How To Test The 1990-1994 Lexus LS400 MAF Sensor

TEST 3: Testing The MAF Signal With A Multimeter

Now that the basics have been checked, we'll check the MAF signal coming out of the sensor and going to the fuel injection computer.

The yellow with green stripe (YEL/GRN) wire, of the MAF sensor connector, is the one that feeds the MAF signal to the fuel injection computer (ECM).

The YEL/GRN wire is labeled with the #3 arrow in the photo above.

As mentioned at the beginning of the tutorial, the MAF sensor produces a signal that can be measured with a multimeter that can read Hertz frequency.

When the engine is at idle, the Hertz frequency will be low. When the engine is accelerated, the Hertz frequency will increase.

In 9 cases out of 10, when the MAF sensor fails, the MAF sensor will produce a Hertz signal that will not increase or decrease as the engine is accelerated or decelerated. This is how you'll know that the MAF sensor is defective and needs to be replaced.

NOTE: To test the MAF signal, the MAF sensor must be connected to its electrical connector. You'll need to use a back-probe or a wire piercing probe to acces the MAF signal. You can see an example of a wire piercing probe here: Wire Piercing Probe.

OK, let's get started:

1. 1

   Place your multimeter in Hertz frequency mode.

2. 2

   Probe the YEL/GRN wire of the MAF sensor with the red multimeter test lead using an appropriate tool.
   
   The YEL/GRN wire is labeled with the #3 arrow in the photo above.

3. 3

   Connect the black multimeter probe to the battery negative (-) post.

4. 4

   Start the engine and let it idle for about 5 minutes till it reaches its normal operating temperature.

5. 5

   Accelerate the engine as you watch the multimeter's frequency readings.
   
   The Hertz frequency reading should increase when you accelerate the engine and should decrease when you decelerate it.

NOTE: If you're using an oscilloscope to the test the MAF signal, you see an example of what the signal waveform should look like here: MAF Sensor Signal Oscilloscope Wave Form.

Let's interpret your test result:

CASE 1: The Hertz frequency reading increased and decreased as indicated. This is the correct and expected test result and tells you that the MAF sensor (on your Lexus LS400) is working correctly.

CASE 2: The Hertz frequency reading DID NOT increase or decrease as indicated. This test result tells you that the MAF sensor, on your 1990-1994 Lexus LS400, is defective.

MAF Sensor Signal Oscilloscope Wave Form