Ignition System Trouble Shooting
Written for American MG Pages but perhaps very relevant
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 ¼” to ½”. 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. 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.
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. 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.
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. 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.
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.
How to Convert from Positive Ground to Negative Ground
NOTE: DIY may be OK and here are the steps – but you are advised to get it done by someone who KNOWS what they are doing!
1) Disconnect the wires at the generator (dynamo)
2) Swap battery connections around
3) Swap Coil leads
4) Swap Heater leads
5) Re-polarize the generator (dynamo)
Re-polarize the Generator (dynamo): To re-polarize the generator, connect a jumper wire from the positive side of the battery and touch it several times to the small terminal on the generator. You will see a small harmless spark. The generator’s magnetic field is now reversed.
Electrical Fuel Pump: Early non-diode containing electric SU fuel pumps are not polarity dependant and so they don’t need the wires switched, a positive 12 volt wire goes to the terminal on the top of the fuel pump, a gounding wire screws into the base flange of the pump body and then attaches to the wall of the trunk floor. Later replacement pumps are polarity independant (usually designated by red tape around the top points cover to indicate a positive ground pump, or black tape to indicate negative ground pump.) The diode inside the top cover of the pump will have to be removed and re-installed in the reverse direction to change these later pumps over to negative ground.
Heater Blower Motor: Swapping heater blower motor leads restores the blower to the correct spin direction.
Wiper Motor: The wiper motor is also polarity independent. No wire changes are required. However, I have read that the thrust is apparently affected and it has been suggested that the armature housing cover be rotated 180 degrees from it’s original position. This can be done by removing the 2 long screws that hold the cover in place.
There has been a lot of talk concerning how to set timing with an unknown engine. I think a basic description of timing might help sort out a lot of this. First, you should know that there are two types of timing in an engine: cam timing and ignition timing. (Three types, if you count injector pulse, but injection timing on gasoline-powered buses is tied to ignition timing and is not separately adjustable so I will ignore it, as should you.) Cam timing is what determines when the valves open and close with respect to the position of the pistons in their bores. It is set when the engine is built- by placing the camshaft and crankshaft in the correct relationship.
It cannot be adjusted on a stock engine. It doesn’t change: if it was right once it will be right for the life of the engine, barring disaster. And by disaster I mean the park-it-where-stops-rolling kind of disaster. (This is a very rare occurrence in VW air-cooled engines. Not so rare in the Rabbit/Golf ones, which drive the cam with a rubber belt which strips if you don’t change it on schedule.) In summary, cam timing is not connected with or affected by the turning of the distributor. What distributors time is spark, or ignition. Why?
At idle, your engine is turning relatively slowly, let’s say 650 rpms. The throttle is closed, so very little fuel and air are being drawn in to the cylinders. This small amount of combustible mixture burns very quickly, so for maximum efficiency, the spark needs to start when the piston is very near top dead center (TDC). If the spark comes too early (i.e. too advanced), the pressure from the ignited mixture will hit the piston while it is still coming up the cylinder and be wasted trying to shove the piston down before it reaches the end of it’s travel. If you try to start an engine whose ignition timing is too advanced, the starter will try to turn the crank one way, and the combustion process will try to turn it the other way, and it will seem as if the starter hasn’t enough oomph to start it. Contrariwise, if the timing is set too late (too retarded), the pressure from the ignited mixture (and the power derived therefrom) will dissipate as the flame front chases the piston down the cylinder bore in the rapidly diminishing pressure of the combustion chamber.
In other words, the piston is already on it’s way towards the bottom of it’s stroke, reducing the effectiveness of combustion. The is very fuel inefficient, since a larger throttle opening at idle (set by the idle speed screw) is needed allow extra fuel in to keep the engine idling. In practical terms, the position of the distributor which yields the highest idle speed is within a very few degrees of where it should be set. (If you retard the timing about 5 degrees from this point, you will be awfully close to spot-on.) Of course, this assumes your carburetor (or Fuel injection)is working well and that the idle mixture is correct.
When driving on the highway, your engine’s timing requirement is different. At higher engine speeds, larger throttle openings and greater loads than idle, you need ignition advance. There are two reasons for this. First, you are burning more fuel so complete combustion takes longer. Second, the combustion time, as a percentage of the time the piston is at or near top dead center is much longer because of the piston speed. What this means is that you have to ignite the charge earlier, while the piston is still coming up, in order to get the full benefit of the pressure against the piston at the right time. Too early or too late timing will have a similar effect at speed as at idle, but greatly magnified and with far more destructive potential. Too retarded timing will give low power, lousy emissions and excessive bore wear. Timing too advanced will cause pinging (a rattling noise usually heard on acceleration), overheating cylinder heads and other problems too scary to contemplate.
I will now pretend that vacuum advance (not fitted to 2 litre Distributors) doesn’t exist, otherwise it will only confuse things. Distributors have little weights inside that swing away from the shaft as it turns faster. As they move out, they rotate the upper part of the shaft which passes through the plate that the points are bolted to so that the rubbing block which opens the points meets the lobes which hit it (and thus open the points) a little earlier.
At what engine speed this advance begins, at what rate it advances and at what engine speed it stops advancing is determined by the shape and mass of the advance weights and the strength of their return springs; at what degree of advance it stops is determined by a limiter on the plate to which the weights are bolted . None of this is meant to be adjusted: the manufacturer sets it up for each engine family it builds, based on compression, cam profile, octane requirements and availability, among other things. Timing the ignition, then, is a matter of getting the timing correct at one end (idle or full advance) and letting the rest of the range look after itself.
If you have a stock distributor and know either the timing at idle or the maximum advance at a given engine speed and you have a good timing light, you will have no trouble setting timing. If not, here are some ways to get in the ball park. I already mentioned that the highest idle speed is very slightly advanced from correct. If you set it there and hear no pinging on acceleration and get good power (ha-ha), you are good. If it pings, retard it little by little until the pinging goes away. If it seems weak, advance the timing a little at a time until you hear pinging or find the engine hard to start, then retard it again.
Warning – When working on your dynamo or indeed at any time whilst the engine is running, keep your hands well away from the fan blades!
Dynamos are often referred to as a generators. If the dynamo has been off the car for a considerable time, or your car has been standing for a considerable time, your Dynamo may require polarizing. Here are two YouTube videos on the subject.
Is My Generator Charging?
If you have a voltmeter, here are some easy tests to make – Measure the voltage across the battery, it should be about 12.6 volts with the engine off. As the engine speeds up, the voltage should increase to somewhere around 14 volts. If it does, the generator is working and charging the battery. If the voltmeter does not go up with the engine running, first check to see that the generator brushes are not worn excessively. If you can, apply a little pressure to the brushes while the engine is running and see if the gen light goes out or the voltmeter reading increases. If it does, the brushes may be worn to the point that they don’t exert enough pressure on the commutator.
If the brushes check OK, do this test: Disconnect the generator from the regulator (probably easiest to simply take the wires off the generator). Connect a jumper from DF on the generator to the generator frame. Now run the engine and measure the voltage from ground to D+ on the generator. NOTE: Some generators have different types of terminals; verify which is DF and which is D+. As you increase the engine speed, the voltage should jump up to +35 volts or so (@3000 RPM). If it passes this test, the generator is good. (Don’t run this test longer than necessary as it will overheat the generator.)
If it fails that test, the generator may need to be polarized. Leave the jumper wire connected from DF to ground. Remove the fan belt. Connect a wire from the battery + terminal to D+ on the generator. The generator shaft should start to spin. Don’t run this way for more than a few seconds to avoid overheating. The generator will now be properly polarized. If the generator did not spin during this motoring test, the generator is likely defective. Put the belt back on and re-test for generator voltage with DF grounded. If the output voltage is still low, the generator is defective.
Why do generators need to be “polarized”. Auto generators need some magnetism to get started. This “residual” magnetism remains in the Field pole pieces even after the engine has stopped. The next time the generator starts up, the residual magnetism creates a small voltage in the Armature windings. Not enough to charge the battery, but enough to allow the Field windings to draw current. As the Field current increases, the pole pieces create even more magnetism. That makes even more voltage in the Armature, and the cycle continues until the generator is capable of producing maximum output.
What happens though to a generator which has been stored a long time or is freshly built? The residual magnetism may have decreased to the point where it can no longer get the generator started producing voltage. In the case of a new generator or one which has been mis-treated, the residual may even be of the wrong direction (North and South poles reversed). Polarization is a simple process used to restore the Field pole residual magnetism and ensure the magnetic direction is correct.
Do regulators need to be polarized?
No, regulators are not polarity-sensitive. But my regulator came with instructions to polarize it… These instructions actually polarize the generator, not the regulator. The regulator mfr simply wants to ensure that your generator will work properly so you don’t blame the regulator.
Generator Diagnostics & Troubleshooting
Generators must spin clockwise to make voltage. They won’t make voltage if they are spun counter-clockwise. Generators always make some voltage when they are spinning. The red Generator light in the dash is an important part of the charging system. The system will not charge if the light is bad or not hooked up. The ignition powers the light, the Generator grounds it when not charging. This makes the light stay on. Here’s how the connections on the voltage regulator go and what they mean: (NOTE: this may depend on your type of regulator)
B = Power out of the regulator to the car and battery
F = Generator Field Winding
WL = Warning Light
D = Generator (actually D is for Dynamo) output
E = Ground to the body (actually E is for Earth)
So, make sure the wide spade connectors are on the “B” terminals. Make sure that there is continuity in the F and D wires. In other words, unplug the D wire from the back of the Generator and unplug it from the voltage regulator. Now take an Ohm meter and touch the leads to each end of the wire, you should get a small amount of resistance which shows the wire isn’t broken. Now do the same test on the F wire.
If those are both okay, hook them back up to the Generator. Now leave them off the regulator. Bridge them with a short piece of wire and using a voltage meter, put the pos. lead of the meter into one of the ends that you’ve bridged. Touch the black test lead on the meter to the battery ground. With the engine running at about 1,000rpm+ you should get 18 volts. (This is called “full fielding the generator”)
If you get 18 volts, the Generator is fine and you have a bad regulator if you get less than 18 volts, you have a bad Generator….for whatever reason. They are extremely easy to repair. New brushes and springs are dirt cheap and take abot 15 minutes to replace. Polish the commutator with fine sand paper while your’re in there and finally, I cannot stress enough the need for clean, tight, and large, engine and body grounds. If you want to check them touch the leads of your Ohm meter to the engine block and to a screw going into the body in the engine compartment somewhere. You should get nearly zero Ohms. Any other reading shows bad grounds. Do the same test between the battery ground POST and a screw going into the body on the inner fender. Do the same test between the “E” terminal on the Voltage Regulator and the body in the engine compartment.
Coil Compatability ListCoil-Compatability-List
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