MSD 6AL 6420 Ignition Control Instruction Manual
MSD 6AL 6420 Ignition Control
MSD 6AL 6420 Ignition Control Instructions
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MSD 6AL 6420 Ignition Control Specifications
| Feature | Specification |
|---|---|
| Model / Part Number | MSD 6AL, PN 6420 (MSD 6 Series) |
| Ignition Type | Capacitive Discharge (CD) |
| Capacitor Charge Voltage | 460 - 480 volts |
| Capacitor Charge Time | Within one millisecond |
| Multiple Spark Duration | 20° of crankshaft rotation |
| Multiple Spark Range | Multiple sparks below 3,000 rpm; single spark above 3,000 rpm |
| Rev Limiter | Built-in Adjustable Soft Touch Rev Control |
| Rev Limit Adjustment | Plug-in RPM modules, available in 100 rpm increments |
| Rev Limit Accuracy | Within 1% of selected rpm |
| Included RPM Modules | 4 modules: 3,000, 6,000, 7,000 and 8,000 rpm |
| Electrical System | Negative ground, 12 volt, with distributor |
| Battery Compatibility | 12 volt and 16 volt batteries |
| Momentary Voltage Tolerance | 24 volts (jump starts) |
| Full Output Supply Voltage | 9 - 18 volts |
| Minimum Operating Voltage | 5 volts |
| Minimum Cranking Voltage (Small Red Wire) | 10 volts |
| Tach Output Signal | 12 volt square wave, 20% duty cycle |
| Power Leads | Two 12 gauge wires (Heavy Red and Heavy Black) |
| Fusing | Internal fuse (no external fuse required) |
| Trigger Inputs | Points/amplifier circuit (White wire) or Magnetic Pickup (Violet + / Green -) |
| Default Cylinder Programming | 8-cylinder; 4 or 6-cylinder even-fire via Red/Blue wire loop cylinder select |
| Mounting Hardware | 4 vibration mounts; 3/16" drill bit for mounting holes |
| Wire Extension Gauge | 10 gauge for power leads, 16 gauge for other wires |
| Circuit Protection | Thick conformal coating of Humi-Seal |
| Quality Control | Four hour burn-in test |
| Included Harnesses | PN 8860 and PN 8861 |
| Battery Requirement Without Alternator | At least 15 amp/hour for every half hour of operation |
| Warranty | One year limited warranty from date of original purchase |
MSD 6AL 6420 Ignition Control Details
The MSD 6AL 6420 is a capacitive discharge ignition. It is built to fix the weak spark that inductive ignitions produce at high rpm. A stock coil has less time to build up energy as engine speed rises. The 6AL stores energy in a capacitor instead, so the coil gets full voltage at every firing. The Soft Touch rev limiter gives a load-free limit by dropping the spark to individual cylinders. It does not cut ignition to the whole engine.
The unit has some compatibility limits. It cannot be used with distributorless ignition systems (DIS) or on vehicles that came from the factory with a CD ignition. It also cannot be used with solid core spark plug wires, because the electromagnetic interference they produce disrupts the MSD. The manual recommends helically wound wire such as MSD Heli-Core or 8.5mm Super Conductor. The 6AL works with most stock and replacement coils. Some race-only coils, such as the Pro Power Coil PN 8201, are not compatible.
For spark plug gaps, MSD offers starting points based on compression ratio. Use 0.050" - 0.060" up to 10.5:1, 0.040" - 0.050" between 10.5:1 and 13.0:1, and 0.035" - 0.045" above 13.0:1. You can then open the gap in 0.005" steps while testing. Route plug wires for cylinders that fire one after the other away from each other. Otherwise, induced voltage can cause detonation. This applies, for example, to cylinders #5 and #7 on a Chevy V8.
The unit can be mounted in almost any position except directly upside down, because moisture would not be able to drain. Keep it away from direct engine heat and do not install it in enclosed spaces like a glovebox. Do not seal the unit. If you are welding on the vehicle, disconnect both heavy power cables and the tach ground wire first. A ballast resistor in the coil wiring should be bypassed. Route the magnetic pickup wires separately and twist them together.
The ignition makes it easy to add a theft deterrent kill switch. If the White wire triggers the unit, grounding the Violet wire through a switch lets the engine crank without starting. If the magnetic pickup triggers the unit, grounding the White wire does the same. A 100V/1A diode is included to fix engine run-on on older vehicles with external voltage regulators. It goes on terminal #1 on Ford, terminal #4 on GM, and the small brown wire on 1973-1983 GM Delcotron alternators.
Some factory tachometers need an adapter. Use the PN 8910 with White wire triggering and the PN 8920 with magnetic pickup triggering. Some fuel-injected foreign vehicles may also need the PN 8910 or PN 8910-EIS to run. Toyotas and Ford Probes always need the PN 8910-EIS. On GM vehicles with an inline filter, bypass the filter if the tach drops to zero.
The manual includes a spark check procedure. With the key On, you can tap the White wire to ground or short the Green and Violet wires together, and a spark should jump to ground from the coil wire set about 1/2" away. The manual also includes wiring diagrams for GM HEI, Ford DuraSpark and TFI, Chrysler, Jeep, Honda, Mallory, Pertronix, timing controls and dual MSD setups.
Frequently Asked Questions (FAQs)
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