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Page created by T Apr 22nd 2009:

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Inside view of an MSD 6AL Capacitive Discharge Multi Spark Discharge Ignition control unit. Click to enlargen. Photo by HPEngines.



The MSD 6AL is a Capacitive Discharge Multi Spark Discharge (as the name implies) ignition with a programmable rev limiter. Below 3,000 RPMIt provides multiple sparks at each trigger. Above 3,000 RPM it provides a single spark. 


 *Important*These ignition systems produce lethal voltages internally and externally. Never allow any part of your body to come in contact with any bare Wire or Contact. 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.  
Reluctor Star. Photo by Jacks. Click to enlargen.


Capacitive Discharge Ignition:

Like all CDI's, this ignition takes 12 volts from the Battery, boosts it to 400 volts and dumps the 400 volts across the ignition Coil whenever a Piston reaches TDC. A high voltage short duration Spark appears across the Spark Plug at this time.  A Device called a Silicon Controlled Rectifier connects the 400 Volt charged Capacitor to the Coil at Spark time.

Magnetic Pickup Phasing:

Phasing of the Pickup wires is different between Bosch HEI and MSD. The small signal wires on connectors 3 and 7 of the HEI Ignition Module (Note: *not* the pink and green wires found on pins 15 and 16 of the HEI Ignition Module but the leftmost 2 Wires as in the Photo by Jacks) need to be swapped over so that the trigger occurs when the Reluctor is approaching the peak and not as it moves away. 
Photo by Jacks showing the module and where the large and small pickup coil connections are made.

Photo by Jacks. Showing the removed Pickup Coil and the 2 small signal wires one with large connector, the other with a small connector to preserve the phasing when used with a Bosch Ignition Module. Click to Enlargen.

Connecting to Holden Blue Motor Type Bosch HEI Distributor:

Thanks for you help T and others. The wiring is like this for blue HEI. No voilet and green wire from the MSD, you use the white wire [for points setup]it connects to the green wire from the dissy and the pink wire you bridge into 12volts with the small red wire from the MSD.She runs really well.
End of submission by Holden Fan. Essentially the original Bosch HEI Ignition Module is kept in place. The small signal pickup wires remain connected to the Bosch HEI Ignition Module. The original coil green wire which connects to pin 15 on the Bosch HEI Ignition Module is connected to the white wire and does the triggering. The MSD 6AL then considers the Bosch HEI Ignition Module to be a set of points. Follow the MSD link below for full details of connecting all wires. 


Safety Precautions:

*Important*  Internally these devices have potentially lethal voltages so any metering of the unit must be done with the unit completely disconnected. Capacitor C15 will have up to 400 volts stored on it so this device should be discharged before work commences.  C15 has a 1 megohm Resistor connected across it so it will eventually discharge. If your Multimeter has a 1000 Volt DC Range, set the Meter to this Range and measure the Capacitor Voltage an hour after removing the unit. If in doubt leave the unit overnight before measuring the Voltage. Once the Capacitor Voltage reads 0 it's safe to work on the unit.  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. Here is a diagram of the internals of a MSD6. Link See page 15 (fig 4) in particular.
The most common thing that goes bad is the two transistors Q15 and Q16 and the fusible link which is a small length of thin wire on the circuit board up near the transformer end.
The transistors are nowadays TIP36 (plastic ones) not the 2N5884 like on the diagram. They are the early all-metal ones. You need a torx driver to get the board out, but they are pretty tame otherwise. End of submission by Circlotron.  Fixing MSD   If you find the inverter transistors have popped, the resistors that go across their base/emitter connections will generally be cactus as well. End of submission by Circlotron.  

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 failure from overloading, typically a load was placed on the unit that was greater than the designer intended.  Replacing a burned component may involve increasing its Power Rating to make sure it won't burn out again. 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. 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.  Sometimes a Fuse is simply a length of Wire soldered to the Printed Circuit Board.   Test for continuity of the Fuse by setting the Multimeter to the Diode or Ohms Range. The Fuse should give a continuous 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 the Meter delivers a solid beep in both directions the Diode may be blown or connected across a Transformer or low value Resistor. Unsolder the Diode from the Circuit Board and test it again.  If it gives a continuous beep in both directions it's blown (short circuited).If it doesn't beep at all in either direction it's also blown (open circuited). 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 connections 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 value against the correct value.  
  Band 1
Band 2
 Band 3
    1st Digit
 2nd Digit
 Black  00
 1 1% 
 Brown  11
 10 2% 
 Red  2 2 100  
 Orange  3 3 1000  
 Yellow  4 4 10000  
 Green  5 5 100000  
 Blue  6 6   
 Violet  7 7   
  8 8   
 White  9 9   
 Gold      5% 
 Silver     10% 
  Example: Brown Black Black Gold = 100 Ohm 5% 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. If the Output Capacitor is shorted the Inverter will not start.The Output Capacitor may become Voltage sensitive meaning that it will pass a Meter test but fail once the 400 volts from the Inverter is applied.  

Integrated Circuits:

Integrated Circuits are probably best tackled by reading the number on them and simply replacing them. They have complex Circuitry inside them that can fail in unpredictable ways.  Only replace the IC after all the other components have been checked and replaced where necessary otherwise an existing fault in the Circuit may cause your new IC to fail.


A Silicon Controlled Rectifier is connected at the output of the Transformer Secondary Winding. The SCR is a special type of Transistor that can switch high Voltages. If this device has failed it will probably be shorted across 2 or more of its legs. This short circuit will prevent the Inverter from starting. Unsolder the legs of the SCR and meter the legs for shorts. These devices can fail under load or high Voltage even though they may pass a Meter test. Replacing the SCR will give the fastest results. The SCR may also be called a Thyristor. As a device the SCR is triggered into the on state by a low Voltage being applied to the Gate leg. The SCR will then stay hard on until the Voltage at its input falls to 0 at which time it will reset. The device will then remain off until another positive pulse is applied to the Gate leg.  

The Inverter:

If you've replaced Resistors that have blown open, the Fuse and any Transistors or Diodes that have been found shorted yet the Inverter won't start, remove the SCR, Output Capacitor and any other components from the Transformer Secondary Circuit.  Now see if the Inverter will start. A high pitched whistle will be evident and DC Voltage will be measured at the Diodes though not the full 400 volts. Should be around 280 volts. REmember that lethal Voltages are present so don't allow any part of you body to touch anything metal. Only the Multimeter probes should make contact. The Black with ground and the Red with the component under test.  If it does start one of the Output Components is shorted. If it won't start one of the Input Components is shorted or the Transformer itself has failed due to an internal Short Circuit. 


Once the Components have all checked out, connect the Ignition back up to the Engine and give it a trial Start.If no response, turn the Ignition off, disconnect the Battery and remove the Ignition. Recheck all the above steps starting with the Fuse.  


HPEngine's Shed

Thread 1

Thread 2

Bosch HEI

MSD 6 Series Information



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