Ignition System Trouble Shooting

by Les Bengtson

Les Bengtson's Classic MG Pages: Articles:


    When your car will not start, it is frustrating. When it suddenly stops as you are driving along, the emotions range from frustration to terror (such as when it happens in rush hour.) There are two basic causes of this type of problem-lack of spark and lack of gasoline. In this article, I would like to focus on the ignition system, how to determine if it is an ignition system problem and how to troubleshoot to find out where the problem lies.

    If the car dies as you are driving along, immediately look at the tachometer. If it has dropped to zero, you have a low tension ignition system fault. If it is falling, but still showing the engine speed as it falls, you have either a fuel system or a high tension ignition fault. This two second procedure can give you a good clue as to where to begin your troubleshooting.

    If you are trying to start your car and it will not start, you may have either a fuel system or an ignition system problem. The easiest way to check is to disconnect the lead going from the coil to the distributor cap at the distributor cap end. Then, using a pair of insulated pliers (or a couple of sticks), hold the end near the engine block and have an assistant crank the engine over. You should get a good spark at the coil lead that will jump a gap of " to ". If it does not, you probably have a low tension (LT) circuit problem.

    To check out the LT circuit, you need a good volt-ohm meter, or VOM. You can use a test light, if necessary, but this is less desirable. An inexpensive VOM can be purchased from Radio Shack, Harbor Freight and most parts stores. Keep it in the original box and put it in the trunk with your tool kit, stored in a plastic bag to prevent moisture or dust from getting at it. The first thing to do is to determine whether you have power going to the coil from the ignition switch. The power going into the coil will go to the terminal marked SW (switch) on earlier coils and + on later coils. The chrome bumper cars will normally have one wire going to this terminal while the rubber bumper cars will have two. The first step is to simply turn on the ignition switch and measure the power input to the coil. You should have 12 V at all times with the CB cars. With RB cars, you will have 12V with the ignition switch in the start position and 6V with the ignition switch in the run position. If you do not have power into the coil, you have a fault between the brown wire going to the ignition switch and the coil. This could be either the ignition switch or the wires. Use the ohm meter function to test the wires by connecting one probe to each end. On a good wire, you will read zero resistance and a bad wire will read infinite resistance. Another method is to use the volt meter function and find out where the volts stop. Remove the steering wheel cowl and check the brown wire for current. If it does not show current, you have bad power input to the ignition switch. If you have good power input, check the various terminals of the ignition switch for power through the switch. You need a wiring diagram, preferably one that has been duplicated and expanded, for this procedure. At this point, it is simply tracing wires until you find where the voltage stops showing on the meter, then replace the wire between the last good point and the one found bad. (Note. Power to the coil, on RB cars when the car is cranking, comes from the starter. If you do not have 12V input with the car cranking, check the starter to coil wire. The car cannot be started under these conditions without jumpering the coil to a good 12V source that has constant power. If the car starts when the key is in the start position, but dies when the key is returned to the run position, you have a white wire circuit problem and that is the area to concentrate on. The white wire circuit provides a 6V input to the coil with the ignition switch in the run position. When it goes bad, you may loose both fuel pump and coil depending on where the break is.) If, however, you find good voltage input to the coil, you proceed to the next step, checking the coil.

    The common check for a bad coil is to "replace with a known good unit" in all of the better service manuals. Not very practical when stuck on the side of the road or when you do not have a "know good coil" handy. A second method is, with the ignition off, use the ohm meter to check the resistance across the coil terminals. Connect one probe to each of the terminals and read the resistance. On a 12V coil, you should read between 3.1 and 3.5 ohms resistance. On a 6V coil, you should read between 1.43 and 1.58 ohms resistance. (The Lucas 12V Sports Coil shows slightly higher resistance than the standard 12V coil, about 5 ohms on the one I tested new.) If you read zero resistance, you have a short in the coil and it is not functioning. If you read infinite resistance, there is a break in the windings and the coil is not functioning. Replace with a known good unit. If the coil tests good, continue checking out the system.

    The next test is to use the volt meter to read the voltage coming from the coil with the ignition switch on. This should be between 6 and 9 volts, depending on model of coil. If it more than this, the coil is shorted internally. If it is less than this, there is too much internal resistance. Once again, replace with a known good unit. If the voltage is within limits, use the ohm meter to check the wire (power off now) between the distributor and the coil terminal. This terminal is marked CB (contract breaker) or - depending on coil vintage. You should show zero resistance. If you show infinite resistance, you have a bad wire. If you show more than a few ohms resistance, you have a broken wire or one going bad. Replace as necessary. When, or if, you have a good wire providing current from the coil to the distributor, you can begin your distributor checks. Maybe.

    If you have an electronic points replacement unit (the so called "electronic ignition") there is not much that the average hobbyist, or even professional mechanic, can check. The practice here is, again, "replace with a known good unit". This is why people who have added these units to a points type distributor should always carry a spare set of points and condenser to install if there are problems. If, however, you have a points type distributor, the tests can continue.

    Turn the ignition switch to the start position, applying power to the system. Check the voltage on the wire coming to the distributor from the coil at the end of the wire, then again at the points. The 25D4 distributor has a tab type connector on the side of the distributor. If the connection is loose or corroded, you will see a voltage drop between the coil and the points. If you have good voltage from the coil wire but low voltage at the points, it is the wire that goes from the terminal on the distributor to the points. I have seen these go bad, but only rarely. Next, with the point closed, check the voltage on both sides of the point's contacts. A drop of more than one volt indicates bad points. While you are examining this area, make sure the base plate ground wire is in good condition. This wire runs from the base plate to one side of the distributor and is connected to the distributor by one of the screws which hold the base plate in place. If it is bad, the grounding of the system is less than optimal and may be the cause of your problem. The other, main ground, for the system is the distributor clamp on the engine. The distributor must be tight (but not too tight) in the clamp and the clamp must be firmly tightened to the engine block for the system to function properly. After these checks have been completed, you should have discovered any LT circuit problems and have corrected them. The only part of the system you have not checked is the condenser. A bad condenser should not prevent the car from starting and running, it only makes it run poorly. It is rare to find a condenser tester today and, once again, the "replace with a known good unit" applies. With the LT circuit tested and functioning, it is time to move on to the high tension circuit.

    The HT circuit consists of the coil, the distributor cap, rotor, coil and plug wires and the spark plugs. The first test in checking the HT circuit is to remove the wire going to the distributor cap from the coil at the cap end. Then, use your insulated handling devise to see if you get spark when an assistant cranks the engine over (as mentioned previously). If, with a known good LT circuit, you do not get a good, strong spark, either the coil or coil lead is bad. Replace the lead and try again. If still no spark, replace the coil. If however, you have a good, strong spark with the original lead (or get one when you replace the original lead), check for spark at the spark plug wire. The best way to do this is to take one of your old spark plug caps into the hardware store and get a long, threaded bolt or screw that fits it. The ones I have checked have been threaded 8-32, a standard size, but brands may vary. Use this device to verify that you have a strong spark at each plug wire. If all the wires have about the same spark, you have demonstrated that the distributor cap and rotor are in good, functional condition and the wires are good. That only leaves bad spark plugs as your source of ignition system problems. If one wire shows no spark or weak spark, it could be a distributor cap/rotor problem or a wire problem. Replace the bad wire with one of the other spark plug wires and retest. If it now shows a good spark, replace the bad wire (I prefer to replace them as a set). If that terminal shows a poor or no spark with a wire that tested good on another terminal, replace the distributor cap and rotor. (As a side note, I cover how to check out the coil and spark plug wires for best performance on another tech article. Right now, we are concerned with basic function rather than best performance. A car will start and run with marginal wires.) If the wires test good, replace all the spark plugs and the car should start. If it does not, you probably have a fuel system problem and would have to troubleshoot that system to correct your problem.

    In summary. The distributor is a two function unit. It creates, through the low tension circuit, a pulsing magnetic field within the primary windings of the coil which is a step-up transformer. It distributes this pulse of higher voltage through the high tension circuit from the coil to the spark plugs by way of the ignition wires, the rotor and the distributor cap. If the ignition system is functioning properly, after your checks, your car should start. If it does not, you need to trouble shoot the fuel system and the ignition timing. Those areas will be covered in separate articles.

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright 2001 by Les Bengtson


CustomPistols.com Home - Classic Sports Car Pages - Tooling for Classic MG Restoration - L. Bengtson Arms Co.


les@custompistols.com