Sometimes called the "Big 3" or "Big 4" or "Big 5" or even "Big 7" wiring upgrade, the idea is to increase the wire size between high-powered components in the engine bay in order to carry extra current, or to carry the same current with lower voltage loss.
This is necessary if you're planning to upgrade your alternator, or if you have accessories - a sound system, winch, lighting, etc. - that require more power than the stock components and wiring can handle.
- Big 3 - this is the "basic" upgrade; all three of these need to be done first, and at the same time.
- Alternator to Battery (+)
- Engine Block Ground to Battery (-)
- Battery (-) to Fender Ground
- Big "More" - depending on who is describing it - adds
- Battery (+) to Fuse Box
- Frame Ground to Battery (-)
- Starter to Battery (+)
- Cylinder Head Ground to Firewall Ground
This step-by-step tutorial will guide you through the process of upgrading all 7 of these wires. Do only the Big 3, or more if you'd like!
Parts
This kit from Outlier Motorsports is really all you need. Big 7 wires (left). Hydraulic crimped lug, glue-lined heat shrink, and label (right).
There are two ways to go about this job. The first - which I recommend - is to purchase a Big 7 Wiring Kit from a reputable source. This will ensure that you get high quality OFC wires (oxygen free pure copper) of the correct length and environmentally sealed, well-crimped terminals. The other option is to create all of the wires yourself, perhaps saving a few dollars (or not, depending on the tools, etc. that need to be purchased).
If you've decided to purchase a kit, I recommend purchasing one of the following kits:
- Big 7 Wiring Kit from Outlier Motorsports - This is the highest quality wiring kit and covers all 7 necessary wires. Everything is the perfect length right out of the box because that's how these guys roll.
- Big 3 Wiring Kit - if you're just wanting to do the basic "Big 3," this is a fine kit. It requires more work as the wires need to be trimmed and the terminals crimped, and you might as well get the Big 7 kit and do the other four wires at the same time, but if you're set on the Big 3...
If you want to create the cables yourself, what you'll need depends on how many wires you want to upgrade and how much current will be carried on the cables.
| Cable | Color | Length (in.) (including lugs) | Up to 150A Alternator | Up to 300A Alternator |
|---|---|---|---|---|
| Alternator to Battery (+) |
Red | 26.5 | 4 ga | 1/0 ga |
| Engine Block Ground to Battery (-) |
Blk | 41 | 4 | 1/0 |
| Battery (-) to Fender Ground |
Blk | 15 | 8 | 4 |
| Battery (+) to Fuse Box |
Red | 20.5 | 8 | 4 |
| Frame Ground to Battery (-) |
Blk | 36 | 4 | 1/0 |
| Starter to Battery (+) |
Red | 61.5 | 4 | 4 |
| Cylinder Head Ground to Firewall Ground | Blk | 15 | 8 | 4 |
- Cable - 10 feet of each read and black is enough to do the Big 7, so just select the wire sizes needed from the list below:
- 1/0 OFC cable - 10 feet red+black
- 4ga OFC cable - 10 feet red+black
- 8ga OFC cable - 10 feet red+black
- Tinned Pure Copper Lug Assortment 8 6 4 2 1/0 Gauge AWG for 5/16, 3/8 Post
- Adhesive-lined Heat Shrink Tubing - always get the adhesive-lined, since it will seal in addition to protecting.
Tools
Not many tools needed to do this job.
If you're installing a kit, you'll need:
- A selection of several sockets - 10mm, 12mm, 14mm. This is a great set, but not cheap.
- A selection of 3-, 6-, ad 9-inch socket extensions.
- A selection of wrenches - 10mm, 12mm, 14mm. This is a good set.
- A set of picks or very small screwdrivers - in order to pry apart small plastic connectors/covers.
- Some electrical tape.
- Floor jack
- 2, 6-ton jack stands
If you're creating your own wires, you'll also need:
- Wire cutters - get a good quality cutter.
- 16 ton hydraulic crimper - for crimping lugs to 8 gauge and larger cable.
- A heat gun for shrinking the tubing.
Doing the Job
With all the correct wire lengths assembled and in hand, it's time to get started. Parts of the process are time consuming, but none are difficult.
Prepping for the Job
This job is made significantly easier if you have access to the engine bay from both above and below.
- Using a floor jack, raise the front of the truck and support it on the frame rails as high as a pair of 6-ton jack stands will allow.
- Remove any skid plates that restrict access to the engine bay from below.
Most of the wiring harness is removed from below, so do yourself a favor and make room for activities.
Removing the Battery
- Loosen - just break tightness - any nuts that secure wires to the negative and positive battery terminals. (Note: Breaking the tightness while the battery terminals are still connected to the battery posts is significantly easier than doing this later.)
- Disconnect the negative battery terminal.
- Disconnect the positive battery terminal.
- Remove the 12mm bolt that secures the battery hold-down; remove the hold-down.
- Remove the battery and battery tray from the engine bay and set aside.
Unbolting the Fuse Box
- Remove the three (3) 10mm bolts that secure the fuse box to the wheel well.
Unbolting the fuse box allows for easier access to access the wires and plugs that will be replaced as part of this job.
Removing the Original Wiring Harness
This guide removes the wiring harness from the battery end (driver side) to the starter end (passenger side). This is personal preference, and the connection points of the wiring harness can be removed in any order.
- Remove the starter wire from the positive battery terminal.
- Remove the fuse box wire from the positive battery terminal.
- Remove the engine block ground/fender ground wire from the negative battery terminal.
Start by removing wires from the battery terminals.
- Remove the fuse box cover.
Removing fuse box cover.
- Remove 10mm nut that secures the alternator positive wire to the fuse box.
- Fold the alternator positive wire up, then push it down through the fuse box to remove it.
Removing the alternator positive wire.
- Remove the connector from the bottom of the fuse box. (Note that this is most easily done with two hands. Use one hand to depress the connector latch, and the other to pull out the connector, leaving the latch attached to the fuse box.)
Fuse box connector from below.
- Remove the 10mm bolt securing a wiring harness bracket to the inner fender.
- Remove the 12mm bolt securing a wiring harness bracket to the driver side frame rail.
Bracket removal.
- (Optional, but makes disconnecting the wiring harness much easier.) Remove the alternator. For instructions and photos of this process, see: Replacing the Alternator
- Remove alternator control connector from alternator.
- Remove 10mm nut that secures wire to the alternator positive terminal.
Disconnecting the alternator from wiring harness.
- Remove 14mm bolt that secures engine block ground wire below the oil filter. (Note: Due to the location under the oil filter - where oil is consistently spilled during filter changes - this area may be thoroughly covered in oil and dirt. Clean it off well prior to removal and after removal in order to ensure a good connection for the ground.)
Looking up from below at the oil filter, and the bolt just below it.
- Use a screwdriver or small pick to unclip the white plastic chase from the metal bracket that runs under the engine pulleys by prying up/toward-front-of-truck on the tabs and then slide the chase down off the bracket.
This chase can often be quite brittle; be careful with it.
- Use a screwdriver or small pick to unclip the black plastic clamp from the metal bracket just forward of the starter by prying up on the tab and then sliding the clamp off the metal bracket. (Note: this is a similar clip as the white chase and the wiring harness clamps on the driver side; inspect those if unsure on removal procedure.)
- Remove the starter solenoid plug from the starter.
- Remove the plastic cover over the starter positive post by prying/pulling it off (no fasteners).
- Remove the 12mm nut that secures the wiring harness to the starter positive post.
The starter is where the wiring harness terminates
- With the wiring harness free, remove it through the bottom of the engine bay.
Extracted!
Measuring and Disassembling the Original Wiring Harness
With the wiring harness removed, it's important to thoroughly measure it before disassembling it in order to replace the appropriate wires. This diagram should help, but make a note of any differences for your particular vehicle.
TODO: missing blue label on frame clamp/mount
Wiring harness measurements. Accurate measurements are critical to ensure that the wiring harness fits onto the existing components, clamps, and chases.
- Using a small screwdriver or pick, pry open each of the clips along the white plastic chase. (Note: This chase can often be quite brittle; be careful with it; don't worry if you break one or two clips.)
- Remove the plastic chase from the wiring harness.
Start from the ends and work towards the middle.
Patience is a virtue.
- Remove all of the electrical tape and wire loom from the wiring harness. (Note: if possible, retain the shape of the harness in order to make assembly of the new harness easier.)
Ready for new wires.
Assembling the New Wiring Harness
Assembling the new wiring harness means both replacing and removing a few of the existing wires.
- Lay the existing wiring harness on the ground.
- Starting at the starter end of the harness, overlay the new starter positive wire.
- Using electrical tape, secure the starter solenoid plug wire to the new starter positive wire on the battery side of where it exits the wire loom, approximately 5" from the plug end of the harness.
- Using electrical tape, secure the starter solenoid plug wire to the new starter positive wire where the two travel through the white plastic chase.
- Starting at the engine block ground lug, overlay the new engine block ground wire onto the harness.
- Using electrical tape, secure the engine block ground wire to the new wiring harness on the battery side of where it exits the wire loom, approximately 8" from the lug end of the engine block ground wire.
- Using electrical tape, secure the alternator plug wires to the main wiring harness on the battery side of where they exit the wire loom, approximately 10" from the alternator plug.
- Note that the following wires will not be included in the new wiring harness:
- Alternator positive wire to battery positive. In the original harness, this wire did not exist. Instead, there was a wire from the alternator positive to the fuse box. The new alternator positive wire will connect directly to the battery.
- Battery negative to fender ground. In the original harness, this wire is spliced into the engine block ground wire at the battery lug. The new battery negative to fender ground will use a dedicated wire.
- Battery positive to fuse box wire. In the original harness, this wire did not exist. Instead, there was a wire from the alternator positive to the fuse box. The new harness supplies power to the fuse box from the battery positive.
Old wires from original harness above, new+old wires in new harness, and additional standalone wires below.
With the wires taped in order to ensure correct lengths, it's time to reattach the looms, chases, and sleeves to complete the harness.
- Begin at the starter positive wire end of the wiring harness.
- Use electrical tape to attach the first length of wiring loom approximately 2.5 inches from the end of the starter positive wire.
- Use electrical tape to secure the starter solenoid plug wire approximately 2" from the end of the wire loom. Tape both sides of the loom where the plug wire exits.
- Use electrical tape to attach the second wiring loom approximately 2.5 inches from the end of the first wiring loom.
- Use electrical tape to attach the protective sleeve between the first two wiring looms.
- Run the starter wires through the white plastic chase and clip the chase together.
- Align the starter-side prong of the white plastic chase with the end of the second wiring loom.
- Use electrical tape to attach white plastic chase to the second wiring loom.
Correct alignment of the white chase prong and wiring loom.
- Align the battery-side prong of the white plastic chase with the end of the third wiring loom.
- Use electrical tape to attach the third wiring loom to the white plastic chase.
- Use electrical tape to secure the engine block ground wire approximately 2" from the end of the third wire loom. Tape both sides of the loom where the plug wire exits.
- Use electrical tape to secure the alternator plug wires approximately 9" from the end of the third wire loom. Tape both sides of the loom where the plug wire exits. (Note: these wires should themselves be protected by a smaller diameter wire loom.)
- Use electrical tape to attach the fourth wiring loom approximately 2 inches from the end of the third wiring loom.
- Use electrical tape to attach the protective sleeve between the third and fourth wiring looms.
Completed harness, all looms installed and taped up.
Installing the New Wiring Harness
Installation of the new wiring harness is - essentially - the reverse of removal, with one exception: installation begins in the middle of the harness.
- Insert the wiring harness into the engine bay from below, in approximately the correct position.
- Clip the two connectors for the white plastic chase into place, thereby positioning the wiring harness in approximately the correct position.
- Plug in the starter solenoid plug.
- Tighten the 12mm nut that secures the starter positive wire. Torque to 70 in-lbs.
Starter positive wire and plug installed.
- Snap the plastic cover onto the starter positive wire.
- Attach the wiring harness clamp to the metal bracket just forward of the starter.
- Tighten the 14mm bolt that secures engine block ground wire below the oil filter. Torque to 25 ft-lbs. (Note: Be sure to clean this area prior to installation.)
- Tighten the 10mm nut that secures the alternator positive to battery positive wire to the positive post of the alternator. Torque to 70 in-lbs. (Note: this wire is not part of the assembled wiring harness, but is much easier to install before the alternator is reinstalled.)
Easier to do on the bench.
- Plug in the alternator control connector to the alternator.
- Completely reinstall the alternator and alternator belt. For the steps and photos of this process, see: Installing the Alternator.
- Reconnect the plug containing the starter solenoid wire and alternator control wires to the bottom of the fuse box.
- install and tighten the 10mm bolt and clamp/bracket that secures the wiring harness to the driver side fender. Torque to "tight."
Installing Additional Cables
With the main harness installed, there are a few more cables to install that are not part of the main harness.
- Feed the battery positive to fuse box wire up through the bottom of the fuse box where the original alternator positive to fuse box wire was removed.
- Tighten the 10mm nut that secures the battery positive to fuse box wire to the fuse box. Torque to 70 in-lbs.
New positive supply wire to fuse box. Feed from bottom (left), secured (right).
- Install and tighten the 10mm bolt that secures fender ground wire to the inside of the driver side fender. Torque to 70 in-lbs. (Note: Be sure to clean this area prior to installation.)
Fender ground, next to battery.
- Install and tighten the 12mm bolt and clamp/bracket that secures the wiring harness to the driver side frame rail. When doing so, capture the frame ground to battery negative wire with the 12mm bolt. Torque to 25 ft-lbs.
Frame ground, directly below the battery.
- Replace the ground wire between the center of the firewall and the engine block with the new, larger ground wire.
Firewall to engine block ground.
Reinstalling the Fuse Box
- Tighten the three (3) 10mm bolts that secure the fuse box to the fender. Torque "tight enough."
Reinstalling the Battery
- Place the battery in the engine bay.
- Using a 12mm socket, secure the battery using the battery clamp.
- Reconnect the battery positive terminal.
- Connect the following lugs to the battery positive terminal
- Starter positive wire from wiring harness.
- Alternator positive wire (not part of wiring harness).
- Fuse box positive wire (not part of wiring harness). (Note: there will now be two wires from the battery positive terminal to the ruse box - the original wire which enters the front inside corner, and the new wire which enters the rear inside bottom corner.)
Three new wires to the positive terminal.
- Reconnect the battery negative terminal.
- Connect the following lugs to the battery negative terminal
- Engine block ground wire.
- Fender ground wire.
- Frame ground wire.
Three new wires to the negative terminal.
That's it. Fire up the truck to make sure everything is working!
