20V Motronic ECU System

1991 200TQ and 1992-95 S4/S6 with 5 Cylinder 20V Turbo Engine:

MASS AIR FLOW (MAF) SENSOR G70

The Mass Air Flow sensor uses a temperature regulated hot wire to measure the "mass" of air flow into the engine to provide load information to the Engine Control unit for setting the ignition timing, fuel injection time, and activation of the evaporative emission solenoid valve as well as idle speed adaptation.

The sensor looks like a cylindrical tube and is mounted between the air cleaner and the turbo intake boot.


1991 200TQ 20V Location

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1992-95 S4/S6 MAF Location

The sensor uses a screen on each end to reduce air turbulence at the measuring point. The platinum wire inside is heated and an air temp sensor is used to maintain a temperature differential of 100C between the air entering the sensor and the temperature of the wire.

As air flows over the wire and is cooled the mass air flow sensor assembly adds more current to maintain the temperature differential and the ECU uses this change to adjust the fuel required. Because the air "mass" is calculated using this technique, no additional corrections are required from changes in air temp and or air density.

Contamination of the hot wire surface can cause problems, so after the engine is shut off the hot wire is heated to ~1000C for one second to burn off any contaminates.

This is why the ECU should not be disconnected from the harness for at least 20 seconds to allow this burn off process to occur. 1991 200TQ 20V: [1] If the Mass Air Flow sensor fails, the engine will continue to run under two conditions.

When the throttle is closed at idle, the engine will run using the idle timing map and a precalculated amount of air. When the idle switch is open during normal running at part load, the ignition timing is set to a fixed value of 20 degrees BTDC and the fuel mixture is leaned [1].

This "Limp Home" mode allows the car to be driven temporarily to the nearest repair shop. 1992-95 S4/S6: [2] During starting the MAF sensor information is not used, the barometric sensor information and temperature dependent fixed values are used for calculating the fuel required.

The self diagnostic system will recognize 2 types of problems in the MAF circuit, break/short to ground (signal too low) or short to positive (signal too high). When a fault is recognized, adaptation for the idle speed and oxygen sensor control is stopped and the evaporative emission control is switched off.

The signal from the throttle position Potentiometer is used to run the engine.

Return to 20V ECU System information index

References: [1] Audi of America, Technical service training publication:
"The New 20V Turbo Engine for the Audi 200 Quattro-publication "
[2] "Motronic Engine Management System for the Audi S4 and the 4.2 Liter Audi V8" All rights reserved.

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