How To Test The MAF Sensor (1990-1995 5.0L V8 Ford Mustang)

The mass airflow (MAF) sensor on your 5.0L V8 Ford Mustang is a hot-wire type, and let me tell you, it's not that hard to test with a multimeter to find out if it's good or bad.

As a matter of fact, you don't even need a scan tool —or any other expensive diagnostic equipment. In this tutorial, I'm going to show you how I troubleshoot the MAF sensor with four simple multimeter tests.

In these tests, we're going to check the MAF sensor's signal output voltage. We'll also make sure it's getting power (12V VPWR) and Ground (PWR GND and MAF RTN).

All of these tests are explained step-by-step so that you can quickly and easily figure out whether the MAF sensor on your Mustang is toast or good.

APPLIES TO: This tutorial applies to the following vehicles:

• 5.0L V8 Ford Mustang: 1990, 1991, 1992, 1993, 1994, 1995.

MAF SENSOR CIRCUIT WIRING DIAGRAMS:

• MAF Sensor And ACT Sensor Circuit Wiring Diagram (1990 5.0L V8 Ford Mustang).
• MAF Sensor And ACT Sensor Circuit Wiring Diagram (1991-1992 5.0L V8 Ford Mustang).
• MAF Sensor And IAT Sensor Circuit Wiring Diagram (1993 5.0L V8 Ford Mustang).

Symptoms Of A Bad MAF Sensor

Your Mustang's fuel injection computer needs to know how much air is entering the engine so it can calculate the correct amount of fuel to deliver.

As you're probably already aware, the mass airflow sensor is tasked with this job. So when it fails, your Ford Mustang's engine is going to be in a world of hurt —and you're gonna see one or more of the following performance problems:

• Check Engine Light: The CEL will be lit on the dash, and the computer will usually store one of these OBD-I diagnostic trouble codes (DTCs):
  • 157: MAF Sensor Circuit Below Minimum Voltage.
  • 158: MAF Sensor Circuit Above Maximum Voltage.
  • 159: MAF Higher Or Lower Than Expected.
• Lack of power: Your Mustang's engine may feel weak or hesitate when you hit the gas, especially under load or at higher speeds.
• Rough idle/stalling: The engine can idle unevenly or shut off on its own.
• Bad gas mileage: A bad MAF affects the PCM's ability to control the air-fuel mixture, making the engine run rich or lean, which kills gas mileage.
• Black exhaust smoke: If the PCM is adding too much fuel because of a bad MAF sensor signal, you may see black smoke coming out of the tailpipe.
• Hard starting: Sometimes the engine will crank but struggle to start.
• Hesitation or surging: You might feel the engine stumble or surge while accelerating or cruising.

MAF Sensor Circuit Descriptions

Your Ford Mustang's MAF sensor is a 4-wire sensor. Two of these wires carry signals to it, and the other two carry signals from it to the fuel injection computer.

To test the sensor itself, we need to know what each of those wires does. In the following table, I'm providing a brief description of each circuit to help with the tests that follow.

Pin	Wire	Description
A	Red (RED)	VPWR –12 Volts Input in RUN or START
B	Black with light green stripe (BLK/LT GRN)	PWR GND –Chassis Ground Input
C	Tan with light blue stripe (TAN/LT BLU)	MAF RTN –MAF sensor Ground (provided by PCM)
D	Dark blue with orange stripe (DK BLU/ORG) –1990 Only	MAF –MAF signal
	Light blue with red stripe (LT BLU/RED) –1991-1995 Only

What Tools Do I Need To Test The MAF Sensor?

Like I mentioned before, you're not going to need any expensive diagnostic equipment to troubleshoot your Mustang's MAF sensor, but you'll need two basic tools. These two are:

• A multimeter: To check the MAF signal voltage output, and verify power and Ground to the sensor.
• A wire-piercing probe or back probe: This tool lets you access the MAF signal voltage inside the MAF signal wire in TEST 1.

Below is the multimeter I've relied on for years to do all kinds of diagnostic tests, and it's never let me down. It has everything you need to check the MAF sensor or any other electrical component on your Mustang (or any other vehicle).

I'm also including a wire-piercing probe kit that I use regularly. It's inexpensive, easy to work with, and doubles as both a piercing probe and a back probe when the job calls for it.

TEST 1: Testing The MAF Signal

To successfully diagnose the MAF sensor as good or bad, we need to know that its voltage signal corresponds to the amount of airflow entering the engine. To make sense of this, let's say:

• When airflow and engine RPM are low, the MAF sensor outputs a low voltage signal. For example:
  • You're at a traffic light and the engine is idling at around 800 to 900 RPM. At this speed, the MAF sensor outputs a voltage signal of around 1 Volt DC.
• When airflow increases and engine RPM goes up, the MAF sensor signal voltage also increases.
  • You step on the accelerator pedal to move the vehicle from that traffic light —airflow increases, engine RPM goes up, and so does the MAF sensor's output voltage.

The cool thing is we can use a multimeter to actually see this voltage signal increasing and decreasing as the engine revs up or down.

And that's exactly what we're going to do in this test section. We're going to tap into the MAF signal wire with our multimeter and check what it's doing at different engine RPMs. Depending on the specific year of your Mustang, this signal wire will be:

• 1990: The dark blue with orange stripe (DK BLU/ORG) wire.
• 1991-1995: The light blue with red stripe (LT BLU/RED) wire.

Nine times out of ten, when the MAF sensor has failed, its signal voltage output will stay stuck at one value no matter what the engine RPM and airflow are.

IMPORTANT: To see the voltage signal in the wire during the MAF signal test, the sensor must remain connected to its 4-wire connector. The only way to check the signal is with a back probe on the connector or a wire-piercing probe on the wire itself. If you don't have one of these tools, this is the one I use and don't hesitate to recommend: Goupchn 4mm Banana to Banana Plug Test Leads Kit (Amazon affiliate link).

Here's the procedure:

1. 1

   Set your multimeter to Volts DC mode.

2. 2

   Attach the black lead of the multimeter to the battery's negative (-) post.

3. 3

   Probe the light LT BLU/RED (or DK BLU/ORG) wire with the red multimeter lead using either a back probe or a wire-piercing probe.
   
   The LT BLU/RED (or DK BLU/ORG) wire connects to terminal D of the MAF sensor connector (see diagram above).

4. 4

   Start the engine and let it idle.
   
   Once the idle steadies out, take note of the reading on the multimeter. Some small fluctuation is normal. This value is your base MAF signal voltage.

5. 5

   Increase engine speed by opening the throttle.
   
   You can do this directly at the throttle body by moving the linkage, or by having a helper press the accelerator pedal.

6. 6

   The MAF signal voltage should climb as RPM rises.

7. 7

   Let go of the throttle so the RPM drops back down.

8. 8

   The signal voltage should decrease as RPM falls.

9. 9

   Repeat the rev-up/release cycle a few times to make sure the readings are consistent.

Here's how to interpret your results:

CASE 1: The signal voltage went up as engine RPM increased and dropped back down when you let off the throttle. This is the expected outcome, and it confirms the MAF sensor is responding the way it should.

If you're diagnosing a trouble code 157 (MAF Signal Below Minimum Voltage) or a code 158 (MAF Signal Above Maximum Voltage), this particular result tells you the sensor is fine right now because its voltage signal is reacting to airflow changes just like it's supposed to.

CASE 2: The signal voltage stayed flat and never changed with engine RPM. In most cases, that points to a bad MAF sensor on your Mustang.

Still, don't swap it out just yet. The next step is to confirm it's getting its 12 Volts (VPWR). Head over to: TEST 2: Making Sure The MAF Sensor Is Getting Power (VPWR).

TEST 2: Making Sure The MAF Sensor Is Getting Power (VPWR)

The MAF sensor's hot wires need 12 Volts (VPWR) and Chassis Ground (PWR GND) to function. The wire that delivers these 12 Volts to the MAF sensor is the red (RED) wire connected to terminal A of the MAF sensor's 4-wire connector.

To check for the presence of these 12 Volts at terminal A, all we'll need to do is a simple multimeter voltage test with the key in the RUN position.

If your test result shows that this voltage is present at terminal A, we'll move on to TEST 2 and make sure the sensor's hot wires are getting Chassis Ground (PWR GND) at terminal B.

IMPORTANT: Be careful when probing the front of the terminal with your multimeter leads. If the probe tip is too thick, it could permanently open up the terminal. If that happens, you'll have to replace the connector.

Let's begin:

1. 1

   Unplug the MAF sensor from its electrical connector.

2. 2

   Set your multimeter to Volts DC.

3. 3

   Attach the black multimeter lead to the battery's negative (-) post.

4. 4

   Carefully probe female terminal number A with the red multimeter lead.
   
   Terminal A should correspond to the RED wire on the MAF sensor's 4-wire connector.

5. 5

   Turn the ignition key to RUN, but don't crank or start the engine.

6. 6

   The multimeter should read between 10 and 12 Volts.

Here's what your test result is telling you:

CASE 1: The multimeter showed between 10 and 12 Volts. That's exactly what we want to see —it confirms the MAF sensor is getting its VPWR power feed.

From here, the next check is to make sure terminal B has a good chassis Ground (PWR GND). Go to: TEST 3: Making Sure The MAF Sensor Is Getting Ground (PWR GND).

CASE 2: The multimeter did not show 10 to 12 Volts. Recheck your connections and repeat the test.

If there's still no voltage, then the MAF sensor isn't the problem —without that 10 to 12 Volt supply, it simply can't function.

At this point, you'll need troubleshoot why the RED wire isn't carrying VPWR and fix it. Once that's resolved, the MAF sensor will work as intended and you'll be able to clear the code.

TEST 3: Making Sure The MAF Sensor Is Getting Ground (PWR GND)

Now that you've confirmed the sensor's hot wires are getting 12 Volts (VPWR) in TEST 2, in this section, we're going to make sure they're also getting chassis Ground (PWR GND).

The wire that delivers this PWR GND to terminal B is the black with light green (BLK/LT GRN) wire of the connector.

This PWR GND is available to the sensor at all times. In other words, it doesn't matter if the key is in the OFF, RUN, or START position —this Ground is always present at terminal B.

IMPORTANT: Be careful when probing the front of the connector or female terminal with the multimeter's metal probe.

Alright, let's begin:

1. 1

   Disconnect the MAF sensor from its electrical connector.

2. 2

   Switch your multimeter to Volts DC.

3. 3

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

4. 4

   Lightly probe female terminal B with the black lead of the multimeter.
   
   Terminal 3 should match up with the BLK/LT GRN wire at the MAF sensor connector.

5. 5

   The multimeter should read between 10 and 12 Volts.
   
   NOTE: Since this is a chassis Ground, it'll be present at all times —whether the key is OFF, RUN, or START.

Let's figure out what your results mean:

CASE 1: The multimeter shows 10 to 12 Volts DC at terminal B. Perfect —that's the result we're after. It confirms terminal B is providing the chassis Ground needed by the MAF sensor's hot wires.

The next step is to confirm that terminal C is supplying the MAF RTN Ground. Let's go to: TEST 4: Making Sure The MAF Sensor Is Getting Ground (MAF RTN).

CASE 2: The multimeter does not show 10 to 12 Volts DC at terminal B. Recheck your test connections and run the test again just to be sure.

If the reading is still missing, then the MAF sensor isn't receiving the chassis Ground it needs for the hot wires to work.

Your next diagnostic move is to track down why this chassis Ground is missing and restore it. Once the BLK/LT GRN wire is supplying Ground again, the MAF sensor should operate normally.

TEST 4: Making Sure The MAF Sensor Is Getting Ground (MAF RTN)

So far, you've confirmed the following:

• TEST 1 –The MAF signal output voltage is stuck at one value, no matter the engine speed.
• TEST 2 and TEST 3 –The MAF sensor's hot wires are getting both power (VPWR) and chassis Ground (PWR GND).

In this last test section, we're going to make sure the tan with light blue stripe (TAN/LT BLU) wire that connects to terminal C is delivering the MAF RTN Ground.

This MAF RTN Ground is provided directly and internally by your Mustang's fuel injection computer. It's only available with the key in the RUN or START position.

To check for the presence of this Ground, we're going to do a multimeter voltage test with the key in the RUN position.

CAUTION: The MAF RTN Ground comes from your Mustang's fuel injection computer (it's not a chassis Ground), so be careful when testing this circuit. Do not apply 12 Volts directly from the battery to it, or you'll fry the computer. The test I'm describing below is a safe and easy way to check for Ground at terminal C.

OK, these are the test steps:

1. 1

   Unplug the MAF sensor from its connector.

2. 2

   Set your multimeter to Volts DC.

3. 3

   Attach the red multimeter lead to the battery positive (+) post.

4. 4

   Carefully probe female terminal C with the black multimeter lead.
   
   Terminal C should match up with the TAN/LT BLU wire of the MAF sensor connector.

5. 5

   Turn the key to the RUN position, but don't crank or start the engine.

6. 6

   The multimeter should register between 10 and 12 Volts.

Here's what your test results are telling you:

CASE 1: The multimeter reads between 10 and 12 Volts. That's the correct outcome, and it confirms the MAF sensor is receiving its MAF RTN Ground through the TAN/LT BLU wire.

You can safely call the MAF sensor bad and in need of replacement if all of the following apply:

• In TEST 1, the output signal stayed locked on one value no matter how much you revved the engine.
• In TEST 2, you verified it's getting its 10 to 12 Volts (VPWR).
• In TEST 3, you confirmed a solid chassis Ground (PWR GND) is present.
• And in this test, the fuel injection computer is delivering the MAF RTN Ground.

If you've confirmed the sensor is shot and it's time to swap it out, I recommend these from a known automotive brand. I've used components from this brand for years and can recommend it with confidence:

1991-1993 5.0L V8 Mustang:

• Walker Products 245-1013 Mass Air Flow Sensor Assembly (Amazon affiliate link).

1994-1995 5.0L V8 Mustang:

• Walker Products 245-1014 Mass Air Flow Sensor Assembly (Amazon affiliate link).

CASE 2: The multimeter does not display 10 to 12 Volts. That means the TAN/LT BLU wire isn't providing the MAF RTN Ground to the sensor.

Go back and double-check your multimeter connections, then run the test again. If the reading is still missing, you can rule out the MAF sensor itself as the source of the trouble code —without that MAF RTN Ground, the sensor simply can't operate.

The next step is to track down why the MAF RTN Ground is missing and fix the issue so the MAF sensor can come back online.

More 5.0L V8 Ford Mustang Diagnostic Tutorials

You can find a complete list of wiring diagrams and diagnostic tutorials for the 5.0L V8 Ford Mustang in this index:

• 5.0L V8 Ford Mustang Index Of Articles.

Here's a sample of the diagnostic tutorials you'll find in the index:

• How To Test The Fuel Pump (1990-1995 5.0L V8 Ford Mustang).
• How To Test The Throttle Position Sensor (1994-1995 5.0L Ford Mustang).
• How To Test For A Blown Head Gasket (1990-1995 5.0L V8 Ford Mustang).
• How To Do And Interpret An Engine Compression Test (1990-1995 5.0L V8 Ford Mustang).