Page created by T Apr 22nd 2009:
- 1 MSD 6AL:
- 1.1 Description:
- 1.2 Safety:
- 1.3 Operation:
- 1.4 Capacitive Discharge Ignition:
- 1.5 Magnetic Pickup Phasing:
- 1.6 Connecting to Holden Blue Motor Type Bosch HEI Distributor:
- 1.7 Repairing:
- 1.8 MSD 6AL Circuit Analysis:
- 1.9 Testing:
- 1.10 Links:
- 1.11 Terms:
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.
Safety:*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.
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.
Connecting to Holden Blue Motor Type Bosch HEI Distributor:
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.
Inside a CDI system is a Circuit called an Inverter. The Inverter converts the 12 volts to 400 volts by pulsing the Battery voltage across a Transformer many times a second and much faster than the Spark Rate. The Inverter uses the ratio of turns between the Primary and Secondary Windings in the Transformer to generate the 400 volts. Because the Inverter runs much faster than the Spark Rate, Spark Energy remains high compared to earlier Ignition Systems that began to lose Spark Energy as the RPM increased.
Single Stage Inverter:
The simplest Inverter is called the Single Stage Inverter. It uses only one Transistor. When power is first applied to the Inverter, the Transistor starts to turn on causing a large Current to flow through the Primary Winding of the Transformer. At the same time a Feedback Voltage is generated in the Feedback Winding of the Transformer. The Feedback Voltage at this time is +ve (the black dots show the Transformer Taps that are in phase) which drives the Transistor as hard into conduction as possible. This makes sure the maximum current flows through the Transformer. Simultaneously a large Voltage is generated in the Secondary (Output) Winding of the Transformer which conducts through two of the Diodes and charges the Capacitor. The Diodes are like Electronic Valves. They make sure that the Transformer Output Voltage always flows in the proper direction to charge the Output Capacitor. Inside the Transformer is the Former. The Former is the item onto which the Transformer wires are wound . The Former can only hold so much Magnetism. Once the maximum amount of Magnetism the Former can contain is reached the Transformer is called Saturated. The instant the Former becomes Saturated the Feedback Voltage changes from the positive direction to the negative direction because the Feedback Winding only generates Voltage while the Magnetic field is building up or collapsing. In other words the Feedback Voltage goes from +ve to -ve and it causes the Transistor to turn completely off. At the instant the Transistor turns off a large Voltage is induced in the Output of the Transformer but in the opposite polarity from when the Transistor was turned on. At this time the other 2 Diodes connect the output Voltage to the Capacitor. The collapsing Magnetic Field induces a strong -ve Voltage in the Feedback Winding making sure the Transistor stays completely off.Once the Magnetic Field has completely collapsed, the Feedback Voltage becomes 0 and the Transistor begins to conduct again restarting the whole cycle. When it is working normally a working Inverter makes a high pitched squeal, similar to the sound a Photoflash makes between photographs.
Dual Stage Inverter:
The Dual Stage Inverter works exactly the same as the Single Stage Inverter. The only difference is that one Transistor turns on while the other is off. When the first Transistor turns off, the second Transistor turns on. Once again control of the Transistors is done by the Feedback Windings F1 and F2. These windings are wound out of phase. When one Winding is producing a +ve Voltage the other is producing a -ve Voltage. When the Transistors change state, the Winding Voltages change with them. The output power from this Circuit is higher even though the unloaded Voltage is the same. In a typical Circuit R1 will be a lower value Resistor than R3 to make sure Q1 turns on first. This makes sure the Inverter starts properly.
A standard CDI Ignition system dumps 400 volts across the Coil each time the Engine reaches TDC on the Firing stroke. This CDI Ignition system dumps 400 volts multiple times across the Coil each time the Engine reaches TDC on the Firing stroke. The Mutli Spark function in an MSD 6AL is generated by a circuit called a Multivibrator which is sometimes called an Astable Mulitvibrator. This circuit is the same as an Electric Bell. When Current is applied to it it turns on and off. If you connected a speaker to it you would hear a buzzing sound like a doorbell buzzer. The Circuit operates a little like the Transistors Q1 and Q2 in the Dual Stage Inverter. The first Transistor turns off the second Transistor for a brief period, then the second Transistor turns on. When the second Transistor turns on again it turns the first Transistor off again. The output of the Multivibrator Circuit makes the SCR turn on and off multiple times generating multiple Sparks for each Piston in the Firing position.
MSD 6AL Circuit Analysis:MSD 6Al Circuit Page 13: The top Circuit shows a connection for Breaker Point Distributors.Page 13: The top Circuit shows a connection for Magnetic Pickup Distributors such as Bosch HEI as fitted to Blue and Black Motor Holdens..Page 13: The bottom circuit shows the various power supplies that are generated from the Battery supply. Page 14: Q1, Q2, D3 and C6 form a differentiator.Page 14: Q4, Q5 and Q6 form a monostable.Page 14: Q7, Q8, Q9, Q10, Q11 and Q12 form a Mutivibrator.Page 14: Q13 buffers the Multivibrator from the Inverter Stage . Page 14: Q17 and Q18 drive the Tachometer. They amplify the signal from the Breaker Points or Magnetic Pickup. Page 15:Single Stage Inverter and CDI Output.
Circuit Diagram and Support Info:
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
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..
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.
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.
|Colour || Band 1||Band 2 || Band 3||Tolerance || |
| 1st Digit|| 2nd Digit ||Multiplier|
| Grey ||8||8|
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 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.
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.