EFI Wiring Made Easy: Install Tips from the Pros

11/22/2023

EFI Wiring Made Easy: Install Tips from the Pros

11/22/2023

Wiring is one of the most critical aspects of many performance upgrades. This is especially true with electronic fuel injection. The internals of an ECU contain delicate microprocessors and many other components that rely on clean, consistent power and a strong, reliable ground. “The foundation of a good EFI system starts with proper mounting of the ECU as well as proper battery connections,” explains Tom Kise of Holley. “If you don't have a solid foundation, everything else is just going to go downhill from there.”


But by following just a few simple rules and guidelines, you can easily wire your EFI system to give you maximum performance and long-lasting reliability. Here are some great tips for installing and setting up the electrical components of a Holley EFI system.

Get Your Car in Shape

EFI systems demand an electrical system that’s in tip-top condition. So, before you do anything, look at the quality of your battery, wiring, and electrical connections. If there’s corrosion, cracked insulation, poorly done splices, or any other flaws, now is the time to address them.

Use the Right Connectors and Splices

To install an EFI system, you’re going to be making some critical electrical connections, and possibly be splicing some wires. So, make sure your work is up to the demands of an EFI system’s sensitive electronic components.


For some of you, that means undoing bad habits. When connecting wires, don’t just twist them together. Instead use the correct size of crimp terminals. And forget about using pliers to crimp the connection – they aren’t suitable for making a strong, electrically sound crimp. Instead, use a crimping tool that’s designed for the job.


Also, never use T-Taps or Scotch Locks. These types of connectors don’t provide a strong connection and are likely to fail sooner than you expect.

Modern EFI systems demand clean, consistent electrical power. Proper installation thus requires high quality connectors installed with proper tools. And while you're at it, take a look at the rest of your car's wiring and electrical components. Any flaws in the system can result in issues down the road, or even damage to the EFI system.


Use a Voltmeter to Find an Ignition Power Source

When wiring an EFI system, you’ll need to find a switched ignition power source. Some people use an old-fashioned test light to find the connecting voltage. The problem with test lights is that they tell you where the power is, but not how much is available. The better option is to use a voltmeter. This will tell you exactly how much voltage you have at a particular source. But make sure that the source you select is clean and is not the accessory circuit of the ignition switch, as this location turns off during crank.

Always Wire the ECU Directly to the Battery

ECUs require clean, steady power to operate correctly. But a lot of automotive systems cause high-voltage surges that can create problems for EFI units. Common sources of such surges include cooling fans, trans brakes, starter solenoids, line locks, and nitrous solenoids.


For this reason, you should always connect the main power and ground for the ECU directly to the vehicle’s battery. By connecting the unit’s main power and ground this way, the battery acts as a capacitor, which filters the power going into the ECU. If you connect the main power from somewhere else on the vehicle, you run the probability of getting a poor ground or “dirty power” into the ECU. This can cause improper ECU operation and worst-case scenario can damage the ECU.


“If you isolate the ECU directly to the battery, the battery acts as a big surge suppressor like the Earth does with lightning,” says Kise. “And the transient flyback voltage gets buffered out when it gets to the battery, which helps absorb and dissipate it.”

Use the Right Battery Terminals

On top-post batteries, ECU cables can be mounted directly to the terminal bolts. But on side-terminal batteries, you need to take an extra step and purchase some terminal adapters that include a stud for mounting accessories.


“On side-terminal batteries, I've seen guys stack numerous terminals underneath both existing battery connections and it just looks like a big jumbled up mess. And that isn’t the best way to go about it. Get some battery terminal extenders from any one of your local parts stores. They’re specifically designed to replace your stock side terminal with a battery terminal bolt and give you a nice convenient mounting location to add additional components and accessories.”


In fact, the ideal way to connect an ECU to a battery is by using side and top terminals, if your battery has both. This allows you to mount the car’s main battery cables to the top terminals, and put the ECU connections on the side terminals. “This is really my preferred method for installation,” says Kise. “It keeps your high-current devices separated from your clean digital electronics.”


And for those of you who have your battery in the trunk, don’t connect the ECU to a distribution stud under the hood. Extend your ECU cables long enough to reach all the way to the trunk and connect directly to the battery.

ECUs should always be connected directly to the battery, to ensure power that's free from spikes caused by other systems in the car. On side-terminal batteries, you'll need to get a set of terminal mounts that have a stud for connecting accessory wiring. The ideal setup is a battery with top terminals and side terminals: The main battery connection can be on top, and the ECU connection can be mounted separately on the side.


Run More Than One Ground Wire

Running proper grounds from the battery to the engine block, frame, and body is just as important as running the ECU’s main power directly from the battery. Connect the main battery ground to the stock location on the alternator bracket with a star washer and make sure the surface is clean. Run a ground cable from the block to the frame and a ground cable from the body back to the battery.


And when running these grounds, don’t skimp on wire. Use at least a 1 gauge wire from the battery to the block and the block to the chassis, and at least 10 gauge wire from the body back to the battery.

Use the Right Wire to Extend the ECU Harness

The harness supplied with Holley EFI units is typically quite long, and it'll support most applications without the need to extend it. But, if you do find it's still too short for your setup and you need to extend it, make sure you’re using adequate wire. In this case, you should use 10-gauge wire or larger.

Position Your ECU Carefully

Wiring your ECU goes hand-in-hand with mounting it, as one often dictates the other. So don’t just slap your ECU haphazardly onto the first piece of open sheet metal you see under the hood.


“It’s important to make sure that you don't mount your ECU too close to high heat sources,” says Kise. “You also want to make sure that you can keep the wiring harness away from moving parts that could cause damage. Another good point to mention is that if you're going to mount your ECU on a vertical surface, you always want to make sure that you mount it with the harness pointing down, to avoid moisture migration that could cause corrosion.”


Kise also says to pay close attention to the mounting hardware included with your ECU. “There are some insulating grommets as well as nylon inserts that go through the body of the ECU that help keep it isolated, as well as some factory supplied stainless steel hardware for installation. It's important that you don’t remove this hardware when you install the ECU. You want to make sure that it floats and it's not touching any direct chassis ground.

Read the instructions carefully. Installing a Holley EFI system isn't difficult, but there are certain specific things that need to be done right. Not doing so can cause poor EFI performance or even worse, damage the unit and void its warranty.


Rely on the Experts

Installing a Holley EFI system is pretty simple. These systems are engineered to be as easy as possible to set up, even for relatively inexperienced users doing it for the first time.


So don’t ignore the expertise Holley engineers have put into the system – look carefully at the instructions and any other documentation included. “Read the instructions and thoroughly understand them before proceeding,” says Kise. “Not following the guidelines and instruction sheets can result in poor vehicle performance, as well as damage to the ECU. And damage caused by improper installation isn’t covered under warranty.”


If you run into problems or you don’t understand something about installing your Holley EFI system, don’t hesitate to contact Holley’s dedicated team of support specialists, at: 1-866-464-6553


You can also find a wide range of additional technical support resources at holley.com/support/fuel_injection

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