Jeff's DIY

Advice on doing your own diagnosis and repair

Advice from the book Can I Do It Myself?

Checking Gasoline Engine Ignition Systems

The ignition system is responsible for producing sparks and delivering them to the spark plugs, which ignite the charge in the cylinder head. There are many types of ignition systems, but all of them require an input on the angle of the crankshaft in order to release each spark at the right time. They also require power to create the spark in a coil and a way to cut the power suddenly to release the spark. In newer models, the timing function is performed by the engine computer using input from a crankshaft sensor. In earlier models, a magnetic pickup coil or breaker points in the distributor was used. We usually start the ignition system check by looking for sparks.

An easy way to check for spark is to pull an ignition wire from the coil, distributor, or spark plug and hold it with an insulator (oven mitt or pliers with plastic grips) close to the terminal you removed it from while someone cranks the engine. If you removed the wire from a spark plug, you can just lay it on the engine. You may have to push the wire into the boot to expose the terminal or use a Phillips screwdriver to reach into the terminal to get the spark out of the boot. Here is what that might look like:

Spark from plug wire


If sparks jump across when cranking the engine, you have fire. If you have fire at the coil/distributor, you must still verify that the ignition wires are properly connected to the spark plugs. This is especially important in cases where the plugs or wires were replaced just prior to the "no start" condition. Look up the firing order specifications for your engine and verify that the wires are connected in order. See my tips on firing order for details. If all of this is good and your engine still won't start, refer back to my tips on no start.

If you do not have sparks, use a (12 volt) tester to check the coil terminal for ignition power. The coil voltage varies from car to car, but should be between 9-15 volts. If the coil terminal is concealed by the ignition module, you will need the wiring diagram to determine which terminal on the ignition module receives battery power. If there is no voltage to the coil (or ignition module) with the key on, check the ignition fuse and relay. Not all vehicles have an ignition relay; but if your car does, you can use my relay tips to interrogate the relay socket. In these designs, the command signal from the ignition switch may be negative or positive. If you aren't sure, see if you can get the wiring diagram for the circuit. If the command signal is not getting to the socket, your ignition switch or ignition wire from the switch are bad. If the command signal is getting to the socket but the relay is not clicking, replace the relay.

Assuming now you have power to the coil but no fire, either the coil is bad or the ignition module is not sending the timing signal to the coil. You can test the coil with an ohmmeter if you have the specifications for your coil. Resistances for the primary winding vary from less than one ohm to over a thousand ohms for some coils. If you have more than one coil, check them all, though generally you should expect the engine to fire if only one of multiple coils is bad. This is a good way to isolate and identify individual coils for replacement. Ignition modules are beyond the scope of this tip, but I can provide a few ideas on where to start. It is a good idea to extract any and all fault codes indicated by the "check engine" light when you are not getting a spark from a powered coil. Please see my tip on Check Engine Light.

Use the fault codes to further troubleshoot the ignition system. It is advisable to get the wiring diagrams for your ignition system to troubleshoot. If you have a fault code relating to the crankshaft sensor, this fault may prevent the computer from timing the spark. Check the wiring and connector to the sensor and make sure the sensor is tightly installed.

Most modern cars have an engine or powertrain control computer/module to send signals to the ignition coils and fuel injectors. If you have an oscilloscope or other meter that can sense a waveform, use it to check the signal to the negative coil terminal(s). If the signals are not rectangular and periodic, suspect your ECM/PCM is bad or is getting bad data from the crankshaft sensor. A regular voltmeter in AC volts mode can also be used to verify a signal is getting to the coil. The meter should register a few volts at the coil terminals while someone is cranking the engine. Knock sensors may, in some cases, also affect the computer output, though these are generally used only to modify the ignition timing while the engine is running with a knock.

If your car is a bit older, it may just have a distributor with a pickup coil and ignition modulator to generate the signals to the coil. In these systems, the modulator charges the coil and then cuts the charge signal when it receives a blip from the pickup coil in the distributor, thereby releasing the spark. The spark then travels from the coil to the distributor and is routed to the appropriate spark plug via the rotor. The signal to the coil should still be rectangular, and if not, suspect the modulator is bad.

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