Tacoma still shutting off when stepping on brakes or turning on lights in the morning

As the title says, my engine will shut off when I step on the brakes (and now when I turn on the lights), either when I’m driving but mainly when I’m cold starting it. After investing several hundred dollars into chasing the issue, it is still unresolved. As of today, I’ve replaced the IAC, the EGR vacuum valve, the MAF, the brake booster, the alternator (upgraded to a 200 amp instead of the stock 80, since I have a decent amount of stuff to lower), and cleaned the throttle body. When I took it to my local mechanic they said they found nothing wrong, when I took it to the Toyota dealership, they presumed it was the battery and replaced it, yet it still shuts off. I’m so lost for words on what it could be at this point, so close to just selling it instead of rolling the dice and seeing if my truck starts in the morning or not.

 

What about checking or swapping out the Start relay in the fusebox under the hood? If a relay is stuck part open or part closed perhaps voltage fluctuation is killing circuit?
As I understand it, there is more than one identical relay in the fusebox and you might swap two for a litmus test.
I know little about tacoma electrical systems but had some very weird symptoms in cold snap that appear to have been this malfunction.
Good luck.

 

Okay I just bought one, will be here later today. I’ll keep you posted tomorrow on if it works. But I’m still open to more ideas just in case.

 

Starter relay is a good idea..I have similar issues and it makes sense

 

The starter relay only allows the truck to start.
It has nothing to do with keeping it running.

If stepping on the brakes or turning on the headlights kills the engine.
I'd be pulling the fuse to those components. If pulling the fuse(s) allows the truck to keep running.
You are on the right path......

 

The only "common" electrical connection that incorporates the brake lights and headlights is the 120 AMP ALT fuse. This the the fuse in the engine compartment junction box and it is bolted in. I have seen these crack and the 4 Runner in that case would stutter on rough roads/bumps... how it did not melt at the crack when the alternator was charging is still a mystery to me. I have also seen where people have twisted it together as a temporay fix that became permanent until it didnt start at all.

 

Something like these? Except at 120?

 

No. Like this.
And the fuse box has to be partially disassembled.
I’d want to do some real diagnostic before tossing more parts at it.

 

I’m not sure how else to diagnose it. The closest availability with an old toyota specific shop is two months out. That’s why I’m forum hopping, hoping to find someone that has the same exact issue as me. Like I said, I’ve already sunk some hefty change into it, in for a penny, in for a pound.

 

/\ this /\

 

Kind of, but you are not accounting for grounds.
Or a couple connectors in common.

If you don't have a good ground. Electricity will try to find one.
And that one it finds, might be the one that causes issues.

I'd start by pulling the 10A Stop fuse. See if issues goes away.
If not....

Then pull the head light fuses, one by one, and see what happens.
We can go from there.....

 

Same as I just asked on the other thread.....

What does an amp probe say when key on / engine off.....are you getting a parasitic draw with key on.....relays, ect

 

Update: replaced the starter relay (I accidentally cracked it when trying to take it off) and the AMP alt fuse, problem still occurring.

 

Man, I don’t know how to put this nicely.
Any time someone suggests throwing parts at a problem, with no explanation of how they concluded that the part needed to be thrown at it, is typically not good advice.

See my prior post. You need to do some basic troubleshooting here.
Throwing parts at a problem, isn’t troubleshooting.

I am willing to help you.
Be we need info to proceed.

See post 11.
It cost you nothing.

 

What kind of electrical accessories are we talking about? Stereo? Lights?

Does the problem go away if you disconnect that stuff?

 

I’ll try that tomorrow morning before I leave for work, it’s highest failure rate is during the cold, once it’s warmed up it doesn’t happen, I’ll give it a whirl though and let you know how it goes. I returned the AMP alt fuse, since the original worked fine. But in the process of looking at the fuse under neath the fuse box I had cracked the starter relay so that was on me.

 

I have two KC light pods hooked up, an updated radio, and a CB radio, all have their own fuses and are wrapped properly as to not interfere with the other wiring I have. I’ll try disconnecting the CB and KCs before I do the radio.

 

When it dies does the instrument panel illuminate like you just turned the key to the ON position or does is stay "dark"?

 

It stays illuminated. Starts back up like nothing ever happened. Radio still plays as well.

 

Like my old high school woodshop teacher would say, KISS. Keep it simple stupid. As @TnShooter already stated, remove the fuses to the components that are causing the issue and go from there. No point in throwing money at it when you can start troubleshooting for free.