How To Test The Camshaft Position Sensor (1998-2003 3.9L V6 Dodge Ram Van)

If you're seeing a P0340 or P1391 diagnostic trouble code (DTC), and the engine isn't starting or is running rough, it's likely the CMP sensor is fried.

Fortunately, the CMP sensor is one of the easier sensors to test on your 3.9L V6 Dodge Ram van —and I'll show you how in this tutorial.

You also don't need any expensive diagnostic equipment —the CMP sensor tests are all done using a standard multimeter. You'll quickly find out if your sensor is working properly or is toast.

I'm also gonna point you to where you can buy a replacement CMP sensor that won't strain your budget —and will get your van back on the road.

NOTE: The camshaft position sensor is also known as the distributor pickup coil.

APPLIES TO: This tutorial applies to the following vehicles since they use the exact same camshaft position (CMP) sensor:

• 3.9L V6 Dodge Ram B1500 Van: 1998, 1999, 2000, 2001, 2002, 2003.
• 3.9L V6 Dodge Ram B2500 Van: 1998, 1999, 2000, 2001, 2002, 2003.

IGNITION SYSTEM WIRING DIAGRAMS:

• Ignition System Wiring Diagram (1998-1999 3.9L V6 Dodge Ram Van).

OTHER CMP SENSOR TESTS:

• How To Test The Distributor Pickup Coil (1989-1991 3.9L V6 Dodge Ram Van).
• How To Test The Camshaft Position Sensor (1992-1995 3.9L V6 Dodge Ram Van).
• How To Test The Camshaft Position Sensor (1996-1997 5.2L, 5.9L V8 Dodge Ram Van).

Symptoms Of A Bad CMP Sensor

If the camshaft position (CMP) sensor quits working, the PCM (Powertrain Control Module) will usually throw one of these diagnostic trouble codes (DTCs):

• P0340: No Camshaft Signal At PCM.
• P1391: Intermittent Loss of CMP or CKP Signal.

But it's not just about the codes. You'll notice other red flags too:

• Cranks but won't start: The engine turns like it wants to fire up, but it never catches.
• Random stalls: Everything runs fine... until it suddenly doesn't. It dies out of nowhere, usually when idling or just as you're stepping on the gas pedal.
• Check Engine Light (CEL): When the PCM logs a P0340 or P1391, that CEL on the dash is gonna light up.
• Rough idle or misfires: A weak cam signal messes with fuel and spark timing. The engine runs rough, stumbles, or misfires —especially at idle RPM.
• Hard starts (hot or cold): If the signal glitches but doesn't disappear completely, the engine will only start after cranking it few times.
• Hesitation or sluggish takeoff: Step on the gas and it feels like someone cut off the power.

What the CMP Sensor Does

Here's what the CMP is up to under the hood:

• Sends out a fuel sync signal:
  • Anytime the engine's cranking or running, the CMP sensor sends a signal —called the fuel sync signal— to the PCM.
  • This tells the computer which cylinder's on the compression stroke so it knows when to fire the injector.
• Works alongside the CKP sensor:
  • The CKP sensor reads engine speed and crankshaft position by watching the flywheel.
  • But it doesn't know which cylinder is which —the CMP sensor handles that part so fuel injection and spark stay in sync.
• Keeps injector timing sharp:
  • The PCM depends on the CMP sensor to know exactly when to open each injector.
  • If that signal drops out, fuel delivery goes out of whack —or just stops.
• Helps nail spark timing (indirectly):
  • While spark timing's handled by the CKP sensor, the CMP sensor backs it up by telling the PCM which cylinder is up next.

Circuit Descriptions Of The Camshaft Position Sensor

Here's a quick breakdown of what each wire does in the table below:

Pin	Wire Color	Description
1	Tan with yellow stripe (TAN/YEL)	CMP Signal
2	Black with light blue stripe (BLK/LT BLU)	Sensor Ground
3	Violet with white stripe (VIO/WHT)	5 Volts DC

Where To Buy The Camshaft Position Sensor

The following links will help you comparison shop for the camshaft position sensor (distributor pickup coil). I think you'll save a few bucks:

TEST 1: Testing The Camshaft Position Signal

As the engine cranks or runs, the CMP sensor sends out a voltage signal that flips between ON and OFF. When it's ON, you'll see 5 Volts DC. When it's OFF, it drops down to zero.

Here's the cool part —you can actually see that flip with a basic multimeter. And that's exactly how we're gonna test it in this section.

If the sensor's shot, it usually stops switching. Instead of flipping between 0 and 5 Volts, it just sits at one voltage the whole time the engine's cranking.

The wire you need to check for this voltage flip is the tan with yellow stripe (TAN/YEL) wire.

IMPORTANT: Don't unplug the sensor. Leave it connected to its engine wiring harness connector while you test the signal. You'll need to either back probe the connector or pierce the wire insulation to get a reading. You can see an example of a wire piercing probe (and where to buy it) here: Wire Piercing Probe.

NOTE: Don't have a multimeter or need to upgrade yours? This is the one I use: Tekpower TP8268 AC/DC Auto/Manual Range Digital Multimeter (at: amazon.com).

Here's what you're gonna do:

1. 1

   Hook the black lead from your multimeter to the battery's negative (-) terminal.

2. 2

   Unplug the ignition coil connector. This keeps the engine from starting while you test —don't skip this step.
   
   NOTE: Leave the distributor cap on. It holds the CMP sensor in place, so it has to stay where it is.

3. 3

   Set your multimeter to DC Volts.

4. 4

   Tap the red lead into the tan/yellow (TAN/YEL) wire coming out of the CMP connector.
   
   NOTE: Don't unplug the sensor. It needs to stay connected to the harness for the signal to show up right.

5. 5

   Once the leads are in place, have someone crank the engine for a few seconds while you watch the meter.

6. 6

   If the sensor's working, you'll see the voltage jump between 5 Volts and 0 Volts DC while the engine's turning.

Here's how to interpret your test results:

CASE 1: Voltage flips between 5 and 0 Volts. That's what we're after. The CMP sensor's doing its job, and it's not the reason your engine won't start.

CASE 2: Voltage stays stuck —no flipping between 5 and 0 Volts. That usually means the CMP sensor isn't working.

But don't swap it out just yet. First, check if it's actually getting power and Ground. Head over to: TEST 2: Making Sure The CMP Sensor Has 5 Volts.