This tutorial will help you to test the manifold absolute pressure (MAP) sensor on the 2002-2007 4.2L Chevrolet TrailBlazer or GMC Envoy.
All you'll need to test it is a vacuum pump and a multimeter. Now if you don't have a vacuum pump, don't worry. I'll show you how to test it without one.
In a nutshell, I'll show you how to test the MAP sensor signal and how to make sure that the MAP sensor is getting power and Ground.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar El Sensor MAP (2002-2007 4.2L Chevrolet Trailblazer) (at: autotecnico-online.com).
Symptoms Of A Bad MAP Sensor
The fuel injection system on your 4.2L Chevrolet TrailBlazer or GMC Envoy is a ‘speed density’ type fuel system.
In this type of fuel system, the fuel injection computer needs to know three things to find out how much air is entering your Chevy TrailBlazer or GMC Envoy's engine and inject the correct amount of fuel into its 6 cylinders.
These 3 things are: The temperature of the air entering the engine, the engine RPM, and the load that the engine is under.
The MAP sensor is the one that provides the engine load information so that the computer can inject the correct amount of fuel for the amount of air entering the engine.
Since the MAP sensor is a critical component of the engine management system, when this bad boy fails you're gonna' see one or more of the following symptoms:
- Check engine light (CEL) is shining nice and bright on the instrument cluster.
- DTC P0107 MAP sensor Circuit Low Voltage.
- DTC P0108 MAP sensor Circuit High Voltage.
- Rough idle.
- ‘Rotten egg’ smell coming from the exhaust.
- Won't pass the state mandated emissions test.
- Bad gas mileage.
- Lack of power, rough idle, or hesitation.
- Engine cranks a long time before starting.
The MAP sensor is a 3 wire type sensor. This means that it has a power wire, a Ground wire and a signal wire. The table below has a brief description of each:
|B||LT GRN||MAP Signal|
TEST 1: Testing The MAP Sensor Voltage Signal
The very first thing that we're going to do is to test the MAP sensor's signal with a multimeter as we apply vacuum to it.
We're not going to start the engine because we're not going to use engine manifold vacuum to test the MAP sensor. What will do is connect a vacuum pump to it so that we can manually apply a vacuum to the MAP sensor.
Now if you don't have a vacuum pump there's no need to worry. Because you can use your mouth to apply vacuum to the MAP sensor via a vacuum hose.
If the MAP sensor on your 4.2L Chevrolet TrailBlazer or GMC Envoy is defective, you'll see that the MAP sensor signal will not decrease as you apply vacuum to it.
To be a bit more specific, the MAP sensor voltage signal will stay stuck in one value no matter how much vacuum you apply to it.
IMPORTANT: The MAP sensor must remain connected to its electrical connector for this test to work. You'll need to use a back-probe or a wire piercing probe to access the voltage inside the wire. You can see an example of this tool here: Wire Piercing Probe.
Let's get started:
Remove the MAP sensor from the intake manifold.
Connect your vacuum pump to the MAP sensor's vacuum inlet port. Reconnect the MAP sensor to its connector if you disconnected it.
Set your multimeter's selector to Volts DC mode and with the red test lead, probe the LT GRN wire of the MAP sensor's connector. This wire is identified by the letter B in the photo above.
Remember, the MAP sensor must remain connected to its 3 wire connector.
Ground the black multimeter test lead directly on the battery negative (-) terminal.
Turn the Key on but don't start the engine. This will power up the MAP sensor and you should see a reading of 4.7 Volts DC on your multimeter.
Now, apply vacuum to the MAP sensor with the vacuum pump (or your mouth). The voltage signal value should decrease.
If you're using a vacuum pump: At 5 in.Hg → 3.9 Volts. At 10 in.Hg → 3 Volts. At 15 in.Hg → 2.1 Volts. At 20 in.Hg → 1.2 Volts.
Release the vacuum. Once released, your multimeter should show the original voltage value.
Repeat this test step several times making sure that each time the voltage decreases/increases as you apply/release vacuum.
Let's take a look at what your test results mean:
CASE 1: The MAP voltage signal decreased and increased as you applied and released vacuum. This is the correct test result and lets you know that the MAP sensor is performing beautifully (it's not defective).
You can also conclude that the MAP sensor is getting power (5 Volts) and Ground too.
CASE 2: The MAP voltage signal DID NOT decrease/increase as you applied and released vacuum. There's a good chance the MAP sensor is defective or it's not getting power or it's not getting Ground.
So, to make sure the MAP sensor is defective the next step is make sure it's getting 5 Volts and Ground. Go to: TEST 2: Verifying MAP Sensor Has 5 Volts And Ground.
CASE 3: Multimeter DID NOT register any voltage. There's a good chance the MAP sensor is defective or it's not getting power or it's not getting Ground.
To find out the next step is make sure the MAP sensor is getting 5 Volts and Ground. Go to: TEST 2: Verifying MAP Sensor Has 5 Volts And Ground.