Testing The MAF Sensor (1998-1999 1.8L Turbo VW Passat)

TEST 2: Making Sure The MAF Sensor Is Getting Chassis Ground

In this test, you'll check the first of two Grounds that the MAF sensor on your Passat needs to produce a signal.

This Ground is a chassis Ground and is available at all times (no need to turn the key the ON position to check for it).

I've labeled the wire that delivers this chassis Ground to the MAF sensor with the number 1 in the photo above.

Alright, this is what you need to do:

1. 1

   Disconnect the MAF sensor from its connector.

2. 2

   Put the multimeter in Volts DC mode.

3. 3

   Connect the red multimeter test lead to the battery positive (+) terminal.

4. 4

   Gently probe the terminal of the connector that connects to the wire identified with the number 1 with the black multimeter test lead.

5. 5

   You should see 10 to 12 Volts on the multimeter.

Let's interpret your multimeter test results:

CASE 1: Your multimeter registered 10 to 12 Volts. This tells you that this path to Ground is OK.

The next step is to make sure that the second Ground the MAF sensor receives is also present. For this test go to: TEST 3: Making Sure The MAF Sensor Is Getting Sensor Ground.

CASE 2: Your multimeter DID NOT register 10 to 12 Volts. Recheck all of your multimeter connections and retest.

If you still do not see these 12 Volts, then this exonerates the MAF sensor as bad since without this Ground it's not gonna' work.

TEST 3: Making Sure The MAF Sensor Is Getting Sensor Ground

Now, we'll check the second Ground the MAF sensor receives. This Ground is provided by the fuel injection computer internally.

The wire that delivers this sensor Ground to the MAF sensor is the one I've labeled with the number 2 in the photo above.

IMPORTANT: When testing this circuit, you've got to be careful not to pass 12 Volts (from the battery) to this wire or you'll fry the fuel injection computer on your Passat. The multimeter voltage test I'm suggesting in the instructions below is a safe way to check for the presence of Ground in the wire.

OK, this is are the steps:

1. 1

   Disconnect the MAF sensor from its connector.

2. 2

   Put the multimeter in Volts DC mode.

3. 3

   Connect the red multimeter test lead to the battery positive (+) terminal.

4. 4

   Gently probe the terminal of the connector that connects to the wire identified with the number 2 with the black multimeter test lead.

5. 5

   Turn the key ON but don't crank or start the engine.

6. 6

   You should see 10 to 12 Volts on the multimeter.

Let's interpret your multimeter test results:

CASE 1: Your multimeter registered 10 to 12 Volts. This is the correct result and confirms the the fuel injection computer is providing the sensor Ground to the MAF sensor.

The next step is to see if the MAF sensor is producing a MAF signal, for this, go to: TEST 4: Testing The MAF Signal.

CASE 2: Your multimeter DID NOT register 10 to 12 Volts. Recheck all of your multimeter connections and retest.

If you still do not see these 10 to 12 Volts, then this exonerates the MAF sensor as bad since without this Ground it's not gonna' work.

TEST 4: Testing The MAF Signal

Now that all of the basics have been checked and you know the MAF sensor is getting power and Ground, the last test is to see if the MAF sensor is actually producing a MAF signal.

It's important that the engine be at its normal operating temperature. So, start the engine and let it reach it's normal operating temperature.

IMPORTANT: The MAF sensor must be connected to its connector to perform this test. So you'll need to use a back-probe or a wire piercing probe to access the signal inside the wire. To see an example of a wire piercing probe, check this link out: Wire Piercing Probe.

Let's get started:

1. 1

   Connect the red multimeter test lead to the wire labeled with the number 4 shown in the photo.

2. 2

   Put your multimeter in Volts DC mode.

3. 3

   Connect the black multimeter test lead to the battery negative (-) terminal.

4. 4

   Start the already warmed up engine.

5. 5

   Note the Volts reading on your multimeter at idle. This reading may be stable (with only small fluctuations) or unstable with very extreme fluctuations. No matter what the instability in the reading, this will be your base reading.

6. 6

   Accelerate the engine as you watch the multimeter's voltage readings.
   
   The MAF signal voltage value should increase as you rev up the engine.

7. 7

   Let the engine return to idle.
   
   The MAF signal voltage value should decrease as the engine returns to idle.

8. 8

   The voltage value should rise/fall every time you rev up/down the engine.
   
   If the MAF sensor is defective, the MAF signal voltage value will stay stuck in one single number as you rev up/down the engine.

Let's find out what your test result means:

CASE 1: The voltage increased/decreased as engine RPM increased/decreased. This tells you that the MAF sensor is good and not the cause of the MAF issue or diagnostic trouble code.

CASE 2: The voltage DID NOT increase/decrease as engine RPM increase/decrease. Recheck all of your multimeter connections and retest.

If you still do not see the indicated voltage rising and falling, you can conclude the mass air flow (MAF) sensor is bad and needs replacement if you have:

• Confirmed that the MAF sensor is receiving power (TEST 1).
• Confirmed that the MAF sensor is receiving chassis and sensor Ground (TEST 2 and 3).
• In this test section, you've confirmed that the MAF signal voltage is stuck in one value as you accelerate/decelerate the engine.