CARLIS WILSON, TECHNICIAN
Testing Today's Automobile
To test the systems one must have the testing equipment and a working knowledge of the system, One is testing. I hope to have some Tips that will help in the testing and collecting information of the automotive systems.
IMPORTANT
Consult the vehicle service manual for specific information and test procedures for any and all service work performed. Do not attempt to service a vehicle without the Manufacturers Instructions and Safety Procedures.
Chasing That Misfire
A misfire can stem from many causes. When attempting to diagnose a misfire it is a good idea to do a "Ignition Scope Test" for some common problems seen in the area of the ignition system.
1. Look at the Primary: coil and distributor dwell to see if it is uniform on each dwell time and the time long enough for good coil saturation.
2. Look at the Secondary: coil kv's, If they are all uniform and less than 15 kv. Check the fire and burn time of each cylinder. If all cylinders has high kv's (20-30 kv) look for a bad coil wire, rotor air gap or corrosion, arcing, and burning on the towers of coil and distributor cap, or a loose spark plug to head and terminal. If one or more is high look for a bad ignition wire, or open path for those high kv patterns. If the kv's is very short, look for a shorted to ground in the path of: spark plug wire, spark plug boot interface. carbon tracking, cracked tip and fowled or carbon-up spark plug.
3. Another cause for spark plug misfire can be found as a cracked tip. This condition is very hard to diagnose. The failure occurs after a small crack develops in the insulator tip of the spark plug, causing the spark to jump a firing gap internal to the ground side of the spark plug.
Ignition Coil Failure
When using the scope look for a good coil oscillations (three is consider good) some coils is able to perform well at low kv's and will misfire at a high demand of kv to ignite the air/fuel mixture. Some coils fail at very cold temperature and others when hot which make it hard to diagnose at shop temperature, (some technicians have stored the coil in the freezer to cool them down before the test). I had a misfire in this week that was a ignition coil failure at 55 mph with the torque converter applied, on a moderate acceleration the engine had a misfire, which could not be simulated in the shop so it was not easy to diagnose.
-By Carlis B. Wilson
