Electrical issues

How do I know if I have a bad relay or if it is something else? I have 2003 Tacoma that is having some funky electrical issues involving the headlights, brake lights, brakes and transmission. Whenever I use the headlights, turn signals, and brakes, my transmission will not shift from a stop and will go into neutral while driving. Anyone know where to start?

 

Welcome to TW…

 

Let's start with some questions that will make it easier for us to help you.

With the truck running, in PARK, and foot ON the brake pedal, can you shift out of park?

Can you also explain in more detail, what you are doing when it goes into Neutral?
Can you replicate the problem by doing the same thing/things?
Like turn on, or off, the headlights?
Does it happen when you turn them on, or off?
Or both?

 

Yes I can, without the headlights on it will drive and shift normally until I hit the brakes, then the transmission will down shift pretty hard the second I hit the brakes. when I turn the headlights on the truck will go into neutral or stay in 1st gear depending on how fast I’m going, the truck will shift and drive normal the second I turn them off. The cautions and turn signals will also make it downshift or go into neutral.

 

I would check your grounds first. Both on the battery, and anywhere connecting the battery to the frame, or near the components to the frame.

Without a return path back to the battery, lots of odd things can happen

 

Is your a 3.4 or a 2.7 engine?

 

This is so bizarre I'm going to ride along to see the solution

 

Bizarre for sure.
The Neutral has me stumped.

 

OP, Does the gear shift indicator light switch to Neutral too?
Or does it just feel like it's in Neutral?
RPM goes up, but truck slows down/doesn't move?

I am wondering if torque converter is being locked / unlocked.
Switching to Neutral on it's own is highly unlikely.

 

So I did a lot of troubleshooting with electrical last night, replaced every ground in the truck and I’ve figured out it’s a brake light relay.after I put the grounds on, I pressed the brake on and off a few times and the issue went away and brake lights came back. Hit the brakes a couple more times and it went back to messed up crazy electrical. Does anyone know which relay in the fuse box controls the brake lights?

 

Google is amazing most of the time.

https://fuse-box.info/toyota/toyota-tacoma-2001-2004-fuses-and-relay

 

What do you mean you "replaced every ground in the truck" and "put the grounds on"? Grounds arent exactly replaceable, they are just places where a wire connects to complete a circuit. Exactly what were you touching and where? Pictures will help us help you. Sure sounds like you've got a loose or shorted connection somewhere.

 

I want to know if you have aftermarket trailer hitch wiring!