TEST 1: Testing For Spark At The Spark Plug Wires

Checking The Spark Plug Wires For Spark. How To Test The GM Distributor Mounted Ignition Module

OK, if you're trying to troubleshoot a misfire and you don't know which one is the one missing, this is your starting point.

If you need to just test the ignition coil, your starting point is TEST 3: Testing The Ignition Coil's High Tension Wire.

If you need to just test the distributor's pick up coil, your starting point is TEST 7: Testing The Pick Up Coil Signal.

If you don't know where to start... start here. This whole thing is designed to test for a misfire or a No Start and this test (TEST 1) will get you started either way.

For those of you that are starting with this test, just a friendly reminder... to get the an accurate result, do not use a regular spark plug in place of a spark tester for the spark test.

Also, don't pull the spark plug wire off of the spark plug either while the engine is cranking (to check for spark) because this will fry the ignition coil.

Doing any of the two above will only cause you more problems and or complications.

OK, let's start:

  1. 1

    Remove the spark plug wire from its spark plug.

  2. 2

    Attach the HEI spark tester (or an equivalent spark tester) to the spark plug wire.

    Don't have an HEI spark tester? Need to buy one? You can buy it here: OTC 6589 Electronic Ignition Spark Tester).

  3. 3

    Attach the HEI spark tester to a good ground point, or use a battery jump start cable to attach it to ground (my preferred method).

  4. 4

    Have your helper crank the engine while you observe the spark tester.

    The spark tester will do one of two things: Spark or Not Spark.

  5. 5

    Repeat the test for all of the remaining spark plug wires.

Let's take a look at what your test results mean:

CASE 1: If you got spark on all of the spark plug wires: This result indicates that the spark plug wires, distributor cap and rotor, ignition module, and ignition coil are working. You have eliminated these as the cause of your NO START Condition.

If your vehicle is still suffering a misfire, go to TEST 8: Other Misfire Causes for some specific testing suggestions.

CASE 2: If you got NO spark from all of the spark plug wires: This result doesn't condemn any specific component just yet.

The next step is to remove the spark plug wire that attaches to the middle tower of the distributor cap (this is the spark plug wire that comes from the ignition coil) to test for spark there. This test will directly test the ignition coil itself and thus eliminate the distributor cap as a possible source of the No spark result. Go to TEST 3: Testing The Ignition Coil's High Tension Wire.

CASE 3: If you got spark on some but not all of the spark plug wires: This test result is more than likely due to BAD spark plug wires or a BAD distributor cap.

It's a common problem for one or two spark plug wires to go bad or for one or two distributor cap towers to go BAD and not let spark thru'. 90% of the time replacing the distributor cap, the distributor rotor and the spark plug wires should solve your problem, BUT to further test this, go to TEST 2.

TEST 2: Testing For Spark At The Distributor Cap

Testing For Spark At The Distributor Cap. How To Test The GM Distributor Mounted Ignition Module

Important: this test is only for when one or several (but not all) of the spark plug wires did not spark!

In this test, you'll be able to confirm that the spark plug wires that did not spark in TEST 1 are BAD or not.

How? By testing for spark directly on the distributor cap towers that feed those spark plug wires.

OK, this is what you need to do:

  1. 1

    Remove the spark plug wire that did not spark from its tower on the distributor cap and place the spark tester directly on the tower.

    I recommend you use a battery jump start cable to hold the spark tester to the tower as shown in the photo at left.

    Also, you must use a small piece of vacuum hose to attach the HEI spark tester to the cap tower being tested.

  2. 2

    Have your assistant crank the engine while you observe the spark tester.

    You're gonna' get one of two results: Spark or No Spark.

  3. 3

    Repeat this test with the others that did not fire off spark (if applicable).

Let's analyze each of these results below:

CASE 1: If you got spark. Then the spark plug wire is BAD, replace them all. This is probably as far as you may need to go since your GM car (or truck, or mini-van, or van) will probably start or solve your misfire problem after replacing these parts.

Here's why: As the spark plug wire ages, its normal resistance to spark increases to the point that the wire can not and does not transmit the spark to the spark plug. This will either cause a misfire, or a lack of power, or a No Start Condition. spark plug wires don't last forever, especially after-market ones (average life-span is 3 to 4 years).

CASE 2: If you got no spark. Then the distributor cap is BAD. Replace the distributor cap and distributor rotor as a Set. This should solve your misfire problem.

Here's why: As the distributor cap ages, the terminals that transmit the spark to the spark plug wires corrode. This corrosion increases the resistance to spark and over time (as more corrosion is created) this same corrosion stops the spark from passing thru' to the spark plug wires.