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Spridget Ignition System

The spridget ignition system, like everything else on our cars is extremely simple once understood. This means it is also very easy to trouble shoot any problems with it

The system comprises of 2 sections. The first section is the "low voltage" side while the second section, the "high voltage" sometimes called the "high tension" side. Below is a diagram of the low voltage circuit. Power is applied to the + of the coil. A circuit through the coil exits on the - terminal and connects to the dissy. Within the dissy, this connects to the wire of the condensor and also to the moving part of the contact breaker (points). The low voltage circuit is completed by a connection through the condensor to earth and also the contact breaker when "closed" will also connect to earth. In normal operation when the points are closed current flows through the coil and saturates the magnetic field within the coil. When they open the colapsing magnetic field induces a high voltage on the High voltage section of the coil. This high voltage is passed into the H.T. wire out of the top of the coil So what does this mean for trouble shooting the low voltage section of the ignition system. We have to consider the system in 2 different conditions, 1 with the contact breaker open, 2 with the contact breaker closed.

Checking with the meter set to Voltage D.C. measure at position A, between + and earth (car body/engine) With the ignition switched on you should always get 12 volts here for early cars and about 8 volts on later cars with Ballasted coils. If the reading here is almost zero volts then you have a problem with the power from the ignition switch, posibly a loose connection on the switch or perhaps a faulty switch. Years ago when theives "hot wired" cars they would simply connect a wire from the positive of the battery to the positive "+" on the coil to overcome the need to switch on the ignition. Of course with this wire in place you still need to operate the starter to start the engine, this would be easy with old spridget by simply pushing the manual button on the starter solenoid, or simply push start the car.

If 12 volts were measured at position A then now move on to position B. Checking with the meter set to Voltage D.C. measure at position B. If the points (contac breaker) are open then we should measure 12 volts at measuring point B. With the points closed the reading will be about Zero. If you have 12 volts at position A and with the points open the value is not 12 volts at position B then most likely the coil is faulty. A very common fault on the low voltage circuit is that the points have been installed incorrectly. Many enthusiasts leave out or poorly install the plastic insulators around the connection post of the points. This results in the points being shorted to earth and the meter at position B will always read almost Zero. That is the same result as the points never opening when the engine is turned over on the starter motor. Possibly the condensor may also be faulty but this would be extremely rare. With that fine now we can move onto the High Tension side.

So each time the contact breaker opens a spark is created in the coil. This spark should travel down the High Tension lead (king lead) from the coil to the centre of the distributor cap. A carbon bush transfers this spark to the top of the rotor arm within the distributor. The spark travels along the rotor arm to each of 4 pins that are connected to each plug lead. The rotor arm turns within the distributor once every 2 revolutions of the engine and should line up with each plug lead when that plug is required to "fire" With the rotor arm facing its correct plug lead the spark jumps across the small gap between the rotor arm and the plug lead pin within the distributor cap, travels along the plug lead to the spark plug. The spark travels down the centre of the spark plug to the tip inside the engine. At the tip we have a centre electode that is connected to the top of the plug and another Electrode connect to the thread of the plug and is therefore earthed to the engine. Therefore the spark travels down the centre electrode and jumps the gap between the 2 electrodes at the tip of the spark plug finding its way to earth and creating a very hot spark to ignite the fuel within the engine cylinder.

Checking the high tension circuit is relatively easy.

The Coil: Connect a plug lead into the end of the coil, put a plug into the end of the plug lead and earth this against the engine or any earth on the car. Connect a wire from the battery plus to the plus connection on the coil and connect another wire to negitive terminal of the coil. The end of the second wire can now be touched onto earth and when released the plug should spark. If you are performing the test correctly and the plug does not spark then the coil is faulty.

When you have proved the coil is fine then connect the coil to the distributor by replacing the king lead. Remove the distributor cap and check out which plug the rotor arm is pointing to. Fit a plug into the end of the plug lead and again earth this plug against the engine or any earth on the car. Again with a wire connecting the plus of the battery to the plus on the coil and another wire to the negative of the coil. Touch this wire to earth and as you release it the plug should spark. If you have performed this test correctly and the plug does not spark Then a problem exists with either the carbon brush in the distributor cap, (can be tested with a meter set on resitance/continuity) or the rotor arm or the distributor cap. A fault in the distributor cap is generally what we refer to as tracking. This is when the high tension spark does not follow the normal metalic route in the cap but finds its own path on the surface of the cap. This tracking can often be seen by running a car in a dark garage. The tracking shows uo as a silvery line on the distributor cap and the only cure is a new cap.

Ignition Timing

When the ignition circuit actually works we have a system that as the contact breaker opens a spark at the tip of a spark plug occurs. Great because that can ignite the mixture in a cylinder and cause an engine to run. What else do we need?? Well it is simple we must have that ignition spark occur at exacrtly the right ime, hence the term ignition timing. Without too much of an explanation about the cycle of an engine I will simply explain how the timing should be set up. Basically the plug needs to fire when the piston is at about the top of it's stroke, but each piston in an engine arrives at the top of its stroke twice for everytime it fires (ignites). So the first thing we must do is make sure the spark occurs when the piston is about at the top of it's correct stroke.

There is a notch in the crank pulley at the front of the engine, this notch lines up with pointers attached to the engine and pointing at the pulley. Amongst the pointers is an extended one that when in line with the notch on the pulley will mean that the engine is at the top of it's stroke with number 1 cylinder and also number 4 cylinder. As was described earlier only one of these two cylinders should be firing and now we need o establish which one is firing. (If we were just checking the timing we would not need to worry about this but to correctly identify which plug lead goes to which plug then we must do the job thoroughly). To identify the correct cylinder we need to remove the rocker cover and inspect the number 1 cylinder rockers 1 and 2 and number 4 cylinder rockers 7 and 8. One pair should be loose and should have a gap, whilst the other pair will be tight and will have no gap. The pair that is loose will be the plug that should be firing. If this is number 4 for example then check which lead the rotor arm is pointing at and that should be connected to number 4 cylinder. It should be noted that the rotor arm turns ANTI CLOCKWISE in normal running and so the next plug lead anti clockwise needs connecting to cylinder 2, continuing clockwise will be 1 followed by number 3. If you have completed this sequence correctly then the plug leads/firing order will now be correct. (Firing order is 1.3.4.2.)

So now we have the right order of plugs firing we now need to set the timing of the spark exactly. There are however a couple of things we should understand before we attempt this relatively simple procedure.

We measure the point of spark timing in degrees that the crankshaft has turned from Top Dead Centre, TDC. Generally these measurements are in the region of 0 to 34 degrees Before Top Dead Centre, BTDC. The other important fact is that the exact spark timing is dependant on engine load and speed, meaning under heavy throttle (high load) the timing needs to be retarded and as engine speed increases (higher revs) the tming needs advancing. To this end the distributor has a couple of gadgets and properties that allow it to achieve these requirements. Everyone will have heard of Vacuum Advance, well that is the piece of the distributor that adjusts the timing depending on engine load. When the throttle is only slightly open (engine on Light load) the vacuum in the inlet manifold is very high and this suction advances the timing in the distributor. The opposite is when the throttle is fully open (engine on heavy load) the vacuum in the inlet manifold is very low and thus this lack of suction reduces the vacuum advance to perhaps zero and thus effectively retards the timing. OK what about engine speed. Another mechanism in the distributor is called the mechanical advance. Starting from no mechanical advance at no RPM, as the RPM increases so does the amount of advance. An average figure would be 24 degrees of advance by the time the engine RPM has reached say 4.5K RPM.

With modern fuels the ideal timing would be set at about 32 degrees BTDC at 4.5K RPM (maximum mechanical advance) with engine under full load (ie no vacuum advance). I say about 32 degrees BTDC because a figure of 28 to 34 degrees BTDC would make very little difference in overall performance. Years ago when our cars were first built, the company knew the full mechanical advance figure and so they would state a figure of x degrees BTDC as a static figure. This would be 32 degrees when - the mechanical advance was added to it which would then achieve the 32 degrees when the engine was at high revs. Nowadays many cars have different distributors installed and so many of he original figures for basic timing are just nonsense and of very little value.

So static figure (8°?) plus maximum mechanical advance (24°?) should equal 32° +/- 2°

In reality then how do we set this figure? Well we have the timing marks on the front cover and a notch on the front pulley. 2 types of timing lights are available. The best type can be preset at an advance figure, this means when set at 32°, the lamp will flash at 32°BTDC and thus in reality we should see the flash at the long pointer on the front cover lining up with the notch on the front pulley. EASY. The second type flashes when the spark occurs and thus when compared to the front pulley will flash at 32° BTDC and so this needs to be identified. It is possible to paint a mark on the front pulley at a place equivalent to 32°BTDC and then this can be used to time the ignition using the basic type lamp.

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