Why Your Chevy 350 Deserves an MSD 6AL Ignition

The small-block Chevy 350 is one of the most popular and versatile engines ever built. Whether you’re running a mild street cruiser or a high-compression race motor, the ignition system is the heartbeat of performance. A weak or inconsistent spark leaves power on the table, causes hard starts, and can lead to misfires under load. The MSD 6AL ignition control (part no. 6420) is a proven upgrade that replaces factory points or HEI modules with a capacitive discharge design. It delivers a series of rapid-fire sparks during each firing event, ensuring the fuel mixture ignites completely even in rich or high-rpm conditions. This article covers the nuts and bolts of installing and troubleshooting the MSD 6AL on a Chevy 350, from wiring diagrams to common failure points.

What Makes the MSD 6AL Different

The MSD 6AL uses a capacitor that stores energy and discharges it through the coil at the exact moment of ignition. Unlike a standard inductive ignition that fades at high rpm, the 6AL produces a full-voltage spark regardless of engine speed. Key features include:

  • Multiple Spark Discharge (MSD): The box fires a series of sparks that last for 20° of crankshaft rotation below 3,000 rpm. This extended burn time lights lean mixtures and reduces plug fouling.
  • Adjustable Rev Limiter: Two rotary dials let you set a hard rev limit from 2,000 to 10,000 rpm (6AL model) in 100 rpm increments. This protects the engine from over-rev damage during missed shifts or driveline failures.
  • Wide Trigger Compatibility: The 6AL accepts points, magnetic pickups (from MSD distributors or GM HEI), and even some aftermarket crank triggers. This makes it adaptable to nearly any Chevy 350 ignition setup.
  • Built-in Tach Output: A dedicated tach driver terminal feeds a clean 12V square-wave signal to aftermarket tachometers, eliminating the need for a separate adapter in most cases.

For a deeper dive into the technology, visit the MSD 6 Series product page.

Tools and Parts Checklist

Before starting, gather everything you need. A missing connector or wrong gauge wire can turn a two-hour job into an afternoon of frustration.

Required Components

  • MSD 6AL box (PN 6420) – includes harness, mounting hardware, and instruction manual.
  • MSD Blaster 2 coil (PN 8202) or MSD Blaster SS (PN 8203) – the 6AL requires a low-resistance, high-voltage coil. Avoid stock coils, which may overheat.
  • Wiring: 12-gauge red and black wires for power and ground, 18-gauge for trigger and tach.
  • Ring terminals, butt connectors, and heat shrink tubing.
  • Fuse holder with 30-amp fuse (MSD recommends a 30A inline fuse on the main power wire).
  • Optional: MSD tach adapter (PN 8910) if your tach needs a different signal.
  • Optional: MSD distributor (e.g., PN 8365) if you’re replacing a points distributor.

Tools

  • Wire stripper, crimper, and diagonal cutters.
  • Screwdrivers (flat and Phillips).
  • Socket set (3/8″ drive, standard and metric).
  • Multimeter (digital, with continuity and voltage testing).
  • Drill and bits if mounting the box requires new holes.
  • Electrical tape or heat gun for shrink tubing.

Installation: Step by Step

Work safely: disconnect the battery negative terminal and let the engine cool. The MSD 6AL stores high voltage even after shutdown; discharge the capacitor by grounding the coil’s positive terminal with a 1k-ohm resistor or by following MSD’s discharge procedure in the manual.

1. Remove the Old Ignition Components

Depending on your Chevy 350’s ignition type:

  • Points system: Remove the coil, points, condenser, and ballast resistor (if present). The 6AL eliminates the need for a ballast.
  • HEI distributor: Remove the stock coil and ignition module from the distributor cap. You’ll keep the distributor housing and magnetic pickup but run the MSD box to fire the coil. Some HEI distributors require a MSD HEI adapter (PN 8870) to connect the pickup wires.

Label the existing wires so you know which was battery positive, tach, and trigger. Take a photo for reference.

2. Mount the MSD 6AL Box

Choose a location away from extreme heat (exhaust manifolds) and moisture. Common spots on a Chevy 350 are the inner fender, firewall (near the brake booster), or a custom bracket on the intake manifold. The box should be mounted with the fins oriented vertically for airflow. Use the supplied screws or self-tapping sheet metal screws. Ensure the box is grounded to the chassis or engine block through its mounting tabs – but you’ll also run a dedicated ground wire (step 4).

3. Wire the Main Power and Ground

The 6AL draws up to 5 amps continuous and peaks higher during start-up. Use 12-gauge wire for both.

  • Red wire: Connect through a 30A fuse directly to the battery positive terminal or to a stud on the starter solenoid that is hot in both crank and run. Do not splice into the ignition switch circuit – the box needs a clean, unfused primary power source.
  • Black wire: Connect to a clean engine ground – the cylinder head, intake manifold bolt, or a dedicated chassis ground. Sand the surface to bare metal.

Do not ground the box through its mounting brackets alone; the dedicated ground is essential for reliable spark.

4. Trigger Input Wiring

The MSD 6AL can be triggered by three signal types. Choose your Chevy 350 setup:

  • Points or electronic distributor with a square-wave output: Connect the small white wire (points trigger) to the distributor’s points wire or the trigger output of an electronic module. The white wire senses a ground pulse.
  • Magnetic pickup (two-wire): Use the green and violet wires (Magnetic trigger). The 6AL’s magnetic input is polarity-sensitive – the green wire goes to the negative pickup lead, violet to positive. If using an HEI distributor, you need the MSD adapter (8870) or you can simply connect the HEI pickup wires to a two-pin Weatherpack connector; MSD sells a pre-built harness.
  • HEI module signal: Some installers keep the HEI module and use the module’s tach output as a trigger – but this is less common and can introduce noise. The cleanest approach is to bypass the HEI module and use the magnetic pickup directly.

Refer to MSD’s official wiring diagram for the 6AL (PN 6420) for exact pinouts.

5. Coil Connections

Use the supplied MSD Blaster coil. Mount the coil using the bracket in a location that keeps the high-voltage tower at least 1″ from metal surfaces to prevent arcing.

  • Orange wire: Goes to the coil’s positive terminal.
  • Brown wire (small black with white stripe on some harnesses): Goes to the coil’s negative terminal.
  • Coil ground: The coil itself may have a mounting bracket that grounds to the engine – ensure it’s making good contact.

Do not install a ballast resistor – the 6AL provides the correct voltage to the coil internally.

6. Tachometer Output

The 6AL has a blue wire (tach output). Connect it to the tach input terminal on your aftermarket tach. If you have a factory tachometer, you may need the MSD tach adapter (PN 8910) to convert the 12V square-wave to the older style signal. In many GM instrument clusters, you can also run the blue wire to the tach filter (if equipped) or directly to the tach’s input.

7. Rev Limiter Settings

Before starting the engine, set the rev limiter. The two rotary dials are labeled “1” and “10”. The dials combine to form a number; multiply by 100 to get the rpm limit. For example, set dial 1 to “6” and dial “10” to “0” for a 6,000 rpm limit. Consult the chart in the manual. For a stock Chevy 350, a limit of 5,500-6,000 rpm is safe. For a built engine, set it according to your cam and valvetrain limits.

8. Final Checks and Startup

  • Double-check all connections: power, ground, trigger, coil, tach.
  • Verify the fuse is installed and the battery is reconnected.
  • Turn the key to “On” (do not crank). Listen for the MSD box – it should emit a soft hum as the capacitor charges.
  • If you have a timing light, confirm that the spark is present at the plugs.
  • Crank the engine. It should start immediately once fuel is present. If not, proceed to troubleshooting.

Troubleshooting Common MSD 6AL Issues on the Chevy 350

Intermittent problems are common with any ignition upgrade. The following guide helps you isolate faults quickly.

No Spark, Engine Will Not Start

  • Check power and ground: With a multimeter, verify 12V at the red wire (fused side) during crank. Test for continuity from the black wire to battery negative. A poor ground is the #1 cause of no-spark.
  • Trigger signal missing: Disconnect the white or green/violet trigger wires. Use a test light. For points/white wire, you should see the light flash when cranking. For magnetic pickup, measure AC voltage between green and violet (should be 0.2-1.0V AC while cranking). If no AC voltage, the pickup or distributor wiring is faulty.
  • Coil primary resistance: Measure the Blaster coil primary resistance between the + and – terminals. It should be 0.7 ohms (Blaster 2) or 0.3 ohms (Blaster SS). Higher resistance indicates a bad coil.
  • Safety shutoff: Some MSD boxes have a built-in anti-theft feature (older models) or are sensitive to voltage spikes. Try a known-good box if available.

Weak Spark or Misfire Under Load

  • Spark plug gap too wide: The 6AL allows you to open the plug gap beyond factory spec. For a Chevy 350 with MSD, start at 0.045″ and go up to 0.050″ for performance. If the gap is excessive (>0.060″), the spark may blow out at high rpm.
  • Incorrect coil: Ensure you are using a compatible MSD coil. A stock coil will overheat and produce weak spark.
  • Ignition timing wrong: Verify initial timing with a timing light. The MSD box does not change timing; the distributor controls that. Check for a slipped distributor or incorrect timing curve.
  • Rev limiter activating prematurely: If the dials are set too low or if there is a miswire, the box will cut spark. Double-check the dial settings. Clean the dial contacts with contact cleaner.

Engine Starts but Stalls or Runs Rough

  • Trigger wire interference: The white or green/violet wires running near high-current cables can induce noise. Route trigger wires away from alternator, spark plug wires, and battery cables.
  • Faulty coil or spark plug wires: Use a multimeter to check plug wire resistance (should be under 1,000 ohms per foot). Check for carbon tracking on the coil tower.
  • Fuel mixture imbalance: A weak spark can mimic a fuel issue. After verifying spark output, check carburetor adjustment or injector pulse.

Box Not Turning On (No Hum, No Tach Signal)

  • Confirm the 30A fuse is intact. If it blows repeatedly, you have a short to ground on the red wire or the box is internally shorted.
  • Measure voltage at the box’s red wire (pins inside the connector). If power is present but the box is silent, the unit may be defective. Contact MSD support.

Advanced Tuning: Getting the Most From Your MSD 6AL

Once the system is running, you can fine-tune it for your specific Chevy 350 build.

Rev Limiter and Launch Control

The 6AL includes a separate two-step rev limiter function (some models) for drag racing. If you have the 6AL-2 or add a two-step module, you can set a lower rpm limit while the clutch or brake is engaged, allowing you to build boost or launch at a consistent rpm. For street use, the standard rev limiter is a safety net.

Coil and Plug Gap Tuning

Because the MSD 6AL produces a higher output voltage, you can open the spark plug gap to spread the flame kernel. Start with 0.045″ and increase by 0.005″ increments until you detect misfire at high rpm. Many enthusiasts find 0.050″ works well with a Blaster SS coil on a Chevy 350 with a mild cam and 9:1 compression. If you run forced induction or nitrous, reduce the gap to 0.035″-0.040″ to prevent blowout.

Timing Curve Modifications

The 6AL itself does not modify timing; that’s the distributor’s job. However, pairing an MSD 6AL with an MSD Pro-Billet distributor (PN 8365 for Chevy 350) allows you to change advance springs and stop bushings to shape the curve. Additionally, you can use an MSD Timing Retard box (PN 8732) for nitrous or boost applications to pull timing under load. The 6AL has a timing retard input (violet/white wire) on some versions; check your manual.

Installation Pitfalls to Avoid

  • Using the ignition switch as a power source: The MSD box demands high current; running it through the switch can overload the circuit and cause voltage drop. Always run direct battery power.
  • Poor grounding: The black wire must go to the engine block, not the chassis. The mounting screws alone do not provide a good enough ground.
  • Ignoring heat management: Mounting the box near the exhaust or inside the engine compartment without airflow can cause the unit to go into thermal shutdown. Keep it away from header tubes.
  • Reusing old spark plug wires: Old wires may have high resistance or insulation breakdown, defeating the high-energy spark. Use quality spiral-core wires (like MSD 8.5mm Super Conductor) to minimize RFI.
  • Incorrect trigger polarity: For magnetic pickups, reversing green and violet leads causes the box to fire at the wrong time (or not at all). Follow the color code exactly.

Conclusion

The MSD 6AL ignition is a transformative upgrade for any Chevy 350, whether you’re chasing tenths at the drag strip or just want a crisp throttle response on the street. With careful installation – paying close attention to power, ground, and trigger wiring – you’ll eliminate the weaknesses of factory ignitions and gain the ability to set a precise rev limiter. Should you run into issues, the troubleshooting steps above isolate the most common problems. Once dialed in, the 6AL provides years of reliable service. For parts and technical support, consult MSD’s official website or trusted retailers like Summit Racing and Jegs for wiring kits and adapters.