Electrical Systems on Meriden Triumphs
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DE-MYSTIFYING ELECTRICAL SYSTEMS ON MERIDEN TRIUMPHS By Pete Kinlyside – TriumphRat.net “OzBloke” This paper attempts to explain the intricacies of Meriden Triumph electrical systems. The idea for the paper was born from my own challenges with these systems, and from the many electrical questions being raised by other owners on the TriumphRat.net Classics web forum. I’ve tried to explain how things work in two formats; the simple non-technical format, and the deeper technical format. It’s not meant to replace workshop manuals, but to supplement them. Hopefully it will help those who want to maintain and fault-find these sometimes frustrating but always amazing older bikes. First, some terms you might see or hear when working on your bike, and which I use in this paper Term Meaning Earth The common return path for electrical current, usually the frame of the bike, plus the engine. Aka: frame, chassis or ground. Voltage The measure of electricity force or “pressure”, measured in Volts Current The measure of electricity flow, stated in Amps Resistance The measure of resistance of a certain component to current flow, measured in Ohms ( Ω ), or Kilohms (1000 Ohms or K Ω ), or Megohms ( 1 million ohms or M Ω ) Short aka. Short circuit. A path for current that bypasses vital components – usually associated with a fault, resulting in high current flow, burnt wires, blown fuse, bad smells, and sometimes fire. Open Circuit A broken path, which does not allow current to flow, Usually associated with broken wires, poor connectors, blown fuse, or faulty components. High-tension The high voltage connection from the secondary of the ignition coil to the spark plug – the spark plug lead. RFI Radio Frequency Interference. Electronic noise radiating from electrical components due to magnetic fields generated by current flowing through a conductor. 1. Ignition Systems This section explains the inner working of Meriden Triumph ignition systems, from the basics such as the theory behind the system, to the more advanced areas like Boyer ignition systems. Ignitions systems are built from a combination of components. For most older bikes, these are: The battery The fuse The ignition switch The engine kill switch The points (aka breaker points, contact points) The condenser (aka capacitor) The coil(s) The spark plug(s) The wires between the various components, in particular the plug leads. For the sake of simplicity, take it for granted that I’m explaining things using a single cylinder at this point. Not much difference with a twin or triple cylinder engine, where most components are duplicated, except when you get into the electronic systems such as Boyer. Simple Science One terminal of the battery is connected to earth. Most modern bikes are negative ground, while the older Meriden Triumphs are positive earth. Positive earth (or positive ground) means that the positive terminal of the battery is directly wired to the frame/engine of the bike to provide the return path for the electrical current. The other terminal of the battery is wired to the fuse, then from the other side of the fuse to the ignition switch, and from there to the kill switch, and from the kill switch to one side of the coil (in the Meriden case, the negative terminal of the coil). With the ignition switch off, no current flows to the coil, and no spark can be produced. With the ignition switch and the kill switch both in the “on” position, 12 volts is applied to one side of the coil. The coil is basically a 1:100 transformer. It has a primary side, being the two screw/blade terminals on top for positive and negative 12 volts connections. The other half is called the secondary or high tension winding. One end of the secondary winding connects to the negative terminal, while the other end connects to the insulated cup connector on the top, into which the plug lead is connected. The set of points, operated by the camshaft, acts as an on/off switch for the positive side of the coil to earth. When the points are closed, current flows from the battery, through the fuse, through ignition and kill switches, through the coil, through the points contacts, to the engine, and finally back to the positive side of the battery via wires and/or frame. At this stage, the condenser, which is wired across the points, is not charged, as the points short it out. Diagram 1 illustrates the components and the current path. Ign Sw Kill Sw Fuse Condenser Battery Spark Plug Current Flow Coil Points DIAGRAM 1 This current flow quickly creates an electromagnet inside the coil. While the points are closed, this magnetic field remains stable. At a point in the rotation of the camshaft, a lobe on the points cam causes the points to open. Current is no longer flowing through the coil, and the magnetic field collapses. This collapsing magnetic field causes a voltage to be generated in the secondary coil. The condenser comes into play now, as it’s no longer being shorted by the points. It has a dual function – to help pump up the primary voltage, and to drastically reduce sparking across the points. The primary voltage goes up to around 300-400 volts, and the secondary voltage leaps to 20-30 thousand volts. This secondary voltage is sufficient to jump the gap between the spark plug centre electrode and the side (ground) electrode, and current flows from the coil secondary, through the spark plug, through the engine to the frame, through the condenser, to the negative terminal of the coil. When the voltage being developed is no longer sufficient to jump the gap, the spark stops. Extra-Technical Explanation As the points open, the magnetic field around the primary winding of the coil begins to collapse. This induces a voltage across the secondary and primary windings. The condenser initially acts as a short until it starts to charge. This stops arcing across the points contacts. As the voltage builds across both the secondary and primary, no current is yet flowing through the secondary, as the current path is not yet establish across the spark plug gap. Around 400 volts can be developed across the primary winding as the condenser becomes fully charged. Depending on the spark plug gap, and the state of the fuel vapour /air mix between the plug electrodes, at a certain voltage, the vapour will ionise, and allow current to pass. This commences the spark. As the current flows through the secondary winding, the condenser discharges through the primary, thereby inducing more voltage across the secondary, which elongates the spark time. This “loop” effect continues in a decreasing cycle until the capacitor no longer holds sufficient charge to induce enough voltage in the secondary to maintain the ionisation of the plug gap, which is when the spark stops. Without the condenser, the spark will be of very short duration, and quite weak. The points will also arc on each opening, causing pitting of the contact surfaces and eventual breakdown. Faults in the ignition system Note: Voltages in the ignition circuit can be harmful, and painful! Do not touch connectors or components with the ignition on. Resistance testing of ignition components must be done with the fuse out and ignition switch off, and preferably with the component completely removed from the electrical circuit. Testing of the condenser can be done by using a multimeter on the highest resistance (20M Ω - 20 Mohms) range. Take one of the bike electrical connections off the condenser, and connect the probes across the component, watching the meter display as you connect the probes. The meter should indicate a low initial resistance, with a rapid increase in resistance as the condenser charges over a period of a few seconds. The meter should, after no more than 5 seconds, indicate infinite resistance. If it shows some steady high resistance (eg 10Mohms), or slowly decreases after initially going high, the condenser needs to be replaced. Make sure your fingers aren’t touching the metal portion of the probes when you do this test, as it will give false readings. The ignition condensers can be tested in either polarity, with the same results. If reversing polarity immediately after a test, the initial meter reading may be false due to the charge on the condenser from the meter. Coils can go faulty in a number of ways. The primary can go open circuit due to vibration or heat (continuous current for extended periods – burnout). The secondary can go open, or can short turns (a current path between layers of the winding, resulting in greatly reduced output), or a short to the outer case. Testing of the coil is a 4 step process. First take all connections off the coil, including the high-tension lead (plug lead). Using a multimeter on low resistance range (200 Ω ), check the resistance between the two low tension (primary winding – 12 Volt) connectors. 6 volt coils should read around 2 to 2.5 ohms. 12 Volt coils should read between 4 and 5.5 ohms. If higher, it could be faulty or a higher voltage (eg 24v) coil. If lower, shorted turns are the most likely culprit – replace. Next, check the secondary winding by a resistance (meter on 20K Ω range) check between the negative 12V terminal and the high-tension output (the brass connector inside the tower where the plug lead goes). This should be around 5 to 6 K Ω . The last check is between either the negative terminal, or the high tension terminal, and the metal casing, with meter on highest Ω range.