Ballast Resistor

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 negative 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.

The Coil

There are three things that you want to check for with the coil. The first is the internal resistance. Disconnect all 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 6,000 and 30,000 ohms. 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 tearing a hole through the case of the V, causing the power to arc from the coil to ground.

The Points

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 oil, 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 or 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 is 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 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 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.

Secondary Circuit

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. When you are going to do is measure the resistance of the wire. You should get a reading of about 8,000 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.


The Spark Plugs

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, gapping 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. It 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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