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How to Find Faulty Ignition Coil on '98 PassatPosted 1/11/2007By Jim Newkirk
Vehicle: 1998 Volkswagen Passat, 1.8L AEB engine The N122 power stage provides the ground-side trigger signal to each of the ignition coils via individual power transistors internal to the N122 power stage and is triggered by the power train control module (PCM). There are two connections at the N122 power stage. The four-wire connector provides the primary ground trigger for each of the four ignition coils. Pin No. 1 triggers coil 1, pin No. 2 triggers coil 2 and so on. The five-wire connector contains the main N122 power stage ground, and the individual trigger signals from the PCM to the N122 power stage. The N122 power stage isolates the PCM from the high current of the coil primary circuits by allowing the PCM to trigger the internal power transistors with a low current signal. The power transistors in the N122 power stage then provide the ground path for the coil primary circuits for each individual coil. Excessive amperage draw from a faulty ignition coil primary circuit can overheat and destroy the internal power transistors of the N122 power stage. The N122 power stage is located on the passenger side of the engine compartment on the plastic intake ducting. Follow these steps to test for correct coil trigger signal: Step 1) Use a test light with the ground lead connected to a 12-volt power source. Backprobe the misfiring coil trigger pin at the N122 power stage (Note: In our example of a No. 2 cylinder misfire, pin 2 of the four-pin connector would be tested.) Observe the test light as the engine is cranked. If the test light blinks, the trigger signal from the N122 power stage is normal and a faulty ignition coil is causing the misfire condition. Step 2) If the test light does not blink, check the PCM trigger signal to the N122 power stage. To verify PCM trigger signal, disconnect the five-wire connector at the power stage. Using a lab scope, probe each of the trigger wires from the PCM while the engine is cranking (the trigger wires are pins 1, 2, 4 and 5). The trigger signal should look the same on all four trigger wires from the PCM. Step 3) If a good trigger signal from the PCM is present on pins 1, 2, 4 and 5 of the five-wire connector, and no trigger signal is present on the four-wire side pin 2, the N122 power stage is faulty and must be replaced. Always replace the coil on the affected cylinder at the same time (in this example, ignition coil No. 2). Step 4) When a coil fails on a vehicle with high mileage, a good preventative maintenance measure is to replace all four coils at the same time. This minimizes the chance of another coil failing and damaging the N122 power stage. The system is less complex than it seems at first glance, and with the right tools, it can be diagnosed quickly and effectively.
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