Ignition Module Troubleshooting

Ignition module troubleshooting is extremely difficult without the use of a professional diagnostic scope. The best way to do this without a scope is the process of elimination.

All ignition control modules receive signals from the crankshaft sensor and on some, the camshaft sensor. The module coverts this signal into a digital signal to the PCM. The PCM is the main engine control computer. In response to the digital signal from the module, the computer determines the necessary engine timing and returns a firing sequence to the module. The module then initiates the firing of the coil by collapsing the primary windings in the coil, which induces a high voltage in the secondary windings, which then fires the spark plugs.

This process is so rapid that it would be impossible to witness this progression in real time on a voltmeter. So here's what to do to get around that. This procedure narrows the field to the module or computer. The odds are much greater that the module or coil will be bad versus the computer.

This procedure requires a service manual from an auto parts store for identification of terminals and specifications. The system is complicated in comparison to older vehicles and testing or shorting the wrong terminal or circuit could easily torch the computer. That is no good since the computer and all these parts are expensive.

Locate the ignition control minus terminal on the control module. Turn the key on and pierce this wire to the ignition control minus with the voltage regulator's red lead. Attach the other lead to a good ground. There should be a reading of 100mv or less if the crankshaft sensor is good. If the reading is off, disconnect the crankshaft sensor connector and test for 12 volts at the sensor. If this is correct, the sensor is likely to be bad. Check the control module battery plus terminal for battery voltage. If there is no voltage, a fuse is blown or a bad wire to the fuse block is present. If there is voltage, the coil must be removed from the control module enough to test. Turn the ignition key off first.

Check the coil by using the ohm side of the voltmeter. Check the positive and negative terminals under the coil for continuity. The exact reading will be in the service manual but for reference, all coils should have between .02 to.075 ohms or resistance here-if there is more, the coil is bad. Touch the coil secondary tower terminal with one of the leads. Leave the other lead on either of the two terminals. The resistance should be between 7,500 to 18,000 ohms. If the reading is off by more than 10 percent the coil is bad. The service manual should be referenced when checking coils because the readings just discussed are dependent of the amount of voltage the coil produces and the manufacturer. If the coils show good, the ignition control module is bad.

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