Providing any high-performance, high-flow fuel pump with a consistent and clean power supply is critical to its performance and longevity. When the relay responsible for powering this high-flow pump receives a trigger voltage that is constantly below its required minimum, a cascade of detrimental effects can occur, ultimately leading to pump failure. In a scenario where the 12V trigger wire to pin 86 of a 40A relay, which requires a minimum of 8.5V to energize, consistently receives a voltage between 2V and 7V, the consequences for the fuel pump can be severe.
The primary and most immediate consequence of an insufficient trigger voltage is relay chattering. An automotive relay operates using an electromagnetic coil that, when energized, closes a set of internal contacts to complete the high-current circuit to the fuel pump. When the voltage supplied to this coil is below its specified minimum, the magnetic field generated is too weak to firmly close the contacts. Instead, the relay will rapidly and erratically open and close, creating a phenomenon known as chattering.
This chattering has several damaging effects that directly impact the fuel pump:
- Intermittent and Unstable Power Delivery: The most direct consequence of relay chattering is an inconsistent power supply to the fuel pump. The pump will be subjected to a rapid on-off cycling of power, causing it to start and stop in quick succession. This erratic operation prevents the pump from reaching its optimal operating speed and maintaining stable fuel pressure. For a high-performance engine relying on the consistent fuel delivery, this can lead to fuel starvation under load, resulting in lean air-fuel ratios, potential engine misfires, and a significant loss of power.
- Increased Heat Generation in the Pump: Each time the fuel pump motor starts, it draws a significant inrush current. The constant and rapid cycling caused by relay chatter subjects the pump’s motor to repeated inrush current events. This leads to a substantial increase in the operating temperature of the pump’s internal components. High-performance fuel pumps such as the Walbro 460, 525 and 535, while designed for robust operation, excessive heat can quickly degrade the motor windings and other internal parts, leading to premature failure.
- Arcing and Pitting of Relay Contacts: The chattering of the relay contacts creates electrical arcing as the contacts make and break the connection to the high-current circuit of the fuel pump. This arcing generates intense heat at the contact points, leading to pitting and the formation of carbon deposits. Over time, this damage increases the resistance across the relay contacts.
- Voltage Drop and Reduced Pump Performance: As the relay contacts degrade due to arcing, the increased resistance causes a significant voltage drop in the power supply to the fuel pump. A fuel pumps flow rate is highly dependent on the voltage it receives. A drop in voltage will lead to a reduction in pump speed and, consequently, a lower fuel flow rate and pressure. This diminished performance can starve the engine of fuel, especially under high-demand situations, negating the benefits of installing a high-flow pump.
- Premature Pump Failure: The combination of excessive heat from repeated inrush currents and the strain of operating with an unstable and lower-than-specified voltage will significantly shorten the lifespan of a fuel pump. The internal components are not designed for such erratic operation, and eventual failure of the motor is a highly probable outcome.
The consistently low trigger voltage to the fuel pump relay sets off a chain reaction of destructive events. The resulting relay chatter delivers a poor-quality, intermittent power supply to the fuel pump, leading to overheating, reduced performance, and ultimately, catastrophic failure of the pump. Therefore, ensuring a stable and sufficient trigger voltage to the relay is as crucial as providing a properly sized power and ground wire for the pump itself.
Here are the primary strategies to defeat this issue and provide a stable trigger for your new relay, ordered from most recommended to least recommended.
Strategy 1: Adjust the Fuel Pump PWM/Trigger Module or Add a Signal Filtering Relay
This is the most robust, electronically sound, and safest method. With the advent of companies like HPTuners, the factory PWM signal can be converted it into a clean, stable 12v+ output suitable for triggering a standard relay simply but adjusting the fuel pump duty cycle to 100%; where this is not possible, use a dedicated independent relay to filter the signal.
- How it Works (Signal Filtering Relay): The module is a small electronic box. You connect the OEM fuel pump wire (the PWM signal) to its input. The module’s internal circuitry recognizes any PWM signal—regardless of its duty cycle or average voltage—as an “ON” command. It then outputs a full, stable +12V from its own power source to Pin 86 of your pumps relay.
- Advantages:
- Reliability: Provides a clean, chatter-free signal.
- Safety: It fully retains the factory ECU’s control. If the ECU commands the pump to shut off (e.g., in a collision or if the engine stalls), the PWM signal disappears, the module turns off its 12V output, and your high-flow pump shuts down as intended.
- Simplicity: They are typically simple to wire in (Power, Ground, PWM In, Trigger Out).
- Disadvantages:
- Cost: This is an additional component to purchase.
Strategy 2: Use a Solid-State Relay (SSR)
A Solid-State Relay uses semiconductors (like MOSFETs) instead of a physical electromagnet to switch the circuit. They have no moving parts and are therefore immune to mechanical chatter.
- How it Works: You would replace your standard 40A relay with an appropriately rated SSR. Many SSRs have a much wider and more sensitive DC input voltage range (e.g., 3-32V DC) and will interpret the OEM’s low-voltage PWM signal as a clean “ON” command.
- Advantages:
- No Chatter: Completely silent and electronically clean switching.
- Durability: No moving parts to wear out.
- Disadvantages:
- Heat: SSRs generate more heat than mechanical relays when under load. A 525 pump draws significant current, so the SSR must be mounted to a proper heat sink to prevent thermal failure.
- Cost & Complexity: Can be more expensive than a standard relay, and sourcing the correct one and implementing a heat sink adds complexity.
- Failure Mode: If an SSR fails, it can sometimes fail in the “ON” position, leaving the pump running continuously.
Strategy 3: Find an Alternative Trigger Source (Use with Caution)
This method abandons the OEM fuel pump wire entirely and finds a different signal wire in the vehicle to trigger the relay.
- How it Works: You tap a wire that is only live when the engine is running. Common sources include:
- Ignition coil or fuel injector power wire.
- A specific “ignition-on” circuit from the fuse box (using an “add-a-circuit”).
- Advantages:
- Low Cost: The cheapest method, often just requiring some wire and a fuse tap.
- CRITICAL Disadvantages:
- MAJOR SAFETY RISK: You are bypassing the ECU’s primary safety function. The ECU is programmed to cut power to the fuel pump if a collision is detected or the engine stalls to prevent fuel from being continuously pumped onto a hot engine or at an accident scene. Using an alternative trigger defeats this safety feature entirely.
- Loss of Priming: You may lose the factory “prime” function, where the pump runs for a few seconds when the key is turned to the “ON” position before cranking.
Strategy 4: Speak with us, Aftermarket Industries
Recommendation
For a reliable and safe system, Strategy 1 is the industry-standard and highly recommended solution. It correctly interfaces modern electronics with aftermarket hardware without compromising the vehicle’s built-in safety protocols.
Strategy 2 is a viable alternative for those comfortable with specifying and heat-sinking solid-state components.
Strategy 3 should be avoided unless you fully understand the safety implications and are building a dedicated track vehicle where other safety systems (like a fire suppression system) are in place. For any street-driven vehicle, compromising the fuel pump cut-off is a significant and unnecessary risk.
Julian Austin – Founder & Head of Engineering