Mass air flow

General MAF 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.

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 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

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