If your vehicle won't start and you know that it is not a lack of fuel or a lack of air, then ignition must be the problem, but where in the ignition system is the issue? What part of the ignition is not doing its job, and how do we find it????
Welcome to the second half of a two part series on troubleshooting a vehicle that won’t start. In the first article
we looked at the basic three requirements a motor’s needs to run: fuel air and ignition. We also looked at the specific troubleshooting of the fuel and air systems but did not get in to the specifics of the ignition due to the size of the topic. For better understand of the ignition system I am not only going to cover troubleshooting, but also explain the basics of how each component works, the idea being that understanding how the parts work, makes them easier to troubleshoot.
Keep in mind that in order to test many of the parts in an ignition system you will need a volt ohm meter, and the information following is under the assumption that you know how to use said meter.
Also one important step in troubleshooting the ignition system is having the key in the on position for many of the tests. Make sure you turn the key back to the off position after each test or you will potentially damage some of the components in the system.
Breaking it down to the individual components
The ignition system can be broken down to following basic parts: spark plug, coil, spark plug wires, distributor, and ignition module or points. There are countless aftermarket options available when it comes to ignition but I am limiting the scope of this write up to the basic ford point system and the ford duraspark system.
As was discussed in the first article, the first step is to check the overall function of the ignition system by testing for spark. To test spark simply disconnect the number one spark plug wire from its spark plug making sure it remains connected to the distributor, attach the wire to a spark plug tester, turn the key to the on position, then crank the motor while checking the tester for spark.
Aftermarket Plug Tester
Homemade Plug Tester
A spark plug is simply two electrodes positioned in such a way that a high voltage spark can jump across them. If these electrodes are corroded, dirty, or misaligned than a spark can not travel between them causing ignition failure. If when you test for spark you are getting a spark at the wire, and you are positive the issue is in the ignition, then it is time to look at the spark plugs and make sure they are not fouled. A fouled spark plug is easy to spot. It will be dark black and will often times have a powdery substance on it. If the plugs are fouled replace them.
As you replace them make sure they are gaped properly. The distance between the two electrodes affects how much fuel is exposed to the spark and how hot the spark is. A wider gap will expose more fuel to the spark but smaller gaps produce a hotter spark. The trick is to set the gap large enough to maximize exposure but small enough to maintain a hot enough spark. Most stock ignition systems achieve this balance at or near.035”.
One of the major advantages of going with a performance ignition is being able to gap the plugs at further distances while still maintaining a hot spark which results in more efficient and explosive use of the fuel in the combustion chamber.
Spark plugs where out with use and should be replace every 10K to 120K miles depending on the quality of the plug, manufacture recommendations and the condition of your motor. The average spark plug has a life span some were between 20K and 30K miles. Motors burning oil or with carburetor issues will go through plugs faster than a motor running efficiently.
A lot of information regarding the over all health of a motor can be determined by looking at a spark plug. If the electrodes look black and wet than the motor is using oil. If the electrodes are flat black and chalky looking than the fuel ratio is to rich, but white electrodes would indicate the opposite issue of a fuel system that is to lean. Ideally you want the electrodes to have a nice golden brown color similar to a toasted marshmallow or coffee with creamer in it.
The coil is the heart of the ignition system. Its only purpose is to convert 9 - 12 volts in to thousands of volts so that there is sufficient voltage to make the jump between the electrodes on the spark plug. The way a coil converts a few volts to thousands of volts is slightly more involved, so I will not bore you with the details, however there is one piece of the process that is essential for you to understand for the sake of troubleshooting the system, and that is, the flow of power from the positive side of the coil to the negative side of the coil MUST BE INTERRUPTED for a high voltage charge to be produced. This will be important for checking to see if the coil is working and for understanding distributor function.
These are two different coils common to Ford ignition systems:
The first step in checking the coil is to simply disconnect the coil to distributor wire from the distributor, connect it to the spark plug tester and then check for spark as before by turning the key to the on position and cranking the motor. If you have an older stile distributor cap with female connections the coil to distributor wire will not connect directly to the spark plug tester, but this is not a problem as the wire can be replaced with one of the regular spark plug wires.
If you are getting spark at the coil, then the coil is not the issue and the problem is most likely the cap and rotor of the distributor. If you are not getting spark the coil may still not be the problem. To further test the coil turn on your volt meter and put it on the DC volts function, then with the key in the on position, place the positive lead from volt meter on the positive terminal of the coil or the positive terminal of the coil connection (as pictured below) and touch the negative lead of the volt meter to a good ground. You should See a volt reading of 9 – 12 volts.
If voltage is not getting to the coil than the problem is either a bad ignition switch or a bad connection some where between the ignition switch and coil. If you replace the ignition wire keep in mind that all older Fords used a ballast resistor between the ignition switch and the coil to drop the voltage down around 9 volts. In most cases the wire is the resister and will need to be replaced with resister wire.
If you are getting voltage to the coil, the next thing to do is test the coil for spark with out the distributor or ignition module as the trigger. To do this simply disconnect the wire attached to the negative side of the coil and attach a jumper wire to the negative terminal. As before the spark plug tester needs to be hooked up, and the ignition must be on. Once everything is ready briefly touch the free end of the jumper wire to the chassis or a good ground. As you pull the lead away from the ground, watch the spark plug tester for a spark.
As I said before it is the interruption of the flow of power that produce a high voltage spark and by touching the jumper wire to ground, and then pulling the lead away, a spark should be produced. If you do not have a spark the problem is the coil and it will need to be replaced. If you have a spark than the problem is in the distributor or ignition module.
Spark plug wires
Bad spark plug wires will almost never result in a motor that will not start, the reason being that all four to eight wires would have to fail at the same time to cause the motor not to start. More likely if your spark plug wires are going bad the motor will run but one or more cylinders will have a miss resulting from a reduction or lack of spark at the plug with the bad wire. There is one exception where a bad wire will cause complete ignition failure and that is the wire between the coil and the distributor. If it goes bad you will loose spark on all cylinders.
When replacing your wires I always recommend going with performance units. It is kind of like driving on a dirt road compared to a freeway. Regardless of what you are driving you can only go so fast on a dirt road, but on a freeway you can go both slow and fast depending on what you are driving. It is the same with spark plug wires, with a stock ignition, cheep spark plug wires will function fine, but if you upgrade to a better coil, distributor, or ignition module you will be limited by the wires.
Performance wires will not hinder the function of a stock ignition and allow you the option to upgrade. Also performance wires tend to have thicker shielding which reduces the risk of a cross fire and they also have a tendency to last longer than stock units.
The quality difference can easily be seen between the stock wire on the bottom and the performance wire on the top:
The easiest way to test your spark plug wires is to put a digital meter on them. You want continuity with a similar amount of resistance on all the cylinder wires.
The distributor has several functions. Its main job is to provide spark to the correct cylinder at the correct time. The cap and rotor work together to connect the coil to the specific cylinder that needs spark. Cap and rotor are parts that see a lot of voltage and they do where out. Before looking any further in to the function of the distributor, verify that the contacts on the cap and rotor are in good shape. I recently had to go through this process of trouble shooting the ignition on my 83 F150 and the problem turned out to be the cap and rotor were so badly corroded that spark was no longer getting to the cylinders at it should have.
Cap with bad contacts
Cap with good contacts
A distributors second function is to either provide or trigger the interruption of current through the coil. There are two main types of distributors found on older Fords, a points type distributor and a duraspark type distributor. If you have a points type distributor the actual disruption of the current through the coil happens in the distributor.
The points act as the switch that opens and closes as the distributor turns. If you have a duraspark unit there will not only be the distributor, but also an ignition module. With a duraspark system there are no points, as it is the function of the ignition module to disrupt the current through the coil.
The advantage of electronic ignition over points is simply the speed and reliability of an electronic switch is far superior to that of a mechanical switch. Points brake down over time which decreases there efficiency and over all function. This will usually result in a loss of performance over time. With electronic ignition the function is the same every time. So as long as the module is good it will perform the same weather it is new or has 100K miles on it.
Trouble shooting a points distributor
As I said above, points where out over time and need to be replaced. The action of rapidly opening and closing to disrupt the circuit slowly burns up the points. The result can be one of two things, either the points will fuse together making it impossible for the circuit to be disrupted, or the surface of the points will no longer become conductive causing the circuit to never be completed. To test for this, with the meter in a continuity function, connect one of the meter leads to the black wire coming out of the distributor and the other led to ground, then rotate the motor.
As the distributor turns the points should open and close creating and then eliminating continuity. If you find the circuit is either never being completed or never opening, the best solution is to replace the points. As with spark plugs, points need to be gaped at a specific amount and this information can be found in a shop manual specific to you vehicle. If the points were the issue then the problem should be solved. NOTE In some cases burnt points can have there connecting surfaces cleaned with sand paper, but this is really only a temporary fix and only mention it as a possible fix if you find your self stuck along side the road.
Troubleshooting a Duraspark distributor
As mentioned before on a duraspark system, the actual disruption of power through the coil is done by the ignition module, but in order for it to know when to switch on and off it needs input from the distributor. This signal is provided by magnetic pick up inside the distributor. A duraspark distributor has three wires in the connector: orange, purple, and black. The orange and purple wires are hooked up to the magnetic pick up and the black is hooked up to the hold down screw inside the distributor and serves the purpose of grounding the distributor to the ignition module.
The first step in testing the distributor is to touch one lead of your meter to the terminal on the end of black wire and the other to the distributor body. This should show a reading of 0 on the meter meaning a good connection between the black wire and the distributor. The next thing to check is the magnetic pickup. This is done by touching one lead of the meter to the terminal on the end of the orange wire and other lead from the meter to the terminal on the end of purple wire.
The reading on the meter should be between 500 to 1,200 ohms. If your reading is in that range switch the meter over to AC volts and spin the distributor shaft. This can either be done by hand if the motor is out f the car or by cranking the motor,with the key in the off position, if the distributor is still installed. When the distributor spins the reading on the meter should be around one volt AC. If you do not have around one volt AC than the magnetic pickup needs to be replaced. Some auto parts stores sell just the magnetic pickup, but many only sell full distributors.
If the distributor checks out the only thing left that could be the issue is the ignition module or the wires that connect it to the other parts of the system. To test the module it must be disconnected from the car and taken to a professional. Most auto parts stores can test your ignition module for free.
If it is not the issue the wires connecting the module to the other parts of the system need to be inspected. A meter is a great way to check for continuity and make sure the wires are not broken, also check the connection terminals for corrosion and make sure there is a good connection.
As you can see the ignition system is a lot more involved than the fuel and air systems, but with a little understanding on how each part works troubleshooting your ignition is really not that heard and can be accomplished with a little testing and systematic elimination of components.