Complete Guide to Chevy 350 Ignition Coil Wiring Schematic

For reliable spark delivery in a General Motors 5.7L Vortec or older small-block V8, route the 12V constant-power feed directly to the heavy-gauge “BAT” terminal on the HEI module–resist the temptation to splice it lower on the harness. The factory-specified 10-gauge wire carries up to 30 amps under crank, so downgrading to 12 or 14 AWG risks voltage sag and hard-start symptoms.

The “IGN” input–typically a 14-gauge purple lead–must receive switched ignition voltage only; tie it straight to the ignition-on circuit bypassing any ballast resistors. If your ECU expects a tach signal, strip the 18-gauge tach output (tan in most harnesses) and run it clean to the ECU’s RPM pin–avoid splicing through aftermarket gauges, as their low-impedance inputs can scramble the waveform.

Ground the module through both the mounting screws and a dedicated 10-gauge wire bolted to bare engine block metal. Painted or coated surfaces create high-resistance paths; scrape them down to clean iron before tightening. For a Pertronix or MSD retrofit, disconnect the original pick-up coil leads and land the new magnetic sensor wires on the corresponding three-pin connector–white/black to (+) trigger, gray to (-) trigger, purple to ECU input. Never leave any terminal floating; a single open circuit will kill spark across all cylinders.

On early 1970s cast-iron distributors lacking a built-in module, mount the external can adjacent to the cap and run the two primary wires (red and white) through a grommet into the engine bay. Terminate the red lead on the ignition switch side of the solenoid, guaranteeing 12 V only when the key is in RUN. The white lead connects to the distributor’s magnetic pick-up; polarity reversal here flips spark timing 180°, so verify continuity with a multimeter before final crimp.

For forced-induction builds pushing boost over 12 psi, upgrade the stock high-voltage tower lead to 8 mm silicone wire and shorten it to 6 inches to minimize inductive losses. Route it away from rubber fuel lines and exhaust manifolds–minimum 2-inch clearance prevents arcing through melted insulation. Test coil resistance at operating temperature: primary should read 0.4–0.6 Ω, secondary 6 000–10 000 Ω. Readings outside these ranges signal internal shorting and mandate immediate replacement.

GM Small-Block 5.7L Spark Inductor Connection Guide

Locate the positive terminal on the distributor’s HEI module–marked with a + symbol–and connect it directly to a 12V source through a dedicated feed wire (typically 10-12 AWG). Use a fusible link or inline fuse rated for 20A to prevent overload. This bypasses the internal resistor, ensuring consistent voltage under high RPM conditions.

Attach the negative terminal (marked –) to the distributor’s ground post or a clean chassis ground point. Avoid daisy-chaining grounds; a single, dedicated path reduces voltage drop and misfire risks, especially in damp or corroded environments. Test ground continuity with a multimeter–resistance should read below 0.5 ohms.

For points-style distributors, use a 1.3–1.5 ohm ballast resistor inline with the +12V feed to the inductor. This stabilizes current flow, preventing premature component failure. Omit the resistor for HEI systems, as they regulate voltage internally via the module’s built-in circuitry.

When retrofitting an aftermarket MSD or Pertronix setup, wire the inductor’s positive lead to the control box’s tach output (labeled TACH or COIL+). Route the negative lead to the box’s ground terminal. Check manufacturer specs–some systems require a 1.5-ohm resistor in series to match OEM specifications.

High-output inductors (e.g., ACCEL, Mallory) often demand 14-gauge or thicker wiring to handle increased current draw. Replace stock connectors with soldered or crimped terminals and heat-shrink tubing to eliminate oxidation-related failures. Avoid twist-and-tape methods; they degrade under heat cycles.

On dual-spark setups (e.g., for supercharged engines), split the +12V feed using a junction block rated for 30A. Each inductor’s negative terminal must ground separately–shared grounds cause crossfire and erratic timing. Label all wires to simplify future diagnostics.

For ignition systems paired with capacitive discharge (CDI) units, verify polarity before connection. Most CDI boxes reject reverse-wired inductors, leading to no-spark conditions. Consult the CDI manual–some require a dedicated power feed, while others share the main battery line.

Inspect the wiring harness for brittle insulation or exposed conductors near exhaust manifolds or moving components. Replace damaged sections with temperature-resistant silicone-jacketed wire (rated for 200°C+). Secure all runs with loom or spiral wrap to prevent chafing, and zip-tie near pivot points to reduce fatigue stress.

Determining Proper Leads for GM Small-Block Engine Spark Output Setup

Locate the positive terminal marked “BAT” or “+” on the OE-style inductive unit first. This post connects directly to the 12-volt feed coming from the ignition switch or ballast resistor, depending on whether the engine runs a points-based or electronic HEI distributor. Use a multimeter set to DC volts; probe the wire while cranking–steady 9-12 volts confirms a clean power lead.

Find the negative post labeled “TACH,” “TACH OUT,” or “-“–this lead links to the distributor’s trigger source. For breaker-point systems, it attaches to the coil side of the points; with electronic distributors, it connects to the control module’s tach output pin. Check resistance between the negative post and engine ground; 0.3-1.0 ohms indicates proper continuity.

Trace the high-voltage cable from the tower to the distributor cap center terminal. Ensure the wire gauge meets 7-8 mm diameter with spiral-core suppression to prevent radio interference. Replace any brittle, cracked, or corroded leads–faulty insulation causes misfires under load, especially above 3,500 RPM.

Verify the ballast resistor value if equipped–typically 1.3-1.5 ohms for stock GM engines. Bypass temporarily with a jumper wire; if engine runs smoother, the resistor may be failing. Electronic systems omit this component, so confirm the setup matches the ignition system type before making connections.

Inspect the harness connectors for the control module (if used). Pin assignments vary: GM HEI modules use a 4-pin connector where pin A is ground, B is switched 12V, C is tach output, and D is signal input from the pickup. Swap any terminals showing oxidation or heat damage–corrosion increases resistance, causing erratic spark delivery.

For aftermarket units like MSD or PerTronix, consult the manufacturer’s pinout guide. Most require a dedicated 12V feed, ground, and tach signal–never assume compatibility with stock wiring. Use heat-shrink tubing on splices; solder connections for durability, then wrap with electrical tape to prevent short circuits near moving parts.

Start the engine and monitor secondary voltage with a spark tester. Weak or intermittent output at idle suggests incorrect lead attachment, ground faults, or a failing ignition amplifier. Scope-pattern diagnostics reveal dwell variation–target 28-36 degrees at 2,000 RPM for optimal performance without point bounce or module overheating.

Step-by-Step Guide to Connecting an HEI Module Spark Source on a Small-Block V8

Begin by locating the positive terminal on the high-energy electronic (HEI) module–it’s marked with a “+” symbol. Attach a 12-gauge wire to this point, routing it directly to the battery’s positive post or an ignition-switched fuse block. Use a fusible link or inline fuse rated at 15 amps to prevent overcurrent damage. Avoid splicing this lead into existing circuits; a dedicated path ensures consistent voltage delivery under load.

Identify the tachometer output terminal on the HEI module–often labeled “TACH” or “R.” Connect this to the engine’s rev counter using shielded 18-gauge wire, grounding the shield at the distributor base to minimize signal interference. If the vehicle lacks a tachometer, cap this terminal with a weatherproof connector to prevent moisture ingress. Never leave it exposed, as corrosion can degrade performance.

• Strip 3/8 inch of insulation from each wire end before crimping or soldering.
• Use heat-shrink tubing over all connections to seal against vibration and dirt.
• Avoid routing wires near exhaust manifolds or sharp edges.
• Verify the battery’s ground integrity–clean, bare-metal contact at the chassis ensures proper module function.

The HEI module’s ground is integrated into its housing, relying on metal-to-metal contact with the engine block. Scrape off paint, rust, or gasket residue where the distributor mounts. Apply dielectric grease to the mating surfaces to enhance conductivity and prevent future corrosion. Tighten the hold-down clamp securely; a loose fit causes erratic spark timing.

For engines with electronic fuel injection or computer-controlled timing, connect a 1k-ohm resistor between the HEI’s “BAT” terminal and the positive lead to suppress radio-frequency interference. This step is critical for avoiding erratic sensor readings in late-model setups. Test for continuity between the distributor case and battery negative terminal–resistance should read below 0.5 ohms.

Final checks:

1. Crank the engine while observing spark output at each plug wire. Bright blue sparks indicate correct voltage.
2. Use a timing light to confirm initial advance matches factory specs (typically 4–8 degrees BTDC).
3. Monitor for misfires under load–hesitation suggests improper ground or weak battery current.
4. Recheck all connections after 50 miles; thermal cycling can loosen terminals.