How To Test The Ignition Coil And Power Transistor (1990-1995 3.0L Pathfindier And Pick Up)

TEST 8: Testing The Power Transistor's Activation Signal

To test the power transistor's activation signal we're gonna' connect the LED light to the white with black stripe (WHT/BLK) wire of the power transistor's 3-wire connector.

As the engine is cranked, the LED light should flash ON and OFF if the power transistor's activation signal is present in the WHT/BLK wire.

NOTE: The power transistor must be connected to its 3-wire connector for this test to work. To access the signal inside the wire you'll need to use a back probe or a wire piercing probe. You can see an example of this tool here: Wire Piercing Probe Review (Power Probe PWPPPPP01).

OK, let's get started:

1. 1

   Connect the red lead of the LED light to the WHT/BLK wire labeled with the number 1 in the photo above.
   
   The power transistor must remain connected to its 3-wire connector. So you'll need to first connect a back probe or a wire piercing probe to the wire and then connect the red lead of LED light to this tool.

2. 2

   Connect the black lead of the LED light to the battery negative (-) terminal.

3. 3

   Have an assistant crank the engine.

4. 4

   The LED light should blink ON and OFF as the engine cranks.
   
   NOTE: Don't worry about what the LED light does before or after your helper starts cranking the engine. The only results you're interested in interpreting are the results obtained with the engine cranking.

Let's interpret your test result:

CASE 1: The LED light flashed ON and OFF. This is the correct test result.

You can conclude that the power transistor is fried and needs to be replaced only if you have:

• Confirmed that the ignition coil does not spark (TEST 4).
• Confirmed that the ignition coil is getting 10 to 12 Volts (TEST 5).
• Confirmed that the ignition coil activation signal is not present (TEST 6).
• Confirmed that the power transistor is getting Ground (TEST 7).
• Confirmed that the power transistor is getting its activation signal (TEST 8).

CASE 2: The LED light DID NOT flash ON and OFF. Without this activation signal the power transistor will not activate the ignition coil.

Recheck all of your connections. If still the LED light does not flash ON and OFF, then this indicates that the camshaft position sensor (located inside the distributor) is defective.

The camshaft position sensor can be easily tested and you can find the tests explained here: How To Test The Camshaft Position Sensor (1990-1995 Nissan Pick Up And Pathfinder ).

Other Causes Of A Misfire

So you've done all of the spark tests and found no problems but your 3.0L V6 Nissan Pathfinder (D21 or Pick Up) still runs rough. Well, here are a couple of suggestions that might help:

1. The valve cover gaskets are leaking engine oil onto the spark plug wells and soaking the spark plugs and spark plug wire boots in oil.
   • Over time, this oil will cause carbon tracks to form on the spark plug's ceramic insulator and on the inside of the spark plug boot. The end result of this will be a misfire.
   • The photo above shows what a carbon track looks like on the inside of the spark plug wire boot and on the ceramic insulator of the spark plug.
2. Engine compression test.
   • One of the most overlooked diagnostic tests to find the root cause of misfire is the compression test.
   • You can find the engine compression test explained here: How To Test Engine Compression (3.0L Nissan).
3. Broken spark plugs.
   • This usually happens at tune-up time (if you have dropped one on the floor).
   • You power washed the engine (this is something that should never be done on any Nissan vehicle).

More 3.0L Nissan Tutorials

You can find a complete list of tutorials in this index: Nissan 3.0L Index Of Articles.

Here's a small sample of the 3.0L Nissan tutorials you'll find in the index:

• How To Test Engine Compression (3.0L Nissan).
• Mass Air Flow (MAF) Sensor Test 3.0L Nissan Quest (1993, 1994, 1995).
• Mass Air Flow (MAF) Sensor Test 3.0L Nissan Maxima (1995-1999).
• How To Test The Starter Motor (Nissan 3.0L, 3.3L) (at: troubleshootmyvehicle.com).
• How To Test Engine Compression (Nissan 3.0L, 3.3L, 3.5L) (at: troubleshootmyvehicle.com).