Your 2.8L Chevrolet S10's (GMC S15's) fuel system is a speed-density type... which means that the fuel injection computer relies heavily on the MAP sensor to know how much fuel to inject into the engine.
So, when the manifold pressure (MAP) sensor fails, your S10 (S15) is gonna' be in a world of hurt. Thankfully, testing the GM manifold absolute pressure (MAP) sensor on your GM 2.8L V6 equipped Pick Up (Blazer) can be accurately done using only a multimeter and a vacuum pump.
This article will show you how to bench test it (although you'll be leaving the MAP sensor connected to its electrical connector) to be able to either condemn the MAP as BAD or eliminate it as the source of the diagnostic trouble code or drive-ability issue your Pick Up (Blazer) is going through.
Contents of this tutorial:
Puedes encontrar este tutorial en Español aquí: Cómo Probar el Sensor MAP Con Multímetro (2.8L V6 GM) (en: autotecnico-online.com).
Symptoms Of A BAD MAP Sensor
The two most obvious symptoms of a bad MAP sensor is that the check engine light will be shining nice and bright and that the engine in your vehicle will idle very rough.
These are some of the other symptoms your GM 2.8L Chevy S10 (GMC S15) Pick Up (Blazer) will experience with a BAD MAP sensor:
- The check engine light (CEL) will be on with a MAP diagnostic trouble code (DTC) stored in the computer's memory.
- If your GM 2.8L Chevy S10 (GMC S15) is 1994 or older, you'll see DTC's:
- 33: MAP Sensor Signal Voltage High.
- 34: MAP Sensor Signal Voltage Low.
- Your vehicle won't start or will have a long cranking time before it starts.
- Black smoke coming out of the tailpipe along with really BAD gas mileage.
- The engine idles rough when running and has a lack of power when accelerated.
TEST 1: Checking The MAP Sensor Signal
As you're already aware, a vacuum is created by the downward movement of the engine pistons (when the engine is running). This vacuum only exists below the throttle plate on down thru' the intake manifold and into the engine cylinders (any air pressure above the throttle plate is just ambient pressure).
The MAP sensor's job is to measure the amount of vacuum in the intake manifold. This vacuum measurement is converted into a DC voltage signal that can be easily tested with a multimeter in Volts DC mode.
To test the MAP sensor, we're not gonna' rely on the engine's vacuum. What we'll do is supply our own vacuum using a vacuum pump. This will give us an extremely accurate test result.
NOTE: You don't have to use a vacuum pump to test the MAP sensor. You can use your mouth to apply vacuum to the MAP sensor (via a vacuum hose). This method won't produce the same test result because applying vacuum with your mouth won't be able to bring down the voltage to 1.1 Volts. The important thing is just to see the voltage go down and then go back up (to its original value) when you apply vacuum with the ‘good ole' lungs’.
OK, to get this show on the road, this is what you need to do:
Remove the MAP sensor from the air cleaner assembly and disconnect the MAP sensor from its vacuum hose.
Connect your vacuum pump to the MAP sensor's vacuum nipple. If you had to disconnect the MAP sensor from its electrical connector to remove it, reconnect it to it now.
With your multimeter in Volts DC mode probe the wire that connects to the MAP sensor connector terminal labeled with the number 2 (in the illustration above).
NOTE: 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. At this point your multimeter should register 4.7 Volts DC.
Now, pump the vacuum pump to apply vacuum to the MAP sensor. The multimeter should register the following voltages at the following vacuum values (they may differ a little on your specific 2.8L S10/S15):
1.) 0 in. Hg ...... 4.7 Volts.
2.) 5 in. Hg ...... 3.9 Volts.
3.) 10 in. Hg .... 3.0 Volts.
4.) 20 in. Hg .... 1.1 Volts.
Repeat this test step several times and each time, you should see the same values on your multimeter.
OK, let's take a look at what your vacuum pump test results mean:
CASE 1: Your multimeter registered the indicated voltages as you applied vacuum: This means that the manifold absolute pressure (MAP) sensor is good and not the cause of the MAP sensor code or problem on your 2.8L Chevy S10 (GMC S15). No further testing is required.
Now, if your vehicle still has the MAP sensor code lighting up the check engine on your instrument cluster.. take a look at the section: MAP Code Won't Go Away for more info.
CASE 2: Your multimeter register voltage, but it did not increase or decrease as you applied vacuum: This confirms that the manifold absolute pressure (MAP) sensor on your S10/S15 is BAD. Replacing the MAP sensor will solve the MAP code issue (33, 34)
CASE 3: Your multimeter registered 0 Volts: This usually means that the MAP sensor is fried. To be absolutely sure, I suggest confirming that the MAP sensor has power and ground. If both (power and ground) are present, the MAP is BAD. To test for power, go to: TEST 2: Verifying The MAP Is Getting Power.