electronic ignition repair & troubleshooting
Electronic Ignition Diagram
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   Most electronic ignition systems will start even if the computer is defective or disconnected. The computer plays an "intervening" role and modifies spark timing by delaying or advancing the pickup trigger signal or module switching of the coil. In spite of the fact that many people blame the computer for anything that goes wrong, the fact is the computer is pretty reliable. Most problems can be traced to opens in the wiring harness or loose or corroded connectors.

  A scan tool is required to check the computer for ignition-related fault codes, unless the system provides a manual flash codes. Refer to a manual for the diagnostic procedure for your vehicle.

  Ignition modules frequently fail for three reasons: heat, vibration or voltage overload. Excessive heat can damage sensitive electronic chips. GM ignition modules and Ford TFI modules, in particular, rely on a layer of grease under the module to help carry heat away from the electronics. If someone replaced the module and forgot to apply the grease, the module may have failed due to overheating. The leads that connect the module to the wiring are also vulnerable to breakage, often as a result of vibration. Finally, a module can be destroyed if the high secondary voltage in the distributor somehow finds a path to the module.

   If you don't find any problems here, check the resistance of the pickup in the distributor (refer to a manual for the exact specs). An open or short here can prevent the pickup signal from reaching the module. The exact test procedures are so varied that we'll summarize by saying that most checks involve using an ohmmeter to measure the resistance of a magnetic pickup or a voltmeter to check voltage readings in the Hall effect sensor circuits. If the readings are out of range, the pickup or sensor needs to be replaced. With Hall effect sensors, it's also important to make sure the sensor is receiving voltage because it can't generate a signal without voltage. Refer to a manual for the specific diagnostic procedure for your vehicle.

  CAUTION: Don't disconnect or unplug any connectors without first making sure the ignition is off. Breaking a connection while voltage is flowing in the circuit can create a voltage surge that may damage electronic components.

  Check all the connectors at the distributor, module and coil to make sure they're tight and corrosion-free. It doesn't take much to disrupt the primary ignition circuit. A visual inspection won't necessarily reveal all such problems because "invisible" corrosion can create enough resistance to disrupt a low voltage circuit. You may have to back probe both sides of a connector with an ohmmeter to see if there is resistance in the link.

  Next, remove the distributor cap (if your engine has one) and inspect the wiring connections on the pickup (and module if the module is located inside the cap as is the case with most GM systems, or on the side of the distributor as is the case with many Ford systems). The flexing of the distributor can sometimes cause hairline cracks in the wires or their insulation creating an open in the circuit.


  Ignition coils are pretty simple (just a set of copper windings around an iron core sealed in plastic), so there's not much that can go wrong with them. They do fail occasionally, but usually the part that fails is something more vulnerable like the ignition module, the pickup in the distributor or the wires that connect to the pickup or the module.

  The coil can be ruled out as a possible cause by checking it's "primary" and "secondary" resistance with an ohmmeter. Primary resistance is checked between the positive and negative coil terminals. As a rule, primary resistance should be two ohms or less. Secondary resistance is tested between the high voltage terminal and the negative terminal. Secondary resistance should be high, ranging anywhere from 8,000 to 20,000 ohms. The exact specs will vary from one application to another, so refer to a manual for the specifications for your vehicle.

  NOTE: On some distributorless ignition systems, the individual coils in the coil pack assembly can be replaced if only one coil is defective. On others, the coils cannot be replaced separately and the entire unit must be replaced if any coil, or the coil module, is bad.

  Most distributor type electronic ignition systems have either a magnetic pickup or "Hall effect" sensor that generates a signal as the distributor shaft rotates. On distributorless ignition systems, the trigger signal is generated by a Hall effect or magnetic crankshaft position sensor (and cam position sensor in many applications). The trigger signal goes to the ignition module, which switches the ignition coil on and off to fire the spark plugs. If the pickup or Hall effect switch is not producing a signal, or if the ignition module is not processing the signal, the coil won't fire.

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