How To Test The 7X Crankshaft Position Sensor (1994-2005 3.1L V6 Buick Century)

The 7X crankshaft position (CKP) sensor is one of the most important sensors on your 3.1L V6 Buick Century. Without its signal, the ignition control module (ICM) doesn't know when to fire the ignition coils, and the engine usually won't start.

The good news is that testing the 7X CKP sensor isn't difficult. In this tutorial, I'm going to show you how to test it using only a multimeter. No scan tool required.

NOTE: On the 1994-1995 Buick Century, the factory service manual and wiring diagrams refer to this sensor as the 3X CKP sensor. Beginning with the 1996 model year, GM refers to the same sensor as the 7X CKP sensor. To keep things simple, I'll refer to it as the 7X CKP sensor throughout this tutorial. Regardless of which name GM uses for your particular model year, it's the same 2-wire crankshaft position sensor covered in this article.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.1L V6 Buick Century: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.

RELATED TUTORIALS: The following tutorial will help you test the ignition coil packs and the ignition control module:

• How To Test The Ignition Coil Packs (GM 3.1L, 3.4L) (at: easyautodiagnostics.com).
• Testing The Ignition Module and Crank Sensor (GM 3.1L, 3.4L) (at: easyautodiagnostics.com).

WIRING DIAGRAMS: You can find the ignition system wiring diagrams here:

• ICM And 3X CKP Sensor Circuit Wiring Diagram (1994-1995 3.1L V6 Buick Century).
• ICM And 7X CKP Sensor Circuit Wiring Diagram (1996 3.1L V6 Buick Century).
• ICM And 7X CKP Sensor Circuit Wiring Diagram (1997 3.1L V6 Buick Century).
• ICM And 7X CKP Sensor Circuit Wiring Diagram (1998-2005 3.1L V6 Buick Century).

Symptoms Of A Bad 7X Crankshaft Position (CKP) Sensor

The 7X crankshaft position sensor works together with the ignition control module (ICM) to determine crankshaft position and engine speed. The ICM uses this information to fire the correct ignition coil and to generate the engine speed signal that's sent to the PCM.

When the 7X CKP sensor stops producing its signal, the ignition control module can no longer determine crankshaft position correctly. As a result, the ignition system stops producing spark, and the engine won't start.

Unlike the 24X CKP sensor, the 7X CKP sensor doesn't usually set a dedicated diagnostic trouble code when it fails. In most cases, the engine simply cranks but doesn't start.

You may notice one or more of the following symptoms:

• Engine cranks but won't start: This is the most common symptom of a failed 7X CKP sensor.
• No spark: None of the ignition coils will produce spark while the engine is cranking.
• Intermittent stalling: In some cases, the engine may stall unexpectedly if the sensor signal is lost.

Circuit Description Of The 7X Crankshaft Position Sensor

The 7X crankshaft position (CKP) sensor is a two-wire magnetic pickup sensor, also known as a variable reluctance sensor. Unlike the 24X Hall-effect CKP sensor, it doesn't require an external power supply or Ground to generate its signal.

Instead, the sensor produces its own alternating current (AC) voltage signal as the crankshaft rotates. As the reluctor wheel passes by the tip of the sensor, the changing magnetic field induces an AC voltage that increases in frequency and amplitude as engine speed increases.

The 3X/7X CKP sensor has a two-wire connector with male spade terminals. The engine wiring harness connector has the corresponding female terminals.

Both wires connect directly to the ignition control module (ICM), which uses the sensor's AC signal to determine crankshaft position and engine speed.

The following table identifies the two wires of the 3X/7X crankshaft position sensor connector.

1994-2005 7X CKP Sensor Connector
Pin	Wire Color	Description
A	Yellow (YEL)	CKP Signal High
C	Purple (PPL)	CKP Signal Low

Where To Buy The 7X Crankshaft Position Sensor

Checkout the following links and comparison shop the 24X crankshaft position sensor for your 1995-1997 V6 Oldsmobile Cutlass Supreme of the 2 best known automotive parts manufacturers (Standard Motor Products and AC-Delco):

TEST 1: Testing For The Crankshaft Position Sensor Signal

For our first test, we're going to check the AC voltage signal generated by the 7X CKP sensor while cranking the engine. We'll do this with a multimeter set to AC Volts mode.

We'll check the signal right at the ignition control module, at the 2-wire connector that plugs into connector 2 (see image above). This connector has a yellow (YEL) wire and a purple (PPL) wire coming out of it.

Before we get started, there's one important thing to keep in mind: the battery needs to be fully charged so the starter motor can crank the engine at its normal speed.

NOTE: Perform this test only if the engine doesn't start. If the engine starts and runs, the 7X CKP sensor is working properly.

CAUTION: You'll be working around a cranking engine, be careful and take all necessary safety precautions.

This is what you need to do:

1. 1

   Set your multimeter to AC Volts mode.

2. 2

   Disconnect Connector 2 from the ignition control module.

3. 3

   Connect the red multimeter test lead to the wire that connects to the terminal labeled with the letter A.

4. 4

   Connect the black multimeter test lead to the wire that connects to the terminal labeled with the letter C.

5. 5

   Have an assistant crank the engine while you observe the multimeter (or oscilloscope).

6. 6

   The multimeter's AC reading should fluctuate from 0.3 V AC to 1.0 V AC continuously as the engine is cranking.
   
   The maximum AC voltage attained will depend on the battery charge condition.
   
   If you're using an oscilloscope, the waveform should look like the one below.

Let's find out what your test results mean:

CASE 1: Your multimeter registered an AC voltage signal. This is the correct test result and it tells you that the 7X CKP sensor is generating a crankshaft position signal.

If you're dealing with an engine no-start due to a lack of spark, then this usually means the ignition control module (ICM) is has failed.

Before condemning the ignition control module, there's one more test you should perform: checking the resistance of the 7X CKP sensor. Go to: TEST 2: 7X CKP Sensor Resistance Test.

CASE 2: Your multimeter did not register an AC voltage signal. Double-check your multimeter connections and repeat the test.

If the multimeter still doesn't register an AC voltage signal while the engine is cranking, don't assume the 7X CKP sensor is bad just yet. One of the most common causes of a missing signal is damage to the sensor's wiring.

IMPORTANT: Before replacing the sensor, inspect its wiring harness carefully. The wires can short to Ground if the insulation has burned through or rubbed against the engine or exhaust. Here's what usually happens:

1. The 7X CKP sensor connects to the ignition control module through a pair of twisted wires about 2 feet (60 cm) long. This wiring harness routes underneath the rear exhaust manifold.
2. Over time, the clips that secure the harness can break or come loose.
3. Once the harness is no longer supported, it can rub against the engine block or touch the hot exhaust manifold, damaging the wire insulation.
4. When one or both wires short to Ground, the AC signal generated by the 7X CKP sensor never reaches the ignition control module.

So, if you didn't get an AC voltage reading in this test, inspect the wiring harness first. If the wiring is damaged, repairing it may solve the problem without replacing the sensor.

If you find that the wiring harness is damaged, you have several repair options:

• Repair the damaged section of the existing harness.
• Replace the harness with a good used one from a salvage yard.

TEST 2: CKP Sensor Resistance Test

As I mentioned in TEST 1 (CASE 1), getting an AC voltage signal from the 7X CKP sensor usually tells you the sensor is working properly and that the ignition control module (ICM) is the likely culprit. In the vast majority of cases, that's exactly what you'll find.

However, over the years, several readers have reported cases where the 7X CKP sensor produced an AC voltage signal during the cranking test but still turned out to be defective.

For that reason, I recommend performing one more test before replacing any ignition system component. This final test checks the internal resistance of the 7X CKP sensor.

Since the 7X CKP sensor is mounted in a hard-to-reach location on the engine block, you'll measure its resistance at the ignition control module connector where the sensor plugs in. This lets you test the sensor without having to remove it or disconnect it at the engine.

NOTE: This test is done on the female terminals of the ICM's connector 2.

Here's what you need to do:

1. 1

   Put your multimeter in Ohms (Ω) mode.

2. 2

   Disconnect Connector 2 from the ignition control module.

3. 3

   Connect the red multimeter test lead to female terminal A.

4. 4

   Connect the black multimeter test lead to female terminal C.

5. 5

   The multimeter's Ohms reading should read 500 to 900 Ohms.

Let's analyze your test results:

CASE 1: Your multimeter showed the indicated Ohms reading. This is the correct test result and it confirms that the CKP sensor is OK.

You can now conclude that the ignition control module is defective (and needs replacement) if you have:

• Confirmed that all six spark plug wires ARE NOT sparking.
• Confirmed it's receiving power (TEST 1).
• Confirmed it's receiving Ground (TEST 2).
• Confirmed the CKP sensor is creating a signal (TEST 3).
• Confirmed, in this test section, the CKP sensor is within the specified resistance.

CASE 2: Your multimeter DID NOT SHOW the indicated Ohms reading. Recheck all of multimeter's connections. Try again.

If the multimeter still does not register the indicated Ohms reading, then you can conclude that the crankshaft position sensor is bad and needs replacement if you have:

• Confirmed that all six spark plug wires ARE NOT sparking.
• Confirmed the ICM is receiving power (TEST 1).
• Confirmed the ICM is receiving Ground (TEST 2).
• Confirmed the CKP sensor IS NOT creating a signal (TEST 3).
• Confirmed, in this test section, the CKP sensor IS NOT within the specified resistance.

More 3.1L V6 Buick Century Diagnostic Tutorials

You can find a complete list of diagnostics tutorials for your 3.1L V6 Buick Century in this index:

• 3.1L V6 Buick Century Index of Articles.

Here's a small sample of the tutorials you'll find:

• How To Test For A Blown Head Gasket (1994-2005 3.1L V6 Buick Century).
• Common Causes Of Spark Plug Failure (1994-2005 3.1L V6 Buick Century).
• How To Test Engine Compression (1994-2005 3.1L V6 Buick Century).
• How To Test The TPS With A Multimeter (1997-2005 3.1L V6 Buick Century).