4x4 Off Roading ›› Mechanical Problems ›› Diagnosing Ignition Systems
Diagnosing Car Ignition System Issues
Often, the diagnostics for fixing problems with ignition system failure, and then fixing them, is a job best left for a professional mechanic.
Nonetheless, once you know how to diagnose a loss of spark in a car engine you have a better chance of getting it started yourself.
Recognising Trouble in Ignition Systems
The first task is to determine whether the engine is getting enough air and a good supply of fuel.
In simple terms, the 'fire' that ignites the mixture of air and fuel in a traditional vehicle engine:
- Is an electric current (e.g. stored in an automobile 12 volt battery).
- Gets replaced by the alternator and then monitored by sensors.
- Is directed by the electronic control unit (ECU) to the spark plugs (located in the engine block cylinders) at the appropriate time.
So, what if something goes wrong somewhere during the ignition process? In short, if the spark fails to reach its intended destination (the spark plugs), the air and fuel will fail to produce combustion in the cylinders.
The engine won't start!
Was the engine running before the car died? If so, the fault is unlikely to be the battery, the solenoid (copper coil), or the starter.
Furthermore, even if one of the spark plugs malfunctions without warning, the engine should still run (albeit with a loss of power) on the other cylinders.
Does it have an electronic ignition system? If so, the problem could be the ignition module. Some vehicles have high-energy ignition systems (operating around 47,000 volts). Thus, it is dangerous to test for a spark by pulling a cable on the distributor or spark plug.
Some older models run with non-electronic ignition systems. Checking the distributor cap may help you determine whether the spark is actually moving to the coil - and then through to the spark plugs.
Important: No matter whether you are diagnosing a distributorless ignition system or an electronic ignition, it is not commonplace for this part to break down.
Running the Diagnostics
Is diagnosing a loss of spark one of the general car repairs you can do yourself? If so, this step-by-step checklist will help you run through the basic tests to find out why the ignition system is failing.
- Assuming you checked the fuel, and unless the fuel gauge is broken, you should feel confident that you don't have a fuel deficiency problem.
- Turning on the headlights, and checking that they burn normal, will also rule out the need for jump starting a car with jump leads.
- A quick scan of the fuse box will show whether you have a blown fuse (e.g. a black or burnt spot usually in the centre). Check it with a light tester to be certain and then replace the fuse(s).
- Some of the mechanical tools you may need will include a test light, ignition multimetre, a spark plug tester, and several different size wrenches.
- It is best not to have fuel pumped into the cylinders and flood the engine after you have disabled the ignition system. Thus, remove the fuel pump fuse (or relay) to disable the fuel system.
Testing for Timing Problems
- Spark plug wires carry a very high voltage. Hence, use caution when conducting any manual testing. After inserting a spark plug tester into the end of the wire (the plug boot), ground it on solid metal somewhere near to the engine. It should be clean and unpainted metal.
- Watch for a spark while someone cranks the engine (e.g. turns the key). Seeing a good spark on each wire probably means you have some kind of timing problem. If there is no spark, the most likely diagnosis is an electrical problem in the ignition system.
- The distributor passes high voltage - generated in the ignition coil - through to the spark plugs (usually in a synchronised pattern). You can trace back from the plug wires to locate the distributor cap. Newer cars, such as most modern SUV makes and models, contain an engine control module (ECM) instead. ECMs coordinate the firing of the spark plugs. Thus, you may ignore any testing of the distributor and distributor cap.
- After removing the distributor cap (either by unclipping it or removing the screws) you can inspect the interior. It should turn while the engine is being cranked. If not, the most likely prognosis is a broken timing belt or chain.
How to Check an Ignition Coil?
- You can transfer power through to the electrical components of an ignition system by turning the ignition key. Thus, you don't need to crank the engine to test which electrical parts and wires are receiving current.
- The ignition coil generates high voltages needed to fire spark plugs. There should be three (3) wires attached to the coil:
- A thin power wire (positive) tracing back to the ignition switch.
- A thin ground wire (negative) going to the ignition module.
- A thick coil wire going to the distributor cap.
- Use a test light to check whether the power wire (attached to the engine coil) is getting power.
- If there is no power, it means the ignition coil is not getting current. Check the wiring for breaks from the ignition switch to the coil and repair any broken wires.
- The wiring up to the coil is working as it should be if you determine the presence of power during this part of the test.
- Use a test light to check the ground wire for power. If it lights up (when the engine is off and the key is on) you have power on the negative side of the coil. If not, there may be a break in the coil wire.
- What if the test light flickers when someone is cranking the engine? In this case you should:
- Check the negative wire for breaks and repair them as necessary.
- Use an Ohm meter to test ignition coil resistance. You will find the resistance values for the primary and secondary coils listed in the service manual.
Distributor and Ignition Module Check
- The pulse generator on a distributor is the point where the wiring enters from the ignition module. Crank the engine after connecting an A/C voltmeter to the pair of wires situated at the pulse generator.
- No current may mean the pulse generator has malfunctioned and you will need to replace it.
- Observing a current may indicate the ignition module is not functioning and so it will need replacing.
- If all the components are operating in the proper manner and the wiring system has no obvious breaks, you should suspect the engine control module (ECM) as being the main cause of your automobile ignition system problems.
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