VW TDI MAF/MAS (Mass air flow sensor) FAQ, for volkswagen TDI (diesel) 1996+
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Introduction

This article discusses the two types of MAF used on the VW TDI and what the car's computer does with the information from the MAF.  The mk3 uses a hot wire, the mk4 and mk5 use a thin film or hot film MAF.  
Related links: 1000q: mk4 MAF testing procedure , 1000q: boost leak test , 1000q: solving limp mode for VW TDI

Why does the car need a MAF?

Modern electronicly fuel metered cars like the VW TDI all use a MAF.   Using a MAF/MAS sensor is one way of directly measuring how much air is passing the MAF sensor (and into the engine).  Other cars may calculate air flow instead of measuring it with a MAF by using air flow sensors and taking into account engine rpm, throttle position, and air temperature.  Knowing how much air the engine is getting is important because the car's computer uses this value to know how much fuel to give the engine.  This is one of the reasons why it's important to have an airtight intake tract after the MAF sensor.  The metered air won't be accurate if it's leaking out a disconnected hose!  Air leaks are called vacuum or boost leaks, see 1000q: boost leak test for more reasons why this is bad and how to check for it in your turbo car.  

A faulty MAF signal will cause the car's computer to give too much or too little fuel.  In fact, a TDI can run without a MAF signal at all, as can many cars.  The car computer knows that there is either a nonsense signal or none at all and uses a default value to keep the engine running.  The computer takes the engine load and speed, modifies the value for some fuel and engine temperature readings and MAF value, etc., to determine fuel injection timing and metering.  A gasoline car will run poorly with no MAF because they have to be within a narrower air/fuel ratio to run smoothly compared to a diesel engine.  In fact, disconnecting the MAF electrical plug is one of the ways you can diagnose a defective MAF in your VW TDI.  See 1000q: mk4 MAF testing for more tips on testing your mk4 MAF.  

The MAF also changed styles after the mk3 generation from a hot wire to a hot film, see below for more details.

Mk3 TDI

The mk3 TDI uses a Pierberg MAF.  This is a hot wire type MAF.  Unlike the mk4 cars, it is unusual for the MAF to fail and is a simpler device.  VW part number is # 074-906-461 (click the link to compare prices)

The hot wire MAF is composed of a thermistor (temperature sensing) wire and an ammeter (measures electrical amps). Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resistance. The colder it gets, the lower its resistance, and the more current that passes through the wire. However, the more current flowing through the wire, the hotter it gets (similar to an electric heater). This sets an upper stable limit of current, since as the resistance of the wire increases, the less current that 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. 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.

The output voltage increases directly with the measured airflow.

To clean it, use a non-residue electrical cleaner.  These are slightly different than brake or carb cleaner.  It's also possible that there is a wiring harness short to the sensor.

Mk4/mk5 TDI

The mk4 and mk5 TDI use a Bosch hot film MAF.  This is a membrane type MAF and can take much faster and more accurate measurements than a hot wire.  It is known for a slow loss of function and causing low power in the TDI and was an extended recall item on pre 2002 mk4 TDIs..  The most recent OEM part numbers are # 038 906 461 C for the ALH engine, #038-906-461-B for the BEW and mk5 BRM pumpe duse engines (same part) but check with your VW part dealer to see if this is current.

MAFs from VW gasser models will bolt on, but I would not use them because they require modification to send the right signal and a new or rebuilt MAF should only cost a little over $100, buy the right sensor for your car.

Here is the location of the MAF outlined in yellow.  The "chip" element is replaceable separately from the "chip+plastic housing", see 1000q: mk4 MAF testing for more troubleshooting tips.

Pin	Description	Wire color	Connected to
1	Temperature sensor output, not used in ALH engine	-	-
2	12V supply input	Yellow/black	relay 109
3	Ground	Brown	pin 4 of the ECU
4	5V reference input voltage	Red/green	pin 50 of the ECU
5	Output voltage	Brown/blue	pin 52 of the ECU

A 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. A control circuit maintains the heating zone at a constant temperature above ambient such that the downstream sensor reads a lower (but constant) temperature, regardless of ambient air temperature. The more air that blows past the upstream sensor, the cooler it is when compared to the downstream sensor. Thus, the change in temperature (ΔT) between the upstream and downstream temperature sensor is proportional to the amount (and direction) of air mass flowing over the membrane. A thin film MAF can also detect a reverse flow of air.

The graph below of a thin film MAF shows how the temperature is steady before and after the heating zone with no air flow but changes with air flow.

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