Mass air flow: Difference between revisions
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==General |
==General information== |
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A mass air flow sensor placed just before the intake manifold measures the instantaneous amount of air entering the combustion chamber. This is done using a thin wire suspended in the air stream. The wire itself is a thermistor, which means that its resistance varies with its temperature. In addition, the more current flowing through the wire, the hotter it gets (similar to an electric heater). Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resitance. This sets an upper stable limit of current, since the higher the resistance of the wire, the less current flows through it. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current. |
A mass air flow (or MAF) sensor placed just before the intake manifold measures the instantaneous amount of air entering the combustion chamber. This is done using a thin wire suspended in the air stream. The wire itself is a thermistor, which means that its resistance varies with its temperature. In addition, the more current flowing through the wire, the hotter it gets (similar to an electric heater). Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resitance. This sets an upper stable limit of current, since the higher the resistance of the wire, the less current flows through it. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current. |
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==S4 specific information== |
==S4 specific information== |
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Revision as of 23:18, 31 July 2006
General information
A mass air flow (or MAF) sensor placed just before the intake manifold measures the instantaneous amount of air entering the combustion chamber. This is done using a thin wire suspended in the air stream. The wire itself is a thermistor, which means that its resistance varies with its temperature. In addition, the more current flowing through the wire, the hotter it gets (similar to an electric heater). Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resitance. This sets an upper stable limit of current, since the higher the resistance of the wire, the less current flows through it. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current.
S4 specific information
The B5 S4's Bosch Motronic ME7.1 ECU uses a mass air flow based fuel injection system to determine how much fuel to deliver to keep the air fuel ratio at an optimal level (also known as stoichiometric) during open loop (engine warm-up and wide open throttle) operation. Note that during closed loop operation (idle and partial throttle), the O2 sensors are used to determine fueling, not the MAF.
B5 S4s came with two different types of MAF sensors - Hitachi and Bosch (part numbers?). Each has totally different characteristics and wiring. A-box cars (MY2000 to some of MY2001.0) came with Bosch MAFs. H-box (part way through MY2001 and later) cars came with Hitachi MAFs. To add to the confusion note that some MY2001.0 cars are A-box and some are H-box. M-box cars are generally considered MY2001.5.
Bosch vs. Hitachi pinout
| A-box/S8/RS4 aka Bosch |
H-box and M-box aka Hitachi | |||
|---|---|---|---|---|
| pin | color | wire | color | wire |
| 1 | blank/unused | none | violet/grey | +5V |
| 2 | green/yellow | +12V (ign/bat) | green/yellow | +12V (ign/bat) |
| 3 | black/green | ground | black/green | ground |
| 4 | violet/grey | +5V | green | signal |
| 5 | green | signal | blank/unused | none |