Testing The Ignition Module And Crank Sensor (GM 3.1L, 3.4L)

This article will help you test the ignition control module (ICM) and 3X crankshaft position sensor on all of the GM 3.1L and 3.4L overhead valve engines (the 3X crankshaft position sensor is also known as the 7X crankshaft position sensor).

So, if you're driving a Chevy, a Buick, a Pontiac, or an Oldsmobile with either one of these engines, not forgetting the Isuzu Rodeo or Trooper, this ‘How To Test The Ignition Module’ and 7X (3X) CKP sensor article applies.

You can find this tutorial in Spanish here: Cómo Probar El Módulo de Encendido Y Sensor Del Cigüeñal (GM 3.1L, 3.4L) (at: autotecnico-online.com).

There are two other articles that may help:

1. If you need to test the 3.1L, 3.4L Coil Packs:
   • How To Test The Ignition Coil Packs (GM 3.1L, 3.4L)
2. If you need to test the 3.8 ignition control module:
   • GM 3.8L Ignition Control Module and Crank (3X, 18X) Sensor Test

To see the complete list of 3.1L, 3.4L test articles, go to the GM 3.1L, 3.4L Index Of Articles.

This is a very easy and straightforward test that can be accomplished in 3 steps. That's right, in three steps. Before I jump into the specific tests, I recommend you read this part of the article. Below you'll find some ‘working theory’ (don't worry, just the nuts and bolts of how everything works), do's and don'ts, and some other good stuff you may want or need to know.

Basic Theory: How The Ignition Control Module Works

In a nutshell, the ignition control module's job is to create a Switching signal for the ignition coils that are sitting on top of it. It's this Switching signal that makes the ignition coils create spark.

Now, before I go any further and if you're wondering what the heck is a ‘Switching signal’, this name simply refers to the fact that the ignition control module ‘switches’ the primary current flowing thru' the ignition coil ON and OFF (Primary Current is the fancy name for the 12 Volts that flow thru' the ignition coil).

The ignition module accomplishes this ‘switching’ ON and OFF, of the Primary Current, by simply interrupting (turning off) the Ground path.

OK, moving on along, for the ignition module to create this Switching signal, it needs to receive a Crankshaft Position (CKP) sensor signal (this signal is usually referred to as the Triggering signal and in GM service literature it's known as the 3X or 7X CKP signal). And well, not only does it need the CKP sensor signal, it needs 12 Volts and Ground.

More specifically, it all starts when you turn the ignition switch (in other words: the key) to crank and start the car or truck (if you're driving an Isuzu), and here's what happens:

1. The ignition module receives 12 Volts.
2. The engine starts to crank and the crankshaft position sensor starts to generate its 7X CKP signal.
3. The ignition module starts to turn ON and OFF the ignition coils in their proper firing order to create spark, after the crank signal arrives. This ON/OFF action is the Switching signal referred to above.
4. Under 400 RPM, the ignition control module controls spark timing without the help of the fuel injection computer.
5. Once the engine starts and the RPMs are over 400 RPM, the fuel injection computer takes over and starts controlling Ignition Timing (albeit thru' the ignition module).

3X (7X) CKP Sensor And 24X CKP Sensor

Early 3.1L V6 engines use only one crankshaft position sensor known as the 7X (3X) crankshaft position sensor, which is connected directly to the ignition control module. Here are some more specifics:

• It's a magnetic reluctor type sensor (two wire type) and produces an AC signal that can be measured with a multimeter (in AC Volts mode).
• On an oscilloscope, it produces an analog waveform.
• The CKP sensor is located on the engine block (the side that faces the transmission) and goes thru' the engine block itself.

Late 3.1L and 3.4L V6 engines use 2 crankshaft position sensors.

• One is located behind the crankshaft pulley on the front of the engine and the other is the same one described above.
• In a two crankshaft position sensor engine, the one that's located behind the crankshaft pulley is known as the 24X CKP sensor and the other is called the same (3X or 7X CKP sensor).

Since the 7X (3X) CKP sensor is the one directly responsible for starting the car, this tutorial will show you how to test only this one. You don't have to worry about the 24X CKP sensor (if you need to test the 24X CKP sensor, you can find the tutorial here: How To Test The 24X Crank Sensor (GM 3.1L, 3.4L) at: troubleshootmyvehicle.com).

In the 3 tests that I'm gonna' teach you, we'll test for all of these signals (except of course the 24X CKP signal). If the cause of your vehicle's no-start condition lays in the ignition system, you'll be able to correctly diagnose and pinpoint the source of the problem.

Symptoms Of A Bad Ignition Control Module Or CKP Sensor

It goes without saying (but I'm gonna' say it anyway) that when the ignition module (ICM) goes bad, your car, or minivan, or Isuzu SUV will not start, but not always.

Every now and then, this type of ignition module fails intermittently. What this means is that the vehicle will start and run most of the time, but now and then it won't.

If you're experiencing this, you'll have to test the ignition module when the vehicle is not wanting to start. This is important, because as long as it starts, the problem does not exist and can not be tested.

So, then, the info that you're gonna' learn in this article applies to testing a module that has fried completely and is keeping the vehicle from starting. Here are some of the most common failure symptoms:

• The car (or minivan or Isuzu SUV) cranks but does not start.
• There is no spark coming out of any of the ignition coils sitting on top of the ignition module.
• You may have taken your ignition control module (ICM) to AutoZone to test it and the friendly boys in orange say it's good, yet the vehicle will not start (all major auto parts stores such as O'Reilly, Pepboys, etc., test ignition modules for free).

What Tools Do I Need?

The most important tool to have is a multimeter. It can be a digital or analog type. You do not need an automotive scan tool. Here are the other recommended tools that will come in handy.

1. 12 Volt test light.
   • You can use the test light instead of a multimeter to test for 12 Volts, only.
2. Wire piercing probes.
   • These tools are time savers and not only that, will help you to keep from peeling too much from the wire to test whatever signal runs thru' it.
   • If you've never seen one of these tools, click here: Wire piercing tool example.
3. A repair manual.
   • This manual will come in handy to fill some of the gaps that this article does not cover.
4. A helper.
   • I recommend someone who isn't gonna' complain and nag about it. I mean, it's stressful enough that the vehicle won't start and you have to fix it!

Do's And Don'ts And Safety Precautions

Anything to do with working on or around a car can be dangerous. Since some of the tests in this article are done with the engine cranking, it goes without saying that you have to be alert, extra careful and use common sense to perform them.

Whomever is gonna' help you crank the vehicle, while you observe the multimeter, has to wait outside of the car or minivan or truck (Isuzu owners) until after you've connected your connections and you're clear of the engine.

After the test is done and you don't need your helper to crank the engine anymore, have him or her step out of the vehicle. All this is to save you from any unfortunate accident that may happen if said helper thinks he or she heard you say "crank it" (or whatever command phrase you use).