Holley Sniper 2 PDM Wiring for LS and LT Swaps: Power Distribution Essentials

The Holley Sniper 2 PDM (power distribution module) is one of the more capable bits of kit in the Sniper 2 ecosystem, and also one of the most misunderstood. Builders who have spent years wiring traditional relay boxes and fuse panels sometimes stare at the PDM connector pinout and wonder where the familiar terminals went. This post covers the Sniper 2 power distribution module installation topology for LS and LT swaps: battery feed, ground strategy, fuel pump relay wiring, starter cutoff logic, and harness routing. Holley Sniper 2 relay wiring differs meaningfully from carb-era or legacy EFI wiring, so it is worth working through the logic before you start crimping.

What the Sniper 2 PDM Actually Does

The Sniper 2 PDM is a solid-state power distribution unit that replaces a traditional relay and fuse block with software-configurable outputs. Instead of running a separate relay for the fuel pump, another for the cooling fan, and a third for the ECU keep-alive, the PDM handles all of them as individually programmable channels. Each channel can be set with its own current limit, inrush tolerance, and fault behaviour inside Holley EFI software.

For an LS or LT swap, this matters because the engine's electrical demands are higher than most classic chassis were designed to handle. A Gen III LS running a return-style fuel system, twin electric fans, and a coil-per-cylinder ignition can pull 60-plus amps at full load. A properly configured PDM routes that load cleanly without the nest of relays that would otherwise live in the engine bay.

The PDM communicates with the Holley ECU over CAN bus. That means the ECU can arm the fuel pump output only when the engine is cranking or running, cutting it automatically on a stall. It also means you can log PDM channel states alongside your engine data, which is genuinely useful for fault-finding.

Input Voltage and Battery Feed

The PDM requires a direct battery feed, not a switched ignition feed. This is the first thing that trips builders up. The module needs unswitched 12V at all times so it can manage its own wake-up sequence and retain fault logs between key cycles.

For a typical LS swap the feed runs from the positive battery terminal through a fusible link or a maxi-fuse (typically 100-150 A depending on total channel load) directly to the PDM main power input. The wire gauge needs to match the anticipated load. For a build running fuel pump, twin fans, and ECU supply through the PDM, 4 AWG (or 25 mm²) is a reasonable starting point for the main feed run. Do not share this feed with the starter motor cable.

Ground the PDM chassis stud to the engine block or to a dedicated ground point on the firewall that itself has a clean, short path to the battery negative. A bad ground on the PDM will produce erratic channel behaviour and difficult-to-diagnose fault codes. On Gen IV LS and LT engines in particular, the factory ground strap locations matter. Run a separate ground strap from the engine block to the chassis if the swap has removed the factory OEM ground points.

Fuel Pump Relay Setup and Sniper 2 Relay Wiring

The Sniper 2 fuel pump relay setup works differently from a traditional switched relay. There is no physical relay for the fuel pump when the PDM is in the system. The PDM output channel assigned to the fuel pump acts as the relay, and the ECU controls it via CAN.

In Holley EFI software the fuel pump output is configured with a prime pulse on key-on (typically 2 seconds to pressurise the rail), then it arms automatically when the ECU sees RPM from the crank sensor. On engine stall the output drops within a second. This is the same safety logic a factory engine management system uses.

For Sniper 2 relay wiring of the fuel pump circuit:

1. Run the fuel pump positive feed from the assigned PDM output channel to the in-tank or inline pump positive terminal.
2. Ground the pump to the chassis or a dedicated ground block near the pump mounting point, not through the PDM.
3. Do not install an additional relay between the PDM output and the pump. The PDM output is current-limited to the channel rating; adding a relay downstream defeats the PDM's short-circuit protection on that channel.
4. If you are running two pumps (for high-horsepower builds), each pump should ideally have its own PDM channel rather than being wired in parallel on one channel. This keeps the current per channel within limits and allows individual fault logging.

For a return-style LS fuel system the pump feed wire runs from the PDM, along the floor of the vehicle (preferably inside a protective sleeve), to the pump. Keep fuel pump wiring as far from the exhaust as the chassis allows. On tight UK-spec right-hand-drive conversions this sometimes means running the harness up the transmission tunnel rather than down the driver's side sill.

Starter Cutoff Strategy

The Sniper 2 ecosystem supports a starter cutoff output on the PDM, which prevents the starter from engaging if certain conditions are not met (engine already running, PDM fault present, roll-control active). For an LS swap this is worth configuring properly rather than bypassing.

The cutoff works by interrupting the signal wire from the ignition switch to the starter solenoid. The PDM output sinks the starter trigger signal to ground when the ECU is not ready. When the ECU arms the system, the output releases and allows the ignition switch signal to pass through normally.

This is not a high-current circuit. The signal wire to the solenoid carries only the trigger voltage; the solenoid itself handles the heavy current. A 14 AWG wire from the PDM cutoff output to the S terminal on the starter solenoid is sufficient for most LS and LT applications. Confirm the signal wire voltage at the solenoid S terminal before assuming the stock ignition switch harness is compatible, particularly on older British chassis where the ignition switch may carry a different voltage path.

Harness Routing for Tight Engine Bays

LS and LT swaps into European and classic British chassis typically leave less engine bay space than a Camaro or truck donor. The PDM itself is not especially large, but where you mount it and how you route the harness from it determines how clean the installation looks and how accessible it is for fault-finding later.

Mount the PDM somewhere with airflow. It generates modest heat under sustained high-current use and should not be packed against the firewall with no clearance. A bracket on the inner wing is a common choice, provided it keeps the PDM away from direct water ingress from the wheel arch.

From the PDM, the main output harnesses split into:

• Engine management feed (to the ECU power input and sensor feeds)
• Fuel system feed (to the pump, and if fitted, the returnless system pressure sensor)
• Cooling feed (to the fan relay or directly to fan motor if the fan draws within channel rating)
• Auxiliary feeds (any ancillaries the builder wants PDM-controlled: nitrous arming, boost controller, lambda heater)

Label every connector before installation. The Sniper 2 PDM connectors are keyed, but the secondary harness connectors to individual components often are not. On a swap into a right-hand-drive car the PDM is usually mounted on the driver's side (right inner wing), which puts the main battery feed on the short side of the car. Keep that main feed run as short as possible to minimise voltage drop.

For the CAN bus connection between the PDM and the Holley ECU, use the supplied twisted pair or source 22 AWG twisted shielded pair. CAN bus is differential signalling and is relatively noise-immune, but do not run it alongside the high-current pump feed or ignition coil wires for any significant length. Separate it by at least 50 mm where possible, or route it through a separate conduit.

If your build integrates an aftermarket gauge cluster or display via CAN, the wiring where the PDM CAN bus meets the gauge cluster CAN bus requires a proper termination resistor at each end of the bus. Two 120-ohm resistors, one at the ECU end and one at the last device on the bus. Missing or mis-located termination resistors cause intermittent CAN faults that are genuinely miserable to diagnose. The Holley Sniper 2 EFI complete integration guide covers the CAN topology in more detail if you are adding gauge displays to the PDM network.

Common Wiring Faults and How to Avoid Them

The majority of Sniper 2 PDM faults seen on LS swaps fall into three categories.

Overcurrent on fuel pump channel. Usually caused by a failing pump drawing more current than the channel is set to allow, or by running two pumps on one channel. The PDM logs the fault and shuts the channel. Check the channel current limit in Holley EFI software and confirm it is set above the pump's rated draw with some headroom, but below the wire gauge rating for that circuit.

CAN communication loss between PDM and ECU. Almost always a termination or wiring fault. Check that the CAN H and CAN L wires are not swapped at either connector, that termination resistors are present and measure approximately 60 ohms across the bus (the parallel combination of two 120-ohm resistors), and that no CAN wiring is open-circuit due to a chafed or pulled terminal.

Ground offset causing phantom faults. When the PDM ground and the ECU ground are not at the same potential, the PDM can report channel faults that do not correspond to any real load problem. Both units should share a common ground point, ideally at a central chassis ground bar that itself connects to the battery negative with a short, heavy cable. The wiring guide for Holley EFI swap kits covers ground strategy for the full harness in more detail.

What to Check Before First Start

Before cranking the engine for the first time after a Sniper 2 PDM installation:

• Confirm main power feed fuse rating matches the anticipated total channel load
• Key-on and check that the fuel pump prime pulse fires (you will hear it for 2 seconds)
• Confirm the fuel pump channel shows active in Holley EFI software during prime
• Check all PDM output channels show no faults in the software fault log
• Confirm CAN bus status shows connected and no error frames accumulating
• Verify starter cutoff releases correctly when the ECU is fully armed (key-on, no fault codes)

The Holley EFI kit install checklist has a full pre-crank sequence for swap installs that is worth running alongside this PDM-specific check.

For fuel system pressure verification, the Holley fuel pump sizing guide covers what rail pressure to expect for LS and LT applications and how to confirm the pump is delivering it correctly.

Engine Sensors and Supporting Electrical Hardware

A well-wired PDM installation is only as good as the sensors feeding data to the ECU and the hardware holding everything in place. If you are still sourcing electrical components for the swap, the engine sensors collection covers compatible sensor options, and the engine fasteners and hardware collection includes the brackets and standoffs useful for mounting the PDM and associated junction blocks cleanly.