How To Test The MAP Sensor (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma)

TEST 2: Making Sure The MAP Sensor Is Getting 5 Volts

The MAP sensor needs 5 Volts and Ground to generate its voltage signal. Both of these are provided by the fuel injection computer.

In this test section, we'll focus on making sure that the terminal labeled with the number 1 is supplying 5 Volts to the MAP sensor with the key in the ON position but with the engine off.

The wire that connects to terminal labeled with the number 1 is the gray (GRY) wire of the MAP sensor's 3-wire connector.

If the MAP sensor is receiving 5 Volts, we'll move on to TEST 3 and make sure the MAP sensor is receiving Ground.

Let's get the ball rolling:

1. 1

   Place your multimeter in Volts DC mode.

2. 2

   Disconnect the MAP sensor from its electrical connector.

3. 3

   Probe the wire that connects to the terminal labeled with the number 1 in the illustration above with the red multimeter test lead.
   
   The wire that connects to terminal 1 is the GRY wire of the 3-wire connector.
   
   NOTE: You can probe the front of the wire's female terminal. If you do, be careful not to damage it with your multimeter's probe.

4. 4

   Now Ground the black multimeter test lead on the battery's negative (-) post.

5. 5

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

6. 6

   Your multimeter should show you a voltage value around 4.5 to 5 Volts DC.

Let's interpret your test result:

CASE 1: The multimeter registered 5 Volts. This is the correct and expected test result.

Now that you have confirmed the MAP sensor is receiving power, your next step is to make sure that it's getting Ground. Go to: TEST 3: Making Sure The MAP Sensor Is Getting Ground.

CASE 2: The multimeter DID NOT register 5 Volts. Without power, the MAP sensor will not function.

Although it's beyond the scope of this tutorial, your next step is to find out why these 5 Volts are missing and restore them. Once you restore power to the MAP sensor, it'll start functioning again.

TEST 3: Making Sure The MAP Sensor Is Getting Ground

So far the two previous tests have confirmed the following:

• The MAP sensor's output voltage signal does not decrease/increase as you apply/release vacuum to the sensor (TEST 1).
• The MAP sensor is receiving 5 Volts (TEST 2).

In this final test section, you'll check if the MAP sensor is receiving Ground from the fuel injection computer.

The wire that supplies Ground to the MAP sensor is the purple (PPL) wire of the MAP sensor's 3-wire connector and connects to the terminal labeled with the number 3 in the illustration above.

IMPORTANT: The fuel injection computer is the component that supplies Ground to the MAP sensor. Be careful and do not short-circuit the Ground wire to 12 Volts or you'll fry the computer. The suggested multimeter voltage test is a safe way to test for the presence of Ground in the wire.

NOTE: The MAP sensor's Ground wire will be one of three colors:

• A purple (PPL) wire.
• A black with red stripe (BLK/RED) wire.
• A black with oragne stripe (BLK/ORG) wire.

These are the test steps:

1. 1

   Place your multimeter still in Volts DC mode.

2. 2

   Disconnect the MAP sensor from its electrical connector.

3. 3

   Probe the wire labeled with the number 3 in the image above, with the black multimeter test lead.
   
   The wire that connects to terminal 3 is the PPL (or BLK/RED or BLK/ORG) wire of the 3-wire connector.
   
   NOTE: You can probe the front of the wire's female terminal. If you do, be careful not to damage it with your multimeter's probe.

4. 4

   Connect the red multimeter test lead on the battery's positive (+) post.

5. 5

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

6. 6

   Your multimeter register 10 to 12 Volts DC.

OK, let's see what your test result means:

CASE 1: The multimeter registered 10 to 12 Volts. This is the correct test result and tells you that the MAP sensor is receiving Ground.

You can correctly conclude the manifold absolute pressure sensor is bad and needs replacement if you have:

• Confirmed the MAP sensor voltage signal does not decrease/increase as you apply/release vacuum to the sensor (TEST 1).
• Confirmed the MAP sensor is receiving 5 Volts (TEST 2).
• Confirmed the MAP sensor is receiving Ground (this test section).

CASE 2: The multimeter DID NOT register 10 to 12 Volts. Without Ground, the MAP sensor will not function.

Although it's beyond the scope of this tutorial, your next step is to find out why the MAP sensor is not receiving Ground and resolve the problem. Once you restore Ground to the MAP sensor, it'll start functioning again.

MAP Sensor Code Won't Go Away

It's not uncommon for the fuel injection computer to think that the MAP sensor has failed when it really hasn't.

This usually happens when another engine performance problem causes the MAP sensor to report abnormal intake manifold vacuum pressures to the fuel injection computer.

So if your MAP sensor is working properly but the trouble code keeps coming back, take a look at the following test suggestions:

1. The vacuum line (or hose) that supplies vacuum to the MAP sensor is cracked and leaking vacuum or is connected to the wrong vacuum port.
2. The engine has several cylinders with very low engine compression causing it to idle rough and thus producing low or erratic vacuum. For this I suggest an engine compression test.
   • How To Test Engine Compression (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
3. The MAP sensor is failing intermittently. Which means that it works fine most of the time, but every now and then it doesn't:
   • I have found that the best way to test these intermittent problems is to slightly tap the MAP sensor with the handle of a screw-driver and see if this tapping screws up the voltage readings as I apply vacuum.
4. The MAP sensor's connector is bad, usually the locking tab is broken and the connector has worked itself loose, causing an intermittent false connection.
5. Your fuel pump is starting to go bad and is not sending enough fuel and/or fuel pressure up to the fuel injectors. I suggest a fuel pump pressure test.