The manifold absolute pressure (MAP) sensor, which is located on the throttle body, can be easily tested with a multimeter. Yup, no scan tool required to find out if it's bad or not.
In this tutorial, I'll explain how to perform the 3 tests you need to do in a step-by-step manner.
NOTE: This tutorial only covers the MAP sensor test on 1992-1996 Ram pickups/vans. For the MAP sensor test on 1997 and newer Ram pickups/vans, see the following tutorial:
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar El Sensor MAP (1992-1996 3.9L Dodge Ram Pickup/Van) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles (since they use the exact same manifold absolute pressure sensor):
- Ram B150 Van 3.9L: 1992, 1993, 1994.
- Ram B250 Van 3.9L: 1992, 1993, 1994.
- Ram B1500 Van 3.9L: 1995, 1996.
- Ram B2500 Van 3.9L: 1995, 1996.
- Ram D150 Pickup 3.9L: 1992, 1993.
- Ram D250 Pickup 3.9L: 1992, 1993.
- Ram W150 Pickup 3.9L: 1992, 1993.
- Ram 1500 Pickup 3.9L: 1994, 1995, 1996.
Symptoms Of A Bad MAP Sensor
The fuel injection computer uses the MAP sensor, the crankshaft position sensor (for engine RPM info), and the intake air temperature sensor to calculate the amount of air the engine is breathing in your Dodge Ram pickup (or van).
Once the computer knows how much air is entering the engine, it can now calculate the amount of fuel to inject into each cylinder.
Since the MAP sensor is a critical component of the engine management system, when it fails you'll see the check engine light lit up by a MAP sensor trouble code:
- Trouble Code 14: MAP Sensor Voltage Too Low (OBD I -1992 to 1994).
- Trouble Code 14: MAP Sensor Voltage Too High (OBD I -1992 to 1994).
- P0107: MAP Sensor Voltage Too Low (OBD II -1996).
- P0108: MAP Sensor Voltage Too High (OBD II -1996).
You're also gonna' see one or more of the following:
- Rough idle.
- ‘Rotten egg’ smell coming from the exhaust.
- Won't pass the state mandated emissions test.
- Bad gas mileage.
- Lack of power or hesitation when you accelerate the engine under load.
- Engine cranks a long time before starting.
MAP Sensor Circuit Descriptions
The MAP sensor needs power and Ground to create its signal. This is the reason it has 3 wires sticking out of its electrical connector.
In the table below, you'll find a short description of what each wire does:
| 1992-1996 MAP Sensor Circuits | ||
|---|---|---|
| Pin | Wire Color | Description |
| 1 | BLK/LT BLU | Sensor Ground |
| 2 | DK GRN/RED | MAP Signal |
| 3 | VIO/WHT | 5 Volts |
The key to successfully diagnosing the MAP sensor is to know that the voltage signal either decreases or increases depending on the amount of vacuum (produced within the intake manifold). This info is then sent to the fuel injection computer 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 MAP sensor test I'm gonna' show you in this tutorial will check to see if the MAP voltage signal actually does increase/decrease as you manually apply vacuum to it.
Where To Buy The MAP Sensor And Save
You can find the MAP sensor in just about any auto parts store, but I think you'll find it cheaper online (and I don't mean buying a cheap Chinese knock-off sensor). Check out the following links and compare.
TEST 1: Testing The MAP Voltage Signal With A Multimeter
What usually happens, when the MAP sensor fails, is that its voltage signal will stay stuck in one value as you apply/release vacuum to it.
We can easily check to see if the signal voltage is stuck in one value or if it's decreasing/increasing by connecting a multimeter to the dark green with red stripe (DK GRN/RED) wire of the connector.
Once the multimeter is connected to the DK GRN/RED wire, we then manually apply vacuum to the MAP sensor with a vacuum pump and see if the voltage signal reacts.
IMPORTANT: The MAP sensor must remain connected to its electrical connector to accomplish this test. 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, go here: Wire Piercing Probe Tool.
Here are the test steps:
- 1
Remove the MAP sensor from its place. Reconnect the MAP sensor to its electrical connector.
- 2
Place your multimeter in Volts DC mode.
- 3
Connect the red multimeter test lead to the DK GRN/RED wire of the MAP sensor harness connector.
- 4
Ground the black multimeter test lead directly on the battery negative (-) post.
- 5
Turn the key to the On position (do not start the engine).
- 6
At this point your multimeter should read a voltage between 4.5 to 5 Volts DC.
- 7
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. - 8
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. - 9
Release the vacuum. Once released, your multimeter should show the original voltage value.
Repeat this step several times making sure that each time the voltage decreases/increases as you apply/release the 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 you that the MAP sensor on your Dodge Ram IS NOT defective.
With this test result you can also conclude 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 is bad and needs to be replaced.
Before replacing the MAP sensor, you need to make sure that it's getting power and Ground. For the next test go to: TEST 2: Making Sure The MAP Sensor Is Getting Power.
