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  Chicagoland MG Club:Tech Tips

Troubleshooting Electrical Problems

If there is anything worse than a car that won't start I haven't found it. But before giving up and having your car towed away (by someone who has no idea what kind of car it is) consider the possibility that YOU, yes YOU, may be able to repair it yourself. Stop and think what has happened. Have you been having intermittent starting problems? If so, you may have a loose connection or dirty contacts somewhere. Did the car just die while you were cruising along? Again it could be a loose connection or maybe a short (although as we all know this is highly unlikely in a British car!). Or has the car been stored for a while? Again, maybe just dirty contacts such as the points. Maybe you're simply out of gas or have a plugged fuel line. Everything can be fixed by a person with average ability, as long as they can figure out what the problem is. About the only thing you need to troubleshoot the ignition system is a multimeter. You can get a cheap analog one (the kind with a meter) for $6.00, but don't waste your money. Buy a digital one and you will be able to understand what the numbers mean which is more than I can say for the analog type. You can buy one mail order for about $13.00. A test light will do in a pinch but it's not a good choice. There are times when you need to know the correct voltage as well as instances that even a small amount of current indicates a problem. Since six volts will light a 12 volt test lamp but one volt won't you can miss a problem and not know it.

In order for the engine to fire it must get a high enough jolt of electricity from the coil to the spark plugs. The first test is whether or not current is getting to the plugs. It's possible to pull one of the spark plug wires from the spark plug and hold it about a quarter inch from the block. When turning over the engine you should see a strong blue spark. The problem that you are going to face is that the rubber boot on the spark plug makes it impossible to perform this test without sticking a screw driver or something else into the boot, and then trying to hold this contraption a quarter inch from the block. A better way is to pull the wire from the spark plug and use a spark tester. This way there is no chance of getting a shock or mistaking the strength of the spark. You can purchase a spark tester for about $7.00 at most auto parts stores. You can also make one for about $2.00. Take an old spark plug and cut off the electrode at the bottom with a hack saw. Then drill a small hole in the metal part of the plug and attach a large alligator clip using a self tapping sheet metal screw. To use the tester attach the alligator clip to a good ground such as the engine block, attach the plug wire to the tester just as you would a spark plug wire and crank the engine. Another advantage of using a spark tester is that it requires the same voltage to fire the tester as is needed to fire the engine. The small gap on a normal spark plug does not require as much current once it is removed from the engine.

If this test is positive, and you are getting a good spark then you know that the ignition circuit is in order, and your starting problem is somewhere else. The most likely suspect would be in the fuel system. You could also have a mechanical problem such as a broken timing chain.

The starting circuit is broken down into two separate circuits. The secondary circuit is made up of the "thick wires" which are the spark plug wires and the wire from the center of the distributor cap. It also includes the distributor cap, the rotor, the spark plugs and one set of windings inside the coil. In other words everything that carries the high voltage.

The primary circuit would be everything that's left. The battery, ignition switch, points, coil wiring, and in some cases the ballast resistor. Starting with the battery which supplies 12 volts, the electricity flows through the ignition switch to the primary side of the coil through the SW (switch or +) terminal on the coil then out through the CB (contact breaker or -) terminal. Connected to the CB terminal is a thin wire that is attached to the distributor, which in turn is attached to the points. The points are really nothing more than a switch. Think of them as the switch on a burglar alarm. When the switch is closed everything is quiet, but when the switch is open the noise starts. When the points are closed the 12 volts of electricity from the battery is flowing merrily on it's way from the SW side of the coil to the CB side of the coil and then into the points. When the points are open 20,000 to 40,000 volts erupt from the top of the coil. The increased voltage coming out of the coil travels through the secondary circuit. It starts it's way down the thick coil wire into the distributor cap, and is distributed to the spark plugs via the rotor to the spark plug wires.

Assuming that you don't have any spark at the spark plug tester, you need to work back from that point. You can take the tests by leaps and bounds, thereby eliminating all components between two points, or test each component separately. I would recommend at least the first time to check each component individually. By doing so you may find a weak component that's adding to the problem, but not directly causing it. If you have some extra time it would be a good idea to run these tests before you have a problem. By doing so you can familiarize yourself with what should happen when everything is working properly.


A good indication of whether or not the complete primary circuit is operating correctly is by the intensity of the spark coming out of the coil wire. To test the spark, remove the coil wire from the distributor and hold the wire a quarter inch from the engine block. With the ignition switch on open, close the points with a screw driver. If the primary circuit is functioning you should get a strong spark coming out of the coil wire. If you're not sure what to look for, the points are made up of two parts. One part looks like a flat spring and it has the "point" attached to one end. This is the movable point. The other point is stationary and is screwed to the bottom of the distributor. If you turn the engine over with the distributor cap off it will become apparent which part is which.

If this test reveals no spark you will have to figure out just how far along the power has traveled from the battery. The first check is to make sure that you are getting power to the coil. With the distributor cap still off, open the points. This can be done by quickly tapping the ignition switch, solenoid button or putting the car in fourth gear and rocking it. Turn the ignition switch on, grab your multimeter and put the positive or negative test lead (depending if your car is positive or negative ground) to the battery side of your coil and the other lead to a good ground. You should get a 12 volt reading. If not, you have a problem with the ignition switch, the battery, the wire between the battery and switch, or the coil. Try jiggling the starter switch and if you now get voltage then either the switch or connections are defective.


The purpose of the ballast resistor is to reduce voltage going to the coil. Not all cars have a ballast resistor. If your coil has three wires connected to it chances are that one of them is for a ballast resistor. If the ballast resistor is bad the car may start but die out immediately. There isn't much you can check. Try grounding the thin wire that runs from the coil to the distributor-it's the CB or - side of the coil. Then with the ignition switch turned to the on position (not the cranking position) measure the voltage from the + side of the coil. You should get a reading of about five to seven volts. If less than five volts it's not getting enough power which may mean a bad ballast resistor.


There are three things that you want to check for with the coil. The first is the internal resistance. Disconnect all the wires going to the coil. Set the multimeter to the lowest ohms scale. Now with the meter connected to the + and - side of the coil you should get a reading of about one and a half to three ohms. Much higher or lower than this indicates a bad coil. Next check the secondary circuit. Set the meter to the high scale and put one lead on either the + or - terminal. Put the other lead into the terminal at the top of the coil. You should get a reading between 6000 and 30,000 ohms. This is one of those tests that I mentioned in the beginning of the article that you should do before you have a problem. Make a note of what your reading is and what scale that you got the reading on. Then in the future when your coil is in question you will know what to expect. The last test is for an internal ground. Set the meter to the high ohms scale and connect one test lead to either the + or - side of the coil and put the other lead onto the case of the coil. The needle should not move at all. If it does the coil is internally grounded and must be replaced. A tip for MGA owners is to make sure that the coil does not rest directly on the generator. The vibration has a nasty habit of wearing a hole through the case of the coil causing the power to arc from the coil to ground.


To test the points all your coil wires should be hooked back up and your ignition switch on. The first check is to make sure that you are getting power to the points. With the distributor cap still off, open the points. This can be done as you did above by turning the engine with the key or solenoid switch. Turn the multimeter to the DC volts scale and touch the probes to the points. One on the movable point and the other end to ground. You should get a reading of about 12 volts. If not, then you are not getting power to the points. If you have 12 volts up to the coil then check the thin wire from the CB (-) side of the coil to the points, you may have a break in it. Assuming you are getting power, you want to check the condition of the points themselves. To test the points, turn the engine until the points are separated. As in the test above, hold the coil wire about a quarter inch from the engine. With a screwdriver touch the movable point to the metal plate below. What you are doing at this point using the screwdriver as the points. If you now get a good spark coming out of the coil wire that you didn't have before it means your points are bad, and they need to be replaced or cleaned. To clean them, close them up (to put tension on them) and put a piece of paper between the points. Pull the paper through a few times. This should remove any oil that has gotten on the points. Although not recommended you may want to substitute fine sandpaper to clean the points. If you weren't able to get a spark even after substituting the points with a screwdriver you'll need to check the condenser.


The last stop along the primary circuit is the condenser. It is a small cylinder about 3/4 of an inch long, with one little wire coming out of the top of it. It's usually mounted on the inside of the distributor, but can be found on the outside of Mallory distributors. Disconnect the wire and hold the condenser so that it doesn't make contact with any metal. With the points are still open, touch the movable point and the base plate with a screwdriver as before. If you get a spark at the screwdriver point you probably have a bad condenser and need to replace it. A further check can be made with the multimeter. This time the condenser should be screwed back down so that it's touching metal but with the little wire still disconnected. Set the meter to the DC volts scale and measure the voltage from the disconnected small wire to the screw on the distributor that the wire is normally connected to. If you get any voltage reading then the condenser is bad and must be replaced.

That's about it for the primary circuit, and most of the time you should have found the problem. The secondary circuit has fewer components and usually is not the cause for starting problems.


In order to check the secondary circuit the primary circuit needs to be functioning correctly. The check of the secondary circuit for the most part is done by visual inspection. Since there are only a few things to check in the secondary circuit it should go fairly quickly. Start by examining the condition of the spark plug wires. If they are greasy or wet, dry them off. If they are brittle or cracked, replace them. To check the internal condition of the wires you will need a multimeter. Connect one lead to each end of the wire and set the multimeter to the ohms scale. What you are going to do is measure the resistance of the wire. You should get a reading of about 8000 to 10,000 ohms per foot. Twist and bend the wire a little while watching the meter. If the reading drops to zero, the wire has a break in it and will need replacing. Next remove the distributor cap and look inside. Make sure there are no carbon tracks inside the cap. Carbon tracks look like black lightening lines that go between the spark plug wire terminals inside the cap. These carbon tracks work like a printed circuit board and conduct electricity, which will short things out. If you find any, the cap needs replacing as well as the spark plug wires. In a pinch you might be able to scrape or wash the tracks off to get the car running. Pull the rotor out of the center of the distributor and sand it down a little and wipe it off. If it's been raining or is damp outside try spraying WD40 on the wires and the distributor cap. Even if the wires appear dry, moisture can penetrate.


Although the secondary circuit is pretty reliable the spark plugs do cause problems. The problem can be as simple as wet plugs from a flooded engine to bad plugs themselves. I experienced a problem once where I had spark at the tester so I spent the next several hours tracking down a problem that didn't exist. The problem was in one spark plug that was bad, and it was enough to keep the engine from firing. For this reason I would recommend pulling all the plugs, cleaning them, gaping them and testing them. Before pulling the plugs be sure to number the wires to correspond with the cylinder that they are attached to. To test the plugs, reconnect them to the spark plug wires and lay them on top of the valve cover. Have someone turn the engine over and watch the plugs to make sure that each one is firing. If you find one plug that isn't sparking, switch it with the one next to it to make sure it's the plug and not the wire. If you are turning over the engine using the solenoid make sure the ignition is turned on.

Well that's about it. It may sound complicated but it really isn't. If you find yourself stuck on the road somewhere it's either do it yourself or wait and wait and wait for help. If you keep a few extra parts, some tools, and a copy of this article, at least you stand a chance of getting back on the road in a reasonable amount of time.

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