Under the Hood: Repeated failure of ignition control module |

Under the Hood: Repeated failure of ignition control module

Question: My 2002 Buick LeSabre has 98,887 miles on it. Its ignition control module is failing. Since November 2014 I have replaced it three times. Can you tell why my control module keeps failing? It stops working without warning.

Answer: Your ignition control module manages your V-6 LeSabre’s three ignition coils. Each ignition coil makes a spark for two companion cylinders in what’s called a waste spark ignition system. Each time one of the coils fires, two spark plugs receive a spark. One cylinder is on its compression stroke and the other is on its exhaust stroke. During the following crankshaft revolution, they reverse roles. It’s a system that predates the simple coil-on-plug systems found on new vehicles.

My question is, when the ignition module fails, does the vehicle not start or does it just run poorly? Typically, when an ignition control module of this type fails it would be one of the three drivers (transistors) within it that controls a specific ignition coil giving up. Should this occur, the other two ignition coils would still function and the engine would run roughly on four cylinders instead of six. It is less likely for this type of ignition control module to fail in such a way that causes the loss of all ignition system function. The common cause of one driver/transistor failing is a shorted ignition coil primary winding. This is a winding of wire within the ignition coil where each loop is separated by a very thin plastic coating. Should the plastic insulation overheat and allow the winding to touch itself, its electrical resistance will be less from end to end (imagine a Slinky with the paint wearing thin, allowing the metal coils to touch each other). This causes an increase in electrical current, which stresses the driver/transistor, causing it to fail. Testing the ignition coil winding resistance is fairly easy. The coils are inexpensive, so I would not hesitate to replace one with marginally low resistance.

If the replacement modules are failing in a manner that prevents the engine from starting, and the replacements are of good quality, I’d ask your service tech to take a very close look at the wiring connector that attaches to the module. Sometimes a faulty terminal connection is jostled/cleaned during a disconnection/reconnection restoring operation, leading one to believe the component was the problem, when it was actually the connection. A careful look at the module’s power, ground and crankshaft position sensor circuits, using a lab scope or graphing multi-meter as the vehicle is driven, may indicate unstable or intermittent readings, which are important for correct system operation. Another possibility is one of the spark plug wires, which contain high voltage, is accidentally routed near the module’s wiring.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif.

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