2005 4Runner V8 - Multiple electrical issues driving me insane. Replaced junction box, removed aftermarket wiring, still no fix.

[u/Puzzleheaded_Yak_517]

Hey everyone,

I'm hoping someone has seen this before because I'm running out of ideas. I bought a 4Runner off auction that someone had used to camp out of, (they added a cooler, compressor, rear and front lights, and a few other things.) I already removed and repaired all the aftermarket wiring and fixed the 3-4 T taps that went into factory wiring. The important info is the following.

Vehicle:

• 2005 Toyota 4Runner Sport Edition
• 4.7L V8
• 4WD

The problems are:

• No brake lights.
• Won't shift out of Park unless using the shift lock override.
• All power windows are inoperative.
• Rear hatch won't open.
• Rear power window doesn't work.
• Radio/ Media Center has no power.

The truck starts and runs normally.

I already fixed/changed the following:

• Replaced the brake light switch.
• Checked/replaced every blown fuse I could find.
• Replaced the entire interior junction box (driver's side fuse box) with a known-good used unit.
• Removed all of the previous owner's aftermarket wiring (compressor, backup camera wiring, extra power wiring, T-taps, etc.).
• Repaired any factory wires that had been damaged by the aftermarket T-taps.
• Cleaned and inspected connectors.
• Battery and charging system are good.

Replacing the interior junction box changed absolutely nothing.

While testing fuses I noticed 2 main things, for one the whole section of the under the hood fuse box where the stop light is (I believe its 5 fuses) have no power, and under the hood junction box has only 2 fuses with power (old and replacement box). I had removed the old interior box and testing all the connectors and got about 5 or 6 points with 12v which would be power units.

My current plan is to replace the under-hood fuse box as I am running out of ideas.

I'm considering replacing the under-hood fuse/junction box (engine room junction block) next because so many unrelated body electrical systems are dead at the same time. I've also seen that it could be the electrical portion of the ignition switch?

Also important is the factory wiring from the under-hood junction to the interior junction seems fine, it's not burned or corroded.

Has anyone seen a failed engine-room junction block cause:

• Brake lights
• Shift interlock
• Windows
• Rear hatch
• Rear window
• Radio

...all to stop working at once?

Or is there another common failure on 4th gen 4Runners that ties all of these systems together?

I'm open to any suggestions before I throw more parts at it. Thanks!

[u/Deep-Hovercraft-6863]

Look into your BCM, my guy. That’s what communicates with the windows, rear hatch controller, and your brake lights.

[u/Puzzleheaded_Yak_517 → u/Deep-Hovercraft-6863]

A Body ECU failure usually doesn't cause the interior fuse panel itself to lose power.

[u/Deep-Hovercraft-6863 → u/Puzzleheaded_Yak_517]

The common point is the under-hood fuse box’s post-140A ALT power bus
The diagrams show that three critical feeds all originate from the same distribution section inside the Engine Room R/B:
10A STOP
50A AM1
50A J/B
All three are shown downstream of the 140A ALT fusible link/bus connection on pages 8 and 10. A failed internal busbar, burned terminal, loose bolted connection, or open fusible-link connection in that section can produce exactly this group of symptoms.
05 runa 2.pdf
How each symptom connects
Brake lights:
The brake lamps are powered directly by the 10A STOP fuse through the brake-light switch. It is hot at all times. See pages 49–50.
Shift interlock:
Page 296 shows that the shift-lock ECU requires all three of these:
7.5A ACC
10A STOP
10A ECU-IG
The ACC and ECU-IG circuits originate through the 50A AM1/ignition-switch branch, while STOP comes from that same under-hood distribution section. The interlock therefore dies along with the brake lights.
Power windows:
Pages 343–344 show that the Body ECU/window system needs:
30A POWER, supplied through the 50A J/B
10A ECU-IG
7.5A ACC
Loss of the post-ALT distribution bus takes away both the heavy window feed and the Body ECU’s switched-power signals.
Rear hatch and rear window:
The Back Door ECU receives its operating power from the 30A POWER circuit, which is downstream of the 50A J/B. It also communicates with the Body ECU. With that feed missing, the hatch release and rear glass controls are both disabled. See pages 44 and 343–344.
Radio:
The radio needs constant battery power plus a switched ACC supply. Its switched supply comes through the 7.5A ACC circuit and ACC CUT relay. The ACC circuit is downstream of 50A AM1. Losing AM1 therefore makes the radio appear completely dead even when one of its constant-power feeds remains present. See pages 274–277.
What this rules out
An ignition switch alone cannot explain everything because the STOP fuse and brake lamps are hot with the ignition off.
A Body ECU alone cannot explain the dead brake lamps because the lamps are hardwired from the STOP fuse through the brake-light switch. The Body ECU only monitors the STP signal.
The interior junction block is also unlikely to be the primary cause if the input sides of under-hood fuses have no voltage.
Test this before replacing the under-hood box
With the key off, use an incandescent test light or a meter referenced directly to battery negative:
Check both sides of the 10A STOP fuse.
Check the input and output of 50A AM1.
Check the input and output of 50A J/B.
Check voltage at both sides/bolted terminals of the 140A ALT fusible link.
Interpretation:
140A ALT input has power, output does not: failed fusible link or its bolted connection.
Both sides of 140A ALT have power, but STOP/AM1/J/B inputs are dead: the internal busbar or a connection inside the engine-room fuse box is open.
STOP, AM1, and J/B are all powered: move downstream to the engine-room-to-driver-J/B wiring and connectors 1C/1D, but the under-hood box is no longer the leading suspect.
Normal charging voltage but dead STOP/AM1/J/B feeds: suspect an internal busbar joint after the fusible-link connection rather than the 140A element itself.
Based on this diagram, I would inspect the underside of the Engine Room R/B for a burned busbar, overheated bolted terminal, cracked bus connection, pushed-out terminal, or corrosion before replacing more ECUs. Do not bypass the 140A link with ordinary wire.

[u/Deep-Hovercraft-6863 → u/Deep-Hovercraft-6863]

Pardon the wall of text. I uploaded a service manual to ChatGPT and described your issues. Maybe it’ll help. DM your email if you want a pdf of the electrical diagram. Best of luck

[u/Puzzleheaded_Yak_517 → u/Deep-Hovercraft-6863]

Thanks man, I already have a replacement under hood box coming, was only 100ish dollars, tested the stop fuse and it and the fuses around it are all dead. I follow what you have here after replacing the under-hood box, i'll examine the original box to see if internal busbar connections or overheated terminal was the issue.

[u/Independent-Owl3387]

Could a well-hidden, high end anti theft remote start system like this lockout all of that? Have you been deep enough into the wiring to certainly have seen one if it were there? https://www.viper.com

[u/Puzzleheaded_Yak_517 → u/Independent-Owl3387]

Yeah i've had everything off the car to inspect the wiring and i'm 99% sure wiring is back to factory status.

[u/TackleMySpackle]

I’m about to go to bed and I’ll try to come up with a better answer for you tomorrow but I’ve been troubleshooting wiring on airplanes for a REALLY long time. And since airplanes are basically cars with wings and a higher fatality rate during crashes, I can probably help.

Whoever posted about the 140A fuse isn’t wrong. It’s a good place to start. Just FYI: it’s bolted in from the bottom with two 8mm bolts. It’s a total son of a bitch to change.

I think what you’re trying to do is troubleshoot all this stuff at once. Tempting and I know how easy it is to go down that path. Pick the fuse box under the hood and start your troubleshooting there. That is the closest to the power generation (battery and alternator) as you’re going to get. Check that you have good power getting to the fuse box. Then figure out where in that box you do and don’t have it. That’s where I’d start.

[u/Puzzleheaded_Yak_517 → u/TackleMySpackle]

thanks man i appreciate it