Hey, guys so I bought my first car 2004 g35 with 100k miles about 3 weeks ago and afterward noticed the brake lights didn't work (blinkers and running lights work). I've been on this forum for the past 2 weeks trying to fix it but nothing has worked so far. I put a new brake light sensor (pressing the plunger manually), replaced the fuse, brake light fluid is fine as well as all other fluids, disconnected waited 10 mins and reconnected the battery, and even put whole new taillights on it and the brake lights still don't work. I took it into a shop/mechanic that deals with all sorts of electronics and gave him a rundown of all the things I had already done to try and fix it. He said "it would take hours to figure out what's wrong and we charge 100 per hour". I told him I would think about it and id be back another day. is there anything I can try to check before biting the bullet and taking it back to the shop? Thanks, a lot guys been loving the g35 community in here so far <3 (also I don't know if this is related but on the dash, there are lights for "VDC OFF" "SLIP" and the "ABS" all turn on when I hit the brakes after I turn on the car)

 

I know you said the brake light fluid is fine but when was the last time you changed it? It has to be changed every 15- 20k miles.

 

Barry
Welcome to this forum
The place to begin is with your vehicles FSM
https://www.nicoclub.com/service-man...F2004%2Flt.pdf

https://www.nicoclub.com/service-manual?fsm=G35+G37+Q40%2FG35%2FSedan%2F2004%2Fpg.p df

"CAUTION:
None of the IPDM E/R-integrated relays can be removed.
SYSTEMS CONTROLLED BY IPDM E/R
1. Lamp control
Using CAN communication line, it receives signal from BCM and controls the following lamps:
– Head lamps (Hi, Lo)
– Parking lamps
– Tail lamps

You may want to consider paying the diagnostic fee at a dealer?
Has this vehicle ever been involved in a flood or did anything get wet due to water leaks?

Good luck

 

Relax...
Check this:
https://g35driver.com/forums/general...ip-lights.html

Also
1. Get a BAFX BLUETOOTH OBD2 Connector from Amazon.
2. Get the Torque Pro application from your phone store and load it on your phone.
3. Read your codes from your car.

 

I am not sure whether vdc slp comes with this failure...but

1. Also check that the plunger on the brake switch is hitting the rubber pad on the brake arm.

Sometimes the pad pushes out on hard braking and the rubber plunger on the switch just goes through the hole where the pad used to be. You can get a new one (it is really just a rubber membrane) or use a 9mm pushpin in there.

Some folks even jb weld a nickel in that location....
A bolt head or even a button/carriage bolt head too thick or high...don't try it...

2. Also the brake arm can get caught during hard braking and held down under the dash. It that case, the brake pedal with stay engaged a bit. You just pull up on the pedal hard to get it unwedged. This is not your problem. I am just putting it out there .

 

Did you check the trunk wiring?
https://g35driver.com/forums/brakes-...ght-all-5.html

 

havent changed with brake fluid since ive had it but wil do

 

just found out my dad was pressing the plunger with his finger and the brake lights still didnt work

 

to my knowledge it's never been in a flood (i will ask the previous owner) it does rain a lot in FL though so maybe :/ ? but the blinkers and running lights work fine
I have a very old obd2 scanner and pulled no codes but have bought a wireless scanner that someone in the comments recommended.

 

Just make sure that pad is on the brake arm.
That plunger needs to hit something when you're dad is not there.....

 

Brake lights have nothing to do with the brake fluid.

Are you sure you checked the CORRECT SWITCH on the brake pedal, there are two, one is for the cruise control, one is for the brake lights and I think the TOP ONE is for the brake lights (can't remember for sure).

The electrical diagram is very basic, follow the power, test with a digital multimeter.




First check fuse 20 in Fuse Block - J/B, make sure it's not blown out.

Then test for power at the brake switch harness on the pedal, pin 3 (red/yellow stripe) should have 12v, when you plug the switch back in you should also have power on pin 4 (pink/blue stripe) when you depress the brakes. Use a paperclip to backprobe the harness with your meter while the harness is plugged in, don't short it out to anything else metal because it's live 12v or you will blow that previous fuse.

Here's a picture of that brake pedal switch harness.



If you have power THROUGH the pedal switch on pin 3 when you press the brake pedal then next go back to the taillight, have someone else step on the brake pedal (or put something heavy on it) while you test for power at either of the tail light harnesses, you should have power on pin 1 (pink/blue stripe) and well as continuity to chassis ground on pin 4 (black).

Here's the tail light harness.

 

Pretty sure this is a trunk wiring issue. The wires in the rubber loom are short and over time get brittle and eventually tear. VDC ABS and SLIP lights came on as soon I touched the brake pedal. Checked the trunk wires and they were torn. It’s a bit of a tricky fix as it requires some sort of extending the wires and soldering to make a proper connection.

 

If you don't have a digital multmeter or have not used one, youtube will have videos to help.
A new skill to acquire.

As said, Do check the wiring in the trunk.

Aside: you can press on the brake pedal by wedging a liter bottle (a regular bottle might work even..I forget) with the seat edge... or cut a 2x4 to length..hell maybe a soup can would work.. then just adjust the seat forward.

 

So we passed power through a wire from the battery straight to the brake light harness and the brake lights then turn on.

 

If you are going to troubleshoot it with battery power instead of a meter, you should be connecting power to pin 4 while brake is not depressed..i.e. simulating pedal is depressed.

If brake lights, problem is switch or before.
If brake does not light, problem is after switch. I.E. the trunk harness wires.