The manifold absolute pressure (MAP) sensor can be very accurately tested with a multimeter to see if it's bad or not without removing it (no scan tool required).
In this tutorial, I'll show you just how in a step-by-step manner. You'll also be able to diagnose the following manifold absolute pressure (MAP) sensor diagnostic trouble codes:
- Trouble Code 14: MAP Sensor Voltage Too Low.
- Trouble Code 14: MAP Sensor Voltage Too High.
- P0107: MAP Sensor Voltage Too Low (OBD II).
- P0108: MAP Sensor Voltage Too High (OBD II).
NOTE: This tutorial only covers the MAP sensor test on 1994 thru' 1996 Ram pickups/vans. For the MAP sensor test on 1997 and newer Ram pickups/vans, see the following tutorial:
Contents of this tutorial:
Basics Of The Manifold Absolute Pressure (MAP) Sensor
The fuel injection computer (known as the PCM - Powertrain Control Module in tech speak) uses the MAP sensor and crank sensor (think engine RPMs) to calculate how much air the engine is breathing in your Dodge Ram pickup (or van).
Once the PCM knows how much air is entering the engine, it can now calculate the amount of fuel to inject into each cylinder to keep it running optimally and to keep it within certain pollution standards.
How is this accomplished? In a nutshell, by measuring the amount of intake manifold vacuum and translating this measurement into a voltage signal.
This voltage signal either decreases or decreases depending on the amount of vacuum (produced within the intake manifold) and is sent to the PCM on the DK GRN/RED wire of the 3-wire MAP sensor connector.
Briefly, the two things you need to keep in mind are:
- Under load when the amount of intake manifold vacuum is high (like when you're accelerating the engine to move your pickup/van from a stand-still, stop, red light, etc.)... the MAP sensor produces a decreasing MAP voltage signal.
- When the engine is under no load and the amount of engine manifold vacuum is low (like when you're waiting for the red light to turn green and your pickup/van's engine is idling)... the MAP sensor produces an increasing MAP voltage signal.
The key to troubleshooting the MAP sensor is to see (with a multimeter) whether this MAP voltage signal actually does increase/decrease as you manually apply vacuum to the MAP sensor's vacuum inlet port (and that's exactly what we'll do in this tutorial).
In the table below, you'll find a short description of what each wire does:
|1994, 1995, 1996 Dodge Ram MAP Sensor Circuits|
|1||BLK/LT BLU||Sensor Ground|
|2||DK GRN/RED||MAP Signal|
TEST 1: Checking The MAP Sensor Signal
What usually happens, when the MAP sensor fails, is that it just simply stops creating a MAP sensor signal.
To be bit more specific... it stops measuring the amount of intake manifold vacuum as the engine runs and produces a ‘fixed’ voltage signal or none at all (remember, this voltage signal has to vary depending on the amount of vacuum within the intake manifold).
The cool thing is that you and I can tap into the DK GRN/RED wire with a multimeter and manually apply vacuum to the MAP sensor with a vacuum pump and see if the voltage signal decreases and then increases (as we release the applied vacuum).
This is a pretty easy test and I'll explain it all in a step-by-step manner in the test steps below.
IMPORTANT: The MAP sensor must remain connected to its harness connector to accomplish this test.
Here are the test steps:
Remove the MAP sensor from its place. Reconnect the MAP sensor to its engine wiring harness connector but leave the vacuum hose off.
NOTE: The MAP sensor connector needs to be connected to the sensor, so you'll need to either back-probe the connector or use a wire piercing probe to get to the signal inside the wire (to see what a wire piercing probe looks like: Wire Piercing Probe Tool).
Place your multimeter in Volts DC mode and connect the red test lead to the DK GRN/RED wire of the MAP sensor harness connector (see image 1 of 2).
Ground the black multimeter test lead directly on the battery negative (-) post and turn the key to the On position (do not start the engine).
At this point your multimeter should read a voltage between 4.5 to 5 volts DC.
Manually apply vacuum with a vacuum pump to the MAP sensor's vacuum inlet port using a suitable vacuum hose.
NOTE: If you don't have a vacuum pump, no problem. You can use your mouth to apply vacuum to the vacuum inlet port.
The multimeter should show an decreasing voltage as you apply vacuum with the vacuum pump.
With about 20 in. Hg of vacuum applied, your multimeter should read about 1.1 to 1.7 volts DC.
NOTE: If you're using your mouth to apply vacuum, and the MAP sensor is good, you won't be able to bring down the voltage down to 1 volt (on lung power). The important thing is to see the voltage decrease when applying vacuum and seeing the voltage return to its original voltage when you release said vacuum.
Let's take a look at what your test results mean:
CASE 1: The voltage decreased and increased as you applied and released vacuum. This tells that the MAP sensor on your Dodge Ram IS NOT defective.
Since the MAP sensor is creating an increasing/decreasing voltage signal on the DK GRN/RED wire, this test result also says that:
- The MAP sensor is getting power on the VIO/WHT wire of its harness connector.
- The MAP sensor is getting ground on the BLK/LT BLU wire of its harness connector.
CASE 2: The voltage DID NOT increase or decrease. This test result usually means that the MAP sensor on your Dodge Ram is bad and needs to be replaced but to make absolutely sure...
I suggest that you verify that it's getting power and ground. Your next test is to make sure it's getting power. For this test go to TEST 2: Makign Sure The MAP Sensor Is Getting Power.