4wd System Operation & Help Thread

I've noticed there's always alot of threads posted asking about the 4wd system in the 2nd gen truck's, usually the dreaded flashing 4wd light and no 4x4 engagement.
I thought I would attempt to shed some light on how the system operates, the sequence of shifts, the logic of the ECU, and some possible steps to pinpoint the part of the system that is causing the issue besides just changing a part and hoping for a fix.
After reading all the service info and doing quite alot of testing on my own truck looking at signals during each shift with a scope I believe I've come up with a good understanding of how the system operates.
Also thanks to @TnShooter and @6 gearT444E for their help in putting this together.
I will be editing and adding to this as I have time.

Shift conditions:

If the conditions below are not correct to make a shift when a shift is requested by the Mode Select Switch the 4wd ECU will beep continuously and flash the 4wd and/or 4LO lights until the switch is returned to the previous position.

Note: Do not turn the key off during a shift or else the system may lock out with an error at the next power-up and the only way to fix it is to manually operate the actuators to a known position.

2H to 4H:
Vehicle speed must be less than 62 MPH (100KM/H)

4H to 4L or 4L to 4H:
Vehicle speed must be less than 3MPH (5 KM/H)
Manual Transmission: Clutch must be depressed (MTN (Pin A23 at the 4wd ECU) at 0v)
Automatic Transmission: Transmission in Neutral (N (Pin A22 at the 4wd ECU) at 12v)



Shift Sequence:

When you go from 2H to 4H the 4wd ECU first shifts the transfer case into 4wd, after it sees the limit switches and 4wd Detection Switch on the Transfer Case Actuator in the correct positions it then locks the Front Axle (ADD), once the ECU sees the limit switches and Detection Switch on the ADD in the correct positions it sees the shift as complete and turns on the 4wd indicator.

When you go from 4H to 2H the 4wd ECU first shifts the transfer case into 2wd, after is sees the Transfer Case limit switches and 4wd Detection Switch in the correct position for 2wd it then unlocks the ADD, once the ADD limit switches and Detection Switch indicate it's unlocked the ECU turns off the 4wd light.

I've made the chart below to show the sequence of operation, the circuits here correspond with the Pin Designations at the 4wd ECU. When a limit switch is closed the voltage at the ECU on it's pin should be 0v, when a limit switch is open the voltage on it's pin should be 12v.


I've also made a pinout table for the 4wd ECU (Note that some of the wire colors differ from year to year but Pin Number and Location are the same) (All voltages are referenced to chassis ground).


4wd ECU Connector Pinout.


Closeup pictures of the connectors with labeling, note if you have the Rear Diff Lock you will have more pins populated but only the ones labeled here are for the 4wd system.




Here is the 2014 full system wiring diagram stitched together as an image (Some wire colors and routing changed between year models but the 4wd ECU, Transfer Case Actuator, ADD, and Diff Lock Actuator pin numbers and designations appear to be the same for all 2nd Gens (05-15))




Facts and observations I've made:

Facts:
The 4wd ECU can NOT communicate with a scan tool, has no diagnostics , and can not generate trouble codes.
The 4wd ECU does not have a constant power feed and resets every time the key is turned off so disconnecting the battery or unplugging the module will not reset it any further.
The 4wd ECU always completes the Transfer Case shift first and then locks/unlocks the ADD when going into and out of 4wd.
The 4wd ECU uses only the 5 limit switches and 3 detection switches to determine the position and state of the Transfer Case and ADD, there are no position sensors used.

Observations:
The 4wd ECU seems to run a short to ground check on all the motor circuits at power-up (I'm assuming it would blow the 4wd fuse at power-up if one of the motor wires was shorted to ground but I haven't confirmed this).
If the shift knob is moved when the key is off the 4wd ECU will attempt to shift to the selected position providing the conditions are met for the selected position and the limit switches are in a valid position.
If any limit switch is out of range at power-up the 4wd ECU will flash the 4wd light and not allow any shifts until the limit switches are in a valid range.
If the 4wd ECU can't complete a shift it will keep trying for a period of time (not sure how long it will try before it times out).
If a limit switch goes out of range during a shift the 4wd ECU will stop where it's at, flash the 4wd and or the 4LO lights, and not allow any further shifts until the limit switches are back in range.



Input Testing:

Checking the state of the limit switches and other inputs/outputs to narrow down a problem can easily be done at the 4wd ECU located just above the glove box.

Accessing the 4wd ECU:

First remove the glovebox by unhooking the glovebox damper (shown below), pressing the tabs in on the sides of the glovebox, and hinging it down until it can be unhooked off the hinges.


Then remove the 10mm head bolt behind the glovebox latch.


And finally remove the upper glovebox trim by gently pulling at the edges.


The 4wd ECU is directly behind it on the right side.




Testing:

Back-probing can be done by using a small sewing needle, t-pin, or dedicated backprobes.
Be careful not to force the pin in so the connector is not damaged.

T-Pin


Backprobes:



Below is a chart of expected limit switch voltages in each mode (taken Key on Engine off referenced to chassis ground).


By comparing actual voltages with expected voltages we should be able to quickly narrow down the part of the system that is causing the issue.

Here is a chart you can print out and input your readings (note you don't need to complete the 4L column unless your specifically having an issue with 4L).



Component Testing:

Transfer Case Actuator:

The Transfer Case Actuator operates the Transfer Case by using a motor to turn the main gear which drives a spring loaded mechanism that turns a small gear which moves the rack in and out to move the shift fork in the transfer case. The 3 limit switches (TL1, TL2, and TL3) tell the 4wd ECU the position of the actuator motor and when to stop it during a shift, the 2 detection switches (4WD and L4) tell the 4wd ECU what position the shift fork is in the Transfer Case (2h, 4H, or 4L). Picture from the Tacoma 4wd Operation Guide (attached below)



Here's where the contacts are in each position. (Image originally from a thread by @TheFlyingPotato, labels added by @6 gearT444E )


Actuator Pinout


ADD Actuator:

The ADD Actuator works in a similar fashion to the Transfer Case Actuator, the motor turns a reduction gear that turns the main gear which moves a spring loaded rack & pinion mechanism that moves the shift fork to the lock and unlock positions. The 2 limit switches (DL1 and DL2) tell the 4wd ECU the position of the actuator and when to stop it during a shift, the ADD detection switch (ADD) tells the 4wd ECU the position of the shift fork in the front axle disconnect (Locked or Unlocked). Picture is from the Tacoma 4wd Operation Guide (attached below), label's by @6 gearT444E


Here's where the contacts are in each position.


Actuator Pinout




Common thing's to look for:

Cracked or disconnected vent hose on the Transfer Case or ADD Actuators.
When the vent hose cracks or falls off water and dirt can enter the actuators and quickly rust, corrode, and destroy the inner components. If you have never checked these it would be a good idea to do so as a preventative.




Bad Ground(s) on rear of cylinder head.
The ground for the limit and detection switches is on the rear of the cylinder head, if this ground is bad the 4wd ECU can't properly determine the position of the 4wd actuators and thus won't function correctly.

EE (V6 1-GR-FE) on rear of passenger side cylinder head shown below.



EH (4cyl 2TR-FE) on rear of cylinder head

 

Other Threads/Posts of Interest:

Timing The Transfer Case Actuator (post's 16, 19, & 20)

How to Manually Shift The Transfer Case / Remove & Clean Transfer Case Actuator

1st and 2nd Gen transfer case information by @gearcruncher

Tip by @MonkeyProof Adding Solder to the ADD Motor for a solid connection

Bad ground on EE causing flashing 4wd lights, no cruise, & EVAP codes (See Posts 236, 237, 254, 257, 259, 272, 274, 275, & 278

Transfer Case Sticking in Neutral Due to Cracked Low Range Hub

Hose clamp rubbed hole in ADD Vent hose

Good video by Ivan (Pine Hollow Auto Diagnostics) diagnosing and repairing the 4wd on an 06 Tacoma.
https://youtu.be/jp4pwCs6RuQ

Good video by The Car Care Nut explaining the 4wd system, also explaining to not shut the key off during a shift at 8:20.

https://youtu.be/Lhx-RMhWyaM

 

Very good write up. This should be a sticky!

 

Thanks for putting this together, should be a great resource for all to use. I see at least 2-3 threads a week about "4x4 or 4LO lights flashing". The dealer isn't very good at diagnosing these issues either so I wouldn't be surprised if you found a toyota tech stopping in here to get some advice!

 

Awesome thread. Sub'd for future reference! The chart of switch positions vs signal is money. Very easy to troubleshoot with.

I especially like these points, contrary to what most people think haha

 

Great compilation of info on the 4wd system. This will be my go-to for any issues and preventative maintenance needs! I second the motion to make this a sticky.

Edited to ask where the transfer case actuator breather terminates. Is it routed into the engine bay? If not is it worth the trouble to extend it?

 

The transfer case vent goes into the case itself, and where the top shifter block off plate is, there is a common vent that gets extended up decently far towards the top of the transmission. You could extend it, but as long as the short tube that connects to the transfer case housing is in tact, there should be no reason that would be required.

 

Thanks!!

 

So I have a pretty good head scratcher. I have all new Toyota actuators, all my voltages read where you want them to be, problem started when I did my clutch a little while ago. I’ve checked all grounds and they work but still no 4x4 dealership was no help, any ideas?

 

What do your 4wd lights do when you try to shift it into 4wd, also do you hear the actuators doing anything when you move the shift knob?

 

Transfer case will engage and I can hear the ecu click, but 4hi just blinks no beeping, no engaging the front diff

 

Have you checked the limit switch voltages with it in the 4H position and blinking and compared them to the 4H column in my chart?

 

So I did a known good swap today with another ecu and same result. I checked the grounds on the back of the engine. I don’t have a multimeter at the moment so I haven’t been able to check the voltages at the ecu. Is there any sensors or something like that on the trans that could have lead to this?

 

there are no sensors on the transmission for the 4wd system, other than the fact that it would need to see neutral or park sensor to shift into 4lo

 

Correct, all the 4WD ECU pins and their functions are listed in the chart below (also included in my first post)

 

yes so that should only affect shifting into 4LO, and is also only a permissive to shift it simply won't shift into 4LO if it doesn't see this signal. As long as the dash indicator for neutral is working that's a good sign your neutral position switch is functional and I'd think it is a rare chance that the wiring between the junction block that also uses this signal and passes to the 4wd ECU is faulted.

 

I’m pretty damn lost lol. I have good voltage at each actuator a good ecu, and still no 4hi, all happened because I did my clutch. Pretty lame having a truck so modded out stuck in 2wd. I’ve manually shifted the T-case and front diff to see if 4hi works and it does work perfectly but there’s definitely some issues with electrical somewhere

 

I think the next step is to check your limit switch voltages when your switch is in 4H and see which one(s) are out of range so we know what circuit to go after.

 

The limit switches are off the ecu correct?

 

No, they are in the actuator.
But can be checked at the 4WD ECU.
You need to back probe the connection to the 4WD ECU.

I bought these at Walmart.
https://www.walmart.com/ip/Dritz-T-Pins-40-Count/48647573