The yellow wire mod is an easy way to add TRAC (auto limited slip diff) in 4LO range as it is disabled by default. It involves splicing a wire under the glove compartment. An improved method, based on this forum post, allows proper signals to the skid control ECU which enables auto LSD in 4LO, without compromising shift patterns.
What is TRAC?
TRAC by default is a limited slip system that regulates engine output (only 2012+ onward).It will reduce will spin and will apply brake pressure. VSC (vehicle stability control) which helps your truck from sliding sideways by apply break pressure as well. This can mean with VSC on and TRAC on, it will cut power in the rear and brake in the front if your Tacoma senses it is sliding or losing traction (ice, sand, etc).
VSC is essentially a yaw rate sensor, which measures the vehicle yaw rate. The yaw rate is the change in direction a vehicle is pointing to in comparison to the direction it is motion to (think sliding sideways on ice). Your Tacoma notices it is sliding when you’re doing some hard core wheeling, and then it starts braking for you!
What you want is to turn off the engine output regulation and turn off VSC but engage the auto LSD, so we can get a bit more traction versus open differentials in the rear.
In the 4 HI range, VSC and TRAC are on by default, you can turn this off by holding the VSC OFF switch for 3 seconds while truck is stopped.
You can see below the different modes that can be operated:
The Original Yellow Wire Mod
The original yellow wire mod enables cutting or putting to ground pin 21 on the first connector under your glove box. This is an input pin, meaning it turns to HI (on) when the truck is shifted into 4LO (it reads from transfer case), then it tells the 4WD ECU its in 4LO yay! When we cut it or connect to ground, the trucks computer doesn’t know if it is in 4LO, so it thinks it is actually in 4HI meaning we can engage auto LSD.
This also makes the 4LO symbol blink on the dash since the computer throws an error. This also alters the shift patterns while in 4LO range, since it thinks the truck is in 4HI.
The Improved Way
The better way of completing this mod is leaving the 4WD ECU alone and instead modding the skid control ECU (since that actually controls TRAC functions).
When the truck is shift from 4HI to 4LO range, it changes the signal sent to the skid control ECU, turning off TRAC functions. For whatever gosh darn reason, Toyota decided for us that we don’t need this functions in 4LO🤷♂️.
In 4LO it pulls pin 9 from HI to LO (12v to 0V), tell to turn off TRAC functions, pretty much telling the skid control ECU to be turned off (no TRAC, no VSC, no auto LSD). So to be able to engage it anytime in any mode (2WD, 4WD HI and 4WD LO), we turn it on all the time! We do this by bringing pin 9 to a 12v source.
Here is the wiring diagram for the skid control ECU. We can see here pin 9 is yellow/black and leads to a “Transfer Indicator SW (switch)”. If I were to put everything together and guess, this is a sensor that checks if the transfer case is in 4LO which then turns off the skid control ECU while in that range.
So what we do is splice the wire on pin 9 and hook it up to 12v! Any 12v source will work.
First thing to do is take off your glove box. Squeeze the sides of it #1 & #2, it will lower until it’s hanging by the right arm.
Pull the arm out from the inside (#3)
The wire we’re after is the 3rd plug down, yellow wire with a black trace (stripe).
I was able to find a steady 12v source from the opposite plug, see below. While there were other pins with similar colours in the plugs (indicating +12v), I went with the easy route.
Problem solved, and no warning lights or compromised performance.
Here’s the download to the 2013 Toyota Tacoma wiring diagrams. This should work for all 2nd gen Tacomas.