How Do I Test the Spark on a 1985 Ford F150 4.9L?

Worn and dirty spark plugs can cause multiple problems.

Testing for spark is an essential step when troubleshooting a Ford F150 that won't start or runs rough. The spark accounts for one of the three elements needed for combustion---fuel and air are the other two elements. Spark testers are sold at auto parts stores. Look for them near the spark plugs and spark plug wires. Spark testers have an adjustable gap that the spark must cross. Read the instructions that come with the tester carefully to avoid damage to the F150 and personal injury.

Things You'll Need

• Spark tester
• Spark plug wire puller
• Volt meter

Instructions

1. Test the battery with a volt meter and charge it if the voltage is below the acceptable range. The battery in a Ford F150 should read between 10 and 14 volts when the motor is not running.

2. Remove the spark plug wire from the plug you think is failing.

3. Attach the spark tester to the spark plug then attach the spark plug wire to the tester.

4. Adjust the gap in the spark tester to about a quarter of an inch. This should be a small enough distance that the spark will occur and large enough for you to see the spark.

5. Have a friend try to start the engine while you look at the gap in the spark tester. The spark will look like a miniature lightning bolt bridging the gap between the posts in the tester.

6. Repeat the test for each spark plug, there may be more then one plug failing.

References

• "Ford Pick-ups & Bronco Haynes Repair Manual"; M. Christman, J. Raffa and J. Haynes; 1996