Troubleshooting Driver Side Stop Light

[VA Mountain FJ40]

Hi,
2012 FJ Cruiser. Driver side rear stop light recently went out. Passenger side stop light illuminates correctly. All other rear lights work—both sides. Sounds like a bulb issue.

Opened hatch to left rear lights. Verified all lights, only stop (brake) lamp not illuminating. Removed the stop lamp plug from the rear light cluster. Replaced old bulb with new Sylvania 921 bulb.

No joy. Tried the other 921 in the blister 2-pack. No joy. Meanwhile the passenger side stop light is operating normally. Sooo… maybe not the bulb?

Not a hint of corrosion anywhere in the 921 connector or entire rear light area… it’s all plastic outside the connector anyway. What about fuse? I have passenger side stop lamp, so fuse must be good, right?

Never a beach truck, corrosion free. Never had a tow hitch or custom wiring. All OEM equipment as original. Wiring hasn’t been fiddled with. No signs of mice, driven daily.

What comes next? Is there a brake light switch somewhere up front? Could a ‘bad’ brake light switch illuminate only one side?
Chip

 

[BMThiker]

Use a multimeter to see if you are getting 12v at the wiring harness.

 

[VA Mountain FJ40]

Use a multimeter to see if you are getting 12v at the wiring harness.

I have 12V to all lamps in the rear driver side EXCEPT the stop lamp. The stop lamp bulb continuity checks out. Not getting 12V at the socket,

The passenger side stop lamp illluminates normally. I am getting 12V to the passenger side stop lamp but not to the driver side.
Can a bad brake switch illuminate only one side?

 

[BMThiker]

I'd guess somewhere in wiring harness that goes to the driver side.

 

[VA Mountain FJ40]

OK. Thanks. Driver side wiring harness upstream of the brake light bulb is the culprit.
I guess I’ll be trouble shooting the left side wiring harness to find a brake light short… Wish I knew more. I do appreciate the response.

 

[VA Mountain FJ40]

Only getting 12V to passenger side brake light when activated. Driver side brake light bulb is good, 12V not reaching driver side brake bulb socket. QED, Somewhere between brake switch and driver side socket is a discontinuity.

Can a malfunctioning brake switch (above brake pedal arm) send 12V only to one side? I guess I can remove and bench test the switch to see if there is a good 12v out. I don’t know where the 12v brake signal splits into driver and passenger sides.

I’m tempted to just tap the passenger side brake light hot wire and run it to the driver side brake lamp wire and be done with it. Or will that upset the ECU?

 

[iwashmycar]

If it hasnt upset it yet, I imagine it would be fine.

Did you remove the small tailight harness completely to check its continuity?

 

[VA Mountain FJ40]

I am not familiar with the small tail light harness. Everything seems to be taped up and folded inside the tiny access port in the corner. Is there a tech reference I can use to figure out questions like:

How many bulbs are on the tail light harness? Did you mean to say brake light harness? (All other lights work besides LHS brake).
Where does this small tail light (brake light?) harness begin and end?
How long is it? Where are the connectors?
Do I have to remove trunk interior panels (beyond the tail light access port) to access the small harness connectors?

Here is the brake light socket.

Here’s what I can see inside the tail light access port:

SOR has spoiled me with their FJ40 parts diagrams where I could look these things up. I don’t know an equivalent reference for the FJ Cruiser.

Where is the connector between the small tail light (brake light?) harness and the rest of the driver’s side harness?

I’m very reluctant to start pulling and unwrapping a bunch of wires I don’t know about when everything except the LHS brake light is working.
On the brake light socket, which wire goes hot… the red wire or the white wire?

 

[VA Mountain FJ40]

The video in the link helped me learn some.
duck://player/zZRwj80QhaE

It looks like the brake light socket wires plug into the tan molex harness connector inside the tail light port. I should be able to follow the brake light socket wiring back to the molex and unplug, and check for 12v on the green/yellow terminal with brake pressed.

If there is 12V at the molex connector, then I just need a new pigtail to the brake bulb socket.
If there is not 12V at the green/yellow terminal, then there is a discontinuity between the molex connector and brake switch…but where?

I validated the 10A stop light fuse in the under hood compartment is good… but it would have to be for the passenger brake lamp to work as is is now.
I have a spare brake light switch, but it is just a disconnect to the brake lights when the pedal is not pressed. If I’ve got brake lights on the right, the existing installed switch has to be working… right?

If I don’t have 12V at the harness connector, don’t know the next step. Reporting back shortly.

 

[VA Mountain FJ40]

Followed LHS brake light socket wires to the molex plug for harness. Unplugged brake light socket pigtail.
Continuity tested the brake light pigtail from molex to bulb socket. No continuity errors.

Mapped out the harness side plug terminals. White/black (ground) on left, green/red (running light) in center, green/yellow (brake light) on right.
With engine on and foot on brake, no 12V across ground and brake or running light terminals at the molex plug in the tail light port.

RHS and center brake lights continue to illuminate as they should.

It’s not the bulb. It’s not the bulb pigtail. It’s not the stop light fuse, or the stop light switch because RHS and center brake lights still work.

I don’t know whether to try to find the discontinuity between the LHS rear light and the stop switch under dash, or see if I can’t find a way to maybe get power from the rear door center brake light? Not sure of my next step.

 

[BMThiker]

I think this is your wiring path.

 

[BMThiker]

Also pertinent. . .

 

[John McVicker]

Google Search

This is what I would do:
A) get a 12v Test Light
B) activate your brake light. Maybe a 2x4 to push the brake pedal
C) be sure the other brake light is on
D) start testing at the rear light that’s not working. Hook Test Light to a good ground
E) now test the wiring on the light that’s not working
F) follow the wiring towards the front & testing with your Test Light

Keep working towards the front until your Test Light lights up. Once this happens you have found where your problem is. Total pain in the ass, but in order to solve your issue, you must find the problem.

 

[BMThiker]

Pulling up the driver side sill panel gives you easy access to that part of the loom. I'd do that before ripping apart the rear cargo area.

 

[gnob]

I think this is your wiring path.
View attachment 4106199
View attachment 4106200

does the actual wiring diagram show a junction in the lr area?

 

[VA Mountain FJ40]

Thank you all very much for the additional information. This will be extremely helpful!