TEST 2: Testing For Spark At The Distributor Cap
Distributor caps can and do go bad. Usually, what will happen is that one or more distributor cap towers will stop transmitting spark to their spark plug wires.
So if one or more of the spark plug wires did not spark in TEST 1, then there's a good chance that:
- The distributor cap is bad.
- The spark plug wire or spark plug wires that did not spark are bad.
The cool thing is that find out what is causing the no-spark test result is easy and it simply involve checking for spark directly on the distributor cap towers of the spark plug wires that did not spark.
What we'll do is remove the spark plug wire from its distributor cap tower and then connect the spark tester to the tower (as shown in the photo above).
Once the spark tester is set-up, we'll crank the engine to see if the spark tester sparks.
If the spark tester sparks, then you can conclude that the spark plug wire (that connects to that tower and that did not spark in TEST 1) is bad.
If the spark tester does not spark, then you can conclude that the distributor cap is bad and the cause of the no-spark test result from the spark plug wire in TEST 1.
Let's get started:
- 1
Remove the spark plug wire (that did not spark) from its tower on the distributor cap.
- 2
Place the spark tester directly on the tower (see photo above).
- 3
Connect the spark tester to the battery negative (-) terminal with a battery jump start cable.
- 4
Ask your helper to crank the engine while you observe the HEI spark tester.
- 5
The spark tester should spark.
- 6
Repeat this test on the other distributor cap towers that did not spark (if applicable).
Let's analyze your test results:
CASE 1: The spark tester sparked. This is the correct test result and lets you know that the spark plug wire that connects to that distributor cap tower is bad. Replace all of the spark plug wires with a new set.
The misfire problem you're troubleshooting should be resolved with a new set of spark plug wires. Now, if the distributor cap and rotor are as old as the spark plug wires, then I recommend that you replace them too.
Here's why: Spark plug wires don't last forever. Eventually they'll stop transmitting spark to their spark plug as their internal resistance increases (to the spark they carry). This will either cause a misfire, or a lack of power, or a no-start condition.
CASE 2: The spark tester DID NOT spark. This test result confirms that the distributor cap is bad.
The misfire problem you're troubleshooting should be resolved with a new distributor cap and rotor. Now, if spark plug wires and spark plugs are as old as the distributor cap and rotor, then I recommend that you replace them too.
Here's why: Eventually the distributor cap towers will stop transmitting spark to their spark plug wires. This happens when the terminals (that are part of the towers) corrode inside the distributor cap. This corrosion increases the resistance to spark and over time (as more corrosion is created) this same corrosion will stop the transmission of spark to the spark plug wires.
TEST 3: Testing The Ignition Coil's High Tension Wire For Spark
The high tension wire that connects the ignition coil to the distributor cap's center tower will eventually fail.
When it does fail, your 5.2L Jeep Grand Cherokee will not start due to a lack of spark (even though the ignition coil is OK).
So if you're reading this test section, you got a no-spark test result from all of the 8 spark plug wires in TEST 1.
Our next diagnostic test is to see if the ignition coil's high tension wire is firing spark (when cranking the engine).
In the photo above I have disconnected the ignition coil's high tension wire from the distributor cap and I've connected the spark tester to it.
If the spark tester sparks, then you can conclude that the distributor cap and rotor are bad and behind the no-spark/no-start problem.
If the spark tester does not spark, then your next step is to go to TEST 4.
Let's get started:
- 1
Disconnect the high tension wire that connects to the center of the distributor cap.
NOTE: The other end of the high tension wire must remain connected to the ignition coil. - 2
Connect the spark tester to the ignition coil high tension wire (as shown in the photo above).
NOTE: You'll connect the spark tester to the end of the wire that connects to the center tower of the distributor cap. The other end of the wire remains connected to the ignition coil. - 3
Ground the spark tester to the battery negative (-) terminal with a battery jump start cable.
- 4
Have your helper crank the engine as you hold the spark tester.
- 5
The spark tester should spark.
Let's analyze your test results:
CASE 1: The spark tester sparked. This is the correct test result.
This test result confirms that the distributor cap and rotor are bad only if you have:
- Confirmed that all of the spark plug wires are not sparking (TEST 1).
- Confirmed that the ignition coil's high tension wire does spark (TEST 3).
To solve your Jeep Grand Cherokee's no-start problem, replace all of the spark plug wires with a new set.
If the spark plugs, the distributor cap, and the distributor rotor are as old as the spark plug wires, I recommend you replace them too.
CASE 2: The spark tester DID NOT spark. This tells you that the distributor is not getting spark.
Then the next step is to check for spark directly on the ignition coil's tower. For this test go to: TEST 4: Testing The Ignition Coil For Spark.