmetrically arranged temperature sensors on either side of the heating zone. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. This sets an upper stable limit of current, since as the resistance of the wire increases, the less current that flows through it. However, the more current flowing through the wire, the hotter it gets (similar to an electric heater). The colder it gets, the lower its resistance, and the more current that passes through the wire. Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resistance. This is done using a thin wire or film suspended in the air stream.Ī hot wire MAF is composed of a thermistor wire and an ammeter. 2.1 Temperature (IAT) effect on MAF readingsĪ mass air flow (or MAF) sensor placed just before the intake manifold measures the instantaneous rate of air entering the combustion chamber. ![]()
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