J3 Connector & Harness

The HOLLEY J3 Connector & Harness (part number 558-403) is a dedicated wiring solution engineered for modern fuel injection and engine management systems. If you're building or upgrading a fuel-injected engine—whether you're swapping a modern powerplant into a classic chassis or fine-tuning an existing EFI setup—this harness provides the critical electrical bridge between your Holley ECU and fuel injectors. Designed by one of the industry's most trusted names in performance fuel systems, this connector assembly eliminates guesswork and compatibility headaches that plague DIY engine builders.

This J3 harness employs precision-molded connectors and color-coded wiring to ensure reliable signal transmission and power delivery to injector banks. The connector configuration is optimized for Holley's EFI systems, managing voltage regulation and ground returns with minimal signal loss—critical for consistent fuel atomization and combustion timing. The harness construction uses automotive-grade insulation rated for underhood heat cycles, vibration resistance, and corrosion protection, ensuring longevity in high-performance environments where engine bay temperatures regularly exceed 150°F. At 1.4 lbs and measuring 10.5 × 2.2 × 11 inches, the package is compact enough for tight engine bays yet provides ample lead length for flexible routing.

Engine builders, restoration shops, and fuel injection retrofit specialists rely on the J3 Connector & Harness when installing or troubleshooting Holley EFI systems. This harness is ideal for carb-to-EFI conversions, engine swaps, and performance tuning applications where factory wiring is unavailable or incompatible. Installation pairs seamlessly with Holley ECUs and injector packs, reducing diagnostic time and ensuring your fuel delivery system fires with precision from cold start to redline. For anyone serious about EFI reliability, this OEM-quality connector solution delivers the electrical backbone your engine management system demands.