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Page created by T Mar 28th 2009:

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The Power Transistor at the top turns the Coil current on and off mulitple times for  each single trigger by the points or other trigger device. Photo by Scott_HG. Click to enlargen.

MSD 5:


The MSD 5 is MSD's entry level Multi Spark Ignition. It is not a CDI System. Merely Transistor Assisted. 

Safety Precautions:

*Important*  Internally these devices have potentially lethal voltages so any metering of the unit must be done with the unit completely disconnected.   Never allow any of the leads to contact any of the others, +ve or ground unless they are being connected for installation. Always make sure the Battery is disconnected and the Ignition Switch is turned *off* before making any connections or disconnections to these units.  


Visual Inspection:

 A failed component can show evidence of burning either on itself, the Printed Circuit Board or both. Any burning is evidence of either bad design because not enough provision was made for the component's heat to be carried away or failurefrom overloading, typically a load was placed on the unit that was greater than the designer intended. Replacing a burned component may involve increasing itsPower Rating. If it's a failed 1/4 watt Resistor a 1/2 watt or 1 watt Resistor with some Heatsinking might be considered.  Failed Transformers have an unpleasant stink, show black and may have varnish oozing from them.If the Transformer cannot be replaced it can sometimes be rewound. Rewinding the transformer involves carefully taking it apart, counting the number of windings of each wire, the grade of wire and noting the direction each Wire was wound in. The direction of the Wire is called the Sense and this determines how the Wire will respond to +ve or -ve Voltage. The Former (the part the Wires are wound onto) can often be reused.   The Transformer is rewound with exactly the same number of turns of each wire, the same grade and in the same sense. Any blown Fuses will indicate a shorted Electronic Device like a Power Transistor or  Diode. 


Check any Fuses for obvious signs of blackness.  Test for continuity by setting the Multimeter to the Diode or Ohms Range. The Fuse should give a continuos beep in both Directions.Get a Fuse of the same type but don't replace it until you've done a full Meter check of the Ignition otherwise you may find your new Fuse blown too.


=Diode Testing:

With the Multimeter set to the Diode range, place the Probes across each Diode. The Meter should give a single beep in the forward direction and nothing at all in the reverse direction. IF it delivers a solid beep in both directions the Diode may be blown or connected across a Transformer or low valuse Resistor. Unsolder the Diode from the Circuit Board and test it again.  If it gives a continuous beep in both directions it's blown.If it doesn't beep at all in either direction it's also blown. When large Diodes fail they usually go short circuit making the Meter give a continuous beep when connected in the forward or the reverse direction.  When small Diodes fail they usually go open circuit making the Meter give no beep at all when connected in the forward or the reverse direction.  

Transistor Testing:

When a Mutlimeter set to the Diode Range is used to test Transistors, a good Transistor behaves like Diodes connected together. There will be one Diode between Base and Emitter. There will be another Diode between Base and Collector. Use the Multimeter between these conections just as if they were Diodes. Disconnect the Transistor Legs if the behaviour is unpredictable.If the legs behave like failed Diodes then the Transistor has blown. When large Transistors fail they usually go short circuit making the Meter give a continuous beep when connected in the forward or the reverse direction. When small Transistors fail they usually go open circuit making the Meter give no beep at all when connected in the forward or the reverse direction. 

Resistor Testing:

Set the Multimeter to the Ohms Range. When the Meter is connected across a Resistor it should give a reading equal to or less than the Resistor's value. The Resistors value is determined by the coloured bands it has. You can also consult the Circuit Diagram and use the Resistor's number to match the Meter's valueagainst the correct value. When in doubt unsolder one leg of the Resistor and take a reading. Make sure you don't touch the Resitor with any part of your body or you will get a false reading. 

Capacitor Testing:

Set the Meter to the Diode Range. Unsolder one leg of the Capacitor you want to test.Connect the Meter to the Capacitor. If the Capacitor has +ve and -ve markings, connect the red lead to the +ve on the Capacitor and the black lead to the -ve of the Capacitor. Don't allow any part of your body to touch the Capacitor legs. Leave the Meter connected for about 20 seconds or until the Meter's Display settles. At this time you are charging the Capacitor. Remove the leads and change the Meter's setting to DC Volts. Reconnect the Meter's leads red to +ve and black to -ve and don't let any part of your body touch the Capacitor's leads. You should briefly see the Meter's Voltage appear on the Meter's Display (typically 9 volts). The Voltage will gradually get smaller with time. If no Voltage appears the Capacitor has failed. Large Capacitors will take a long time to charge and a long time to discharge. Small Capacitors will take a short time to charge and a short time to discharge.






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