A spark tester is a device used to test if the ignition system in a vehicle is working. Specifically, it is used to test the spark plug wires to see if they are sending the appropriate charge to the spark plugs themselves in order to start the engine. This is a simple method of diagnosing the source of any problems within the ignition system.
A spark tester is often similar in appearance to a spark plug. The main difference is that a spark tester is equipped with a metal clip that is either at the end of the tester or on a wire that extends out of the tester. The tester also usually has a longer, tubelike shape and is lighter in color.
A spark tester connects to a spark plug wire much like a regular spark plug. In this case, however, the tester is installed into the wire, while the wire is usually connected to the spark plug after installing the plug in the engine. To connect a spark tester, remove the wire from the engine (grasping it by the boot) and then insert the tester into the boot.
Once the spark tester is connected to the spark plug wire, it must be grounded. The tester's metal clip connects to a metal ground in order to help generate the spark that it's supposed to. Any metal part of the car will do, although a location that is visible while seated in the driver's seat is needed.
Once the tester is properly installed in the spark plug wire and grounded to a metal ground, crank the engine with the ignition key and look for a spark to appear at the tester. A strong, blue spark is what you want to see.
If no spark or a weak, yellowish one appears at the tester when cranking the engine, the spark plug wire is likely bad and needs to be replaced. If a strong, blue spark appears, the wire is good and any problems are being caused by the spark plug or another part of the ignition system.
