How To Test The MAF Sensor (1991-1995 1.9L Ford Escort, Mercury Tracer)

The mass airflow (MAF) sensor on the 1991-1995 1.9L Ford Escort (or Mercury Tracer) can be tested easily with just a multimeter —you don't need a scan tool or any other expensive diagnostic equipment for this job.

In this tutorial, I'll walk you step by step through the four multimeter tests you can do to find out if the MAF sensor is good or bad.

You'll check the MAF signal voltage, verify it's receiving VPWR power, and confirm both Grounds (PWR GND and MAF RTN) are present.

APPLIES TO: This tutorial applies to the following vehicles:

• 1.9L Ford Escort: 1991, 1992, 1993, 1994, 1995.
• 1.9L Mercury Tracer: 1991, 1992, 1993, 1994, 1995.

Common Symptoms Of A Bad Ford MAF Sensor

Your Ford Escort or Mercury Tracer's fuel injection computer (PCM) needs to know how much air is entering the engine to calculate the correct fuel injection. And you guessed it —the MAF sensor is the one tasked with measuring that airflow.

When the MAF sensor fails, you'll usually see the check engine light illuminated along with one or more of these symptoms:

• On the 1991-1995 OBD I vehicles, you'll see one of the following codes:
  • Code 157: MAF Sensor Circuit Below Minimum Voltage.
  • Code 158: MAF Sensor Circuit Above Maximum Voltage.
  • Code 159: MAF Higher Or Lower Than Expected.
• On the 1996 OBD II vehicles, you'll see one of the following codes:
  • P0102: MAF Signal Low Input To PCM.
  • P0103: MAF Signal High Input To PCM.
• MAF sensor malfunction that DOES NOT light up the check engine light (CEL).
• Lean and/or Rich code(s).
• Fuel trim code(s).
• A tremendous lack of power upon acceleration.
• Black smoke coming from the tail-pipe.
• Bad gas mileage.
• Vehicle may idle rough and stall.

Circuit Descriptions Of The Ford MAF Sensor's Connector

The MAF sensor on your Ford Escort or Mercury Tracer uses a 4-wire connector. The table below identifies each circuit and its role, information you'll need for the diagnostic checks ahead.

Pin	Wire	Description
A	White with red stripe wire (WHT/RED)	VPWR –12 Volts Input in RUN or START
B	Black with dark green stripe (BLK/DK GRN)	PWR GND –Chassis Ground Input
C	Green with yellow stripe (GRN/YEL)	MAF RTN –MAF sensor Ground (provided by PCM)
D	Brown with black stripe (BRN/BLK)	MAF –MAF signal

IMPORTANT: All of the tests are ON CAR TESTS, do not remove the mass airflow sensor from the vehicle or from its plumbing.

TEST 1: Testing The MAF Signal

Your first step in testing the MAF sensor is to tap into the brown with black stripe (BRN/BLK) wire —which carries the MAF signal— with your multimeter, and monitor how it responds to engine airflow.

Here's what you should see during this MAF signal test:

• When you rev the engine and airflow increases, the MAF signal voltage should climb right along with it.
• When you let off the throttle and the RPMs drop, the MAF signal voltage should drop as airflow decreases.

No matter how many times you rev the engine and let off the throttle, the MAF signal voltage should climb as RPM goes up and drop back down as the engine settles to its base idle speed.

If the MAF sensor on your Ford Escort or Mercury Tracer is toast, the signal voltage will stay stuck at one reading —no matter how much you rev the engine up or down.

NOTE: The MAF sensor needs to stay connected to its wiring harness while you test it. That means you'll need a wire-piercing probe to tap into the MAF signal wire. If you don't already have one, here's the one I use and recommend: Goupchn 4mm Banana to Banana Plug Test Leads Kit (Amazon affiliate link).

These are the test steps:

1. 1

   Switch your multimeter to Volts DC.

2. 2

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

3. 3

   Connect the red lead to the brown with black stripe (BRN/BLK) wire using a wire-piercing probe.

4. 4

   Start the engine and let it idle.
   
   Once idle smooths out, note the voltage reading on the multimeter. A little fluctuation is normal. This first reading is your base MAF signal voltage.

5. 5

   Rev the engine by opening the throttle.
   
   You can do this at the throttle body by moving the linkage, or by having a helper step on the accelerator pedal.

6. 6

   The MAF signal voltage should rise as RPM increases.

7. 7

   Release the throttle to let the RPM come back down.

8. 8

   The voltage should drop as the RPMs fall.

9. 9

   Repeat the accelerate/release cycle a few times to verify the results are consistent.

Time to interpret your test result:

CASE 1: The signal voltage rises and falls as you open and close the throttle. This tells you the MAF sensor is operating correctly.

If the MAF sensor were faulty, the voltage would stay fixed at one value regardless of engine RPM.

CASE 2: The signal voltage does NOT respond to throttle movement. This usually means the MAF sensor is toast. It could also mean the MAF sensor isn't receiving power or Ground.

Before calling the MAF sensor defective, we need to confirm its getting power and then Ground. For the first of these test, go to: TEST 2: Making Sure The MAF Is Getting VPWR.

TEST 2: Making Sure The MAF Is Getting VPWR

The hot-wires on your Escort or Tracer's MAF sensor require a steady 12 Volt supply to operate. Without this VPWR feed, the hot-wires can't heat up and the PCM won't receive an usable signal.

For our next step, we're gonna confirm this VPWR power supply is available at female terminal A of the MAF sensor's 4-wire connector with your multimeter.

With key ON engine OFF (KOEO), you should see 10-12 Volts DC at terminal A. Anything lower, or no voltage at all, points to a VPWR power supply problem that must be fixed (and not a bad MAF sensor).

Alright, these are the test steps:

1. 1

   Disconnect the MAF sensor from its engine wiring harness connector.

2. 2

   Put your multimeter in Volts DC mode.

3. 3

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

4. 4

   Gently probe round female terminal A with the red multimeter lead.
   
   NOTE: Confirm that terminal A connects to the white with red stripe (WHT/RED) wire on the MAF sensor's 4-wire connector.

5. 5

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

6. 6

   The multimeter should display 10 to 12 Volts.

Let's take a look at what your test result means:

CASE 1: The multimeter shows 10-12 Volts. This confirms the VPWR feed is present and working.

With power verified, the next step is to confirm the sensor has a solid chassis Ground at terminal B in: TEST 2: Making Sure The MAF Is Getting PWR GND (Chassis Ground).

CASE 2: The multimeter shows no voltage or less than 10 Volts. That means the sensor isn't being powered. A MAF without VPWR can't heat its wires and can't generate a viable MAF signal.

This result points directly to a VPWR power supply fault. Your next step is to diagnose why the VPWR feed is missing and repair the circuit so the sensor can come online.

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

With VPWR confirmed, the next step is to verify the MAF has a reliable chassis Ground path. Without it (even if the VPWR feed is present), the hot-wire circuit inside the sensor is dead.

Ford designates this chassis Ground as PWR GND. Unlike the VPWR supply, it isn't switched —in other words, it's present at all times, whether the key is in OFF, RUN, or START position.

To test the PWR GND circuit, we'll connect our multimeter to terminal B (which connects to the black with dark green (BLK/DK GRN) wire of the connector).

Alright, here's what we need to do:

1. 1

   Unplug the MAF sensor from its engine wiring harness connector.

2. 2

   Set your multimeter to Volts DC.

3. 3

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

4. 4

   Probe female terminal B with the black multimeter test lead.
   
   NOTE: Check that terminal B corresponds to the BLK/DK GRN wire at the MAF sensor's 4-wire connector.

5. 5

   Your multimeter should show between 10 and 12 Volts.
   
   NOTE: Because this is a chassis Ground, it will always be present, whether the key is ON or OFF.

Let's take a look at what your test result means:

CASE 1: The multimeter shows 10-12 Volts. This confirms the sensor has a good chassis Ground.

With the PWR GND circuit checked, the next step is to check the second Ground circuit —the MAF RTN supplied by the PCM. Continue to: TEST 4: Making Sure The MAF Is Getting MAF RTN Ground.

CASE 2: The multimeter shows no voltage. That points directly to a missing chassis Ground. Without it, the sensor's hot-wires can't operate.

Your next move is to troubleshoot the PWR GND supply wire to find why this chassis Ground is missing and restore it so the circuit can function.

TEST 4: Making Sure The MAF Is Getting MAF RTN Ground

Here's what your test results are telling you so far:

• In TEST 1, the signal voltage did not react to changes in airflow.
• In TEST 2, you verified the sensor is receiving VPWR power.
• In TEST 3, you confirmed a solid chassis Ground (PWR GND).

Your final diagnostic test is to verify the PCM is supplying the MAF RTN Ground to the MAF sensor at terminal C.

Terminal C connects to the green with yellow stripe (GRN/YEL) wire (which connects directly to your Escort or Tracer's fuel injection computer).

NOTE: This Ground originates inside the fuel injection computer (PCM). Never apply battery voltage to this circuit —doing so will deep-fry the PCM instantly. The multimeter test I'm outlining below is a safe method for confirming the MAF RTN Ground at terminal C.

Let's get started:

1. 1

   Disconnect the MAF sensor from its electrical connector.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

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

4. 4

   Probe female terminal C with the black multimeter test lead.
   
   NOTE: Check that terminal C actually corresponds to the GRN/YEL 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

   You should see 10 to 12 Volts on the multimeter.

Let's take a look at what your test result means:

CASE 1: Multimeter shows 10-12 Volts. This test result confirms the PCM is supplying the MAF RTN Ground.

If you've reached this point and every other check has passed, the sensor is defective and needs replacement. Just to be clear, this conclusion is correct only if all of the following are true:

• TEST 1 →The MAF signal did not react (stayed stuck in one value) regardless of engine airflow (RPM).
• TEST 2 →The RED wire delivers 10–12 Volts (VPWR feed).
• TEST 3 →The BLK wire provides a solid chassis Ground (PWR GND).
• And in this step → The PCM supplies the MAF RTN Ground through terminal C.

CASE 2: Multimeter shows no voltage. This points directly to a missing PCM-supplied Ground. Without the MAF RTN Ground, the MAF sensor can't function.

Your next troubleshooting step is to see why this MAF RTN Ground is missing and repair the circuit so the sensor can function.

More 1.9L Ford Escort And Mercury Tracer Diagnostic Tutorials

You can find a complete list of 1.9L Ford Escort and Mercury Tracer tutorials in this index:

• 1.9L Ford Escort Index Of Articles.

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

• How To Test The Fuel Pump (1991-1996 1.9L Ford Escort, Mercury Tracer).
• Common Causes Of Spark Plug Failure (1991-1995 1.9L Ford Escort, Mercury Tracer).
• How To Test For A Blown Head Gasket (1991-1995 1.9L Ford Escort, Mercury Tracer).
• How To Test Engine Compression (1991-1995 1.9L Ford Escort, Mercury Tracer).