1st Gen 4WD Issues Diagnosis/Prevention Guide

This guide is intended to help diagnose and fix 95+% of 4WD issues, on V8/auto trucks. If this thread doesn’t help you much, or you have feedback on the guide, let me know. I'll continue to update and improve this guide as needed. Very open to feedback. Thanks!

STEP 1: UNDERSTANDING YOUR 4WD SYSTEM

The 4WD systems on our trucks consist of five main systems. Understanding how these systems work individually and with each other is key to diagnosing accurately. These systems include:

• 4WD switches on dashboard
• 4WD computer control unit
• Transfer case shift actuator
• Front differential shift actuator
• Transfer case shift position switches

Dashboard Switches
The switches on the dashboard of the truck are pretty simple and are not frequently the cause of 4WD problems. The 4HI switch can be pressed anytime as long as the vehicle is traveling straight, on level ground, and is at a speed below 55 mph. The 4LO switch can be pressed when the truck is already in 4HI, stopped, in neutral, and without the wheels turned.


4WD Control Computer
Often referred to as the “4WD ECU” the 4WD computer is located on the right side of the passenger footwell, underneath the plastic kick panel. (see image HERE) This computer takes information from the dashboard switch, both shift actuators, and position switches to determine what is happening and what action needs to happen. This computer also houses the relays, which can be heard clicking when a 4WD shift is prompted.

4WD computer failures are not common but have happened in the past.


Transfer Case Shift Actuator
The transfer case shift actuator is located on the rearward side of the transfer case. (see image HERE) It’s job is to shift the transfer case between 2WD HI, 4WD HI, and 4WD LO. It receives signals from the 4WD computer to shift between the three aforementioned positions. Unlike the Front Differential Actuator, there are no internal position switches. It is what engages and disengages the front driveshaft, which goes between the transfer case and front differential. The actuator itself consists of a small electric motor and a set of reduction gears. These gears move a shift selector shaft in and out of the transfer case, which controls the components inside. The actuator also features a spring mechanism that allows the gears inside the actuator to turn even if the shift selector shaft cannot move. The spring mechanism maintains tension on the selector shaft, ensuring that a proper shift is accomplished once the selector shaft is able to move. The ADD actuator also features a similar mechanism.

4WD failures often are due to a faulty transfer case actuator.


Front Differential Shift Actuator (ADD)
The Front Differential Shift Actuator’s official name is the Automatic Disconnecting Differential, or ADD for short. For this guide I will be using the term ADD. The ADD is located on the back of the front differential, on the passenger side. See photo HERE. It’s job is to connect and disconnect the front wheels to the front differential by moving a collar back and forth to engage a splined shaft, which connects the two front wheels together. On the inside of the ADD actuator, there are metal contacts that complete circuits when the actuator is in one position or another. This tells the 4WD computer what position the actuator is in. The actuator itself consists of a small electric motor which turns a set of reduction gears. As mentioned above, the actuator also features a spring mechanism, which allows the actuator to move, even when a shift cannot be accomplished. This mechanism holds tension on the splines between the differential and axle, ensuring that a shift is accomplished once the splines are lined up. Having the front wheels disconnected from the front differential during normal 2WD operation can increase fuel mileage and reduce wear on components.

ADD failures are common, just like the transfer case actuator.


Transfer Case Shift Position Switches
The transfer case shift position switches are attached to the top/rear of the transfer case actuator. These are simple switches that complete or disconnect circuits, in order to tell the 4WD computer what position the transfer case is in.

Shift Position Switches failures are not common.


STEP 2: DIAGNOSING YOUR 4WD ISSUE

Typically, 4WD issues boil down to one of two things: being stuck in 2WD, or being stuck in 4WD. It’s easy to tell when one of these things is happening.


Stuck in 2WD
Being stuck in 2WD can be easy to see. The truck should drive totally normally, with smooth turns, braking, acceleration, etc. On dirt or gravel, you should be able to spin at least one of the rear tires with a stomp on the gas pedal. Another dead giveaway- the 4HI light flashes when the button is pressed.


Stuck in 4WD
This situation is a bit more unfortunate than being stuck in 2WD. Again, it’s often easy to tell when 4WD is stuck on. If the 4HI light on the dashboard wasn’t enough of a sign, the bucking, hopping, or tire chirping around turns should be.


First Steps Of Diagnosis
The first part to diagnosing your 4WD issue is understanding what is happening, and what should be happening. This can be one of the most straightforward ways to determine what part is broken or acting up.

Things to check first

• Ensure 4WD fuse (in dashboard) is good
• Make sure both actuators and position switches are plugged in tightly, and that wires aren’t burnt/melted/chewed/torn

What should happen in 2WD

• The front driveshaft should spin freely by hand
• When spinning one of the front wheels, only that wheel should turn.
• No lights on the dashboard

What should happen in 4WD

• Front driveshaft should be locked, unable to turn
• Both wheels should spin when one is spun.
• 4HI or 4LO light on dashboard is on

After testing these criteria, you can use the process of elimination to determine what is and isn’t working as it should.

When to suspect a shift actuator
A faulty shift actuator is often the first thing to be suspected. Failures among shift actuators are the most common culprit when it comes to 4WD issues. If your truck isn’t known for electrical issues, the shift actuators should be the first culprit.

It is possible to “bench test” the ADD actuator. This process is outlined at the bottom of this guide, in the “tips, tricks, and links”. This can further confirm that a part is/isn’t working properly.


When to suspect the position switches
Although significantly less common, the transfer case position switches can fail. Since they are less common, replacing the position switches is not advised until both the transfer case actuator and ADD have been replaced with good new parts, or confirmed to work reliably.

One giveaway that the position switches are bad is if the transfer case shifts fine, but the ADD does not shift, even with a new ADD actuator. This is because the 4WD computer will not shift the ADD until it can confirm that the transfer case has successfully shifted. Since it cannot tell what position the transfer case is in, it will not shift the ADD.

When to suspect the 4WD computer
It is hard to specifically say “this is when the 4WD computer is bad”, as there have been only a handful of cases on here where replacing the computer has solved the problem. I personally would be hesitant to replace the 4WD computer unless both actuators and position switches have been confirmed to be good.

However, if your truck has a history of significant corrosion or has had issues with water leaks in the past, check the computer and it’s wires for corrosion.

The above information in theory will fix your 4WD issue. If you are still having issues, please let me (@FiatRunner) know so I can help!

STEP 3: PURCHASING THE CORRECT PARTS

Purchasing the correct part can be key to ensuring consistent and reliable 4WD operation for years to come. I, including others, strongly recommend only using OEM parts. Non-OEM parts, while cheaper, are never as long lasting, and sometimes simply don’t work.

Listed below are the part numbers for the correct shift actuators and position switches.

ADD Shift Actuator:
41400-34013

Transfer Case Shift Actuator:
36410-34015

Transfer Case Position Switches:
84222-12010

When ordering these parts, make sure to purchase from a reliable source. Buying parts from Amazon or ebay is not advised. There are thousands of counterfeit products on those websites, which are nowhere near the same quality as OEM parts.

If you need help purchasing the correct part using Toyota’s part lookup system, check out this guide: https://www.tundras.com/threads/howto-use-the-toyota-parts-system-to-find-order-parts.113808/


STEP 4: REPLACING/FIXING YOUR BROKEN PART

Once you’ve determined what part is faulty or acting up, you can often go one of two routes:

1. Attempt to disassemble your part and fix it
2. Replace the part

Attempting to fix your actuator

Some members have been successful in removing the plastic covers on the actuators, and cleaning/removing corrosion on the inside. Do this at your own risk! Removing the plastic covers also comes with the risk of mis-timing the gears on the inside. This can unsync your two actuators and confuse your 4WD computer, but it’s still possible. Videos explaining the process are linked below.

https://www.youtube.com/watch?v=3TqnZqi4Iw4&t=939s
https://www.youtube.com/watch?v=TmuEVv1neVQ

Front Actuator Replacement

https://www.youtube.com/watch?v=ePU4x8cLdg8&t=598s

Transfer Case Actuator Replacement

https://www.youtube.com/watch?v=Bux-qK3Ze-A



STEP 5: PREVENTING FUTURE 4WD ISSUES

Now that you have fixed your 4WD issues, it is recommended to take care to prevent them from happening in the future. Preventing future issues can be very easy and simple.

Shift Actuator Vent Hoses
One of the most common causes of corrosion inside of the 4WD actuators, both Transfer Case and ADD, is due to the vent hose dry rotting, cracking from age, or becoming dislodged. On both the transfer case and ADD actuators, there is a small nipple protruding from the plastic covers. This attaches to a small rubber hose, which acts as a vent. When this hose becomes cracked or damaged with age, water/salt/dirt/etc can get inside of your actuator and corrode the metal contacts and motor components. Checking to ensure your vent hose is properly attached can be crucial to actuator survival.

Use Your 4WD!!!!!!!!!!!
The other (and most common) reason for 4WD failure is from a lack of use. Parts get stuck, corroded, seized with age and lack of use. It is recommended to drive in 4WD for a handful of miles every month to properly lubricate and heat cycle the 4WD components. However, don’t just throw it into 4WD at any time. If you’re not in a low-traction situation, (e.g. rain, snow, dirt/gravel) only drive in a straight line. Turning on dry pavement with 4WD engaged will put excessive force and strain on driveline components, which can damage them. I like to cycle my 4WD in and out a few times, why not?


TIPS, TRICKS, AND LINKS

FIPG
Form-In-Place-Gasket is Toyota’s gasket maker. Yes, it’s worth it. It’s better. Be sure to use it when attaching your ADD actuator. Part #00295-00103

Both 4HI and 4LO lights flashing all the time
The 4WD computer is recognizing that the transfer case actuator is not plugged in.

Bench testing ADD actuator
The front ADD actuator can be bench tested using a 12v power supply. Connect wires to pins 1 and 5 of the electrical connector. Apply voltage. To reverse actuator direction, swap wire polarity. If the actuator is working correctly, the actuator should move back and forth quickly and easily. Be careful not to hold the wires on for too long, I'm sure it's easy to fry the connections inside.

Not working? You might be viewing the connector from the wrong side. Ensure that the connector’s lock tab is facing upwards and you are counting left to right. See photo HERE

4WD wiring diagrams
4WD 2UZ FE (1).pdf

Video Showing How ADD Works: (Start at 4:00)
https://www.youtube.com/watch?v=rnS4bgFhWCM&t=296s

 

Great write up! Should be a sticky.

 

Nicely done. Definitely needs to be a sticky.

 

I’ll link it in the Megathread in a bit!

 

4WD Photo Dump

Location of 4WD control computer (w/ labels)


Location of transfer case actuator, also shows location of position switches


Transfer case actuator with plastic cover removed


ADD actuator with cover removed


ADD actuator showing shift fork


ADD actuator showing electrical connector pin numbers

 

Amazing write up. Fantastic job.

 

@FiatRunner what you're doing here is SO valuable! I really appreciate you taking the time. I plan on keeping my 4x4 1st Gen for the rest of my life, and having great references for repair is essential to that goal. Great work!

 

Thank you! Fixing my own 4WD was a huge debacle and so many times I’ve thought to myself “why don’t we have a guide for this?”

 

Why isn't this a sticky?

Attn: @ColoradoTJ

 

Yay!

 

Thanks guys!

 

@FiatRunner
3 positions of the TC shaft.
(With the actuator pulled by removing the small gear)
Fully out =?
Middle =?
Fully in =?
Not going to mess with it now that winter set in here, but I have the blinky 4lo/won't go into 4lo issue.
A greater part of me feels like this could end up spiraling/breaking something... when 4hi would have been good enough. But if I ever want to seriously "wheel" this thing, plow, or pull some stumps (do stupid shit), I'm going to want 4lo.

 

I honestly can’t remember for sure what the positions are. Don’t want to tell you something that might be wrong, but I think it was:
Fully out = 2HI
Middle = 4HI
Fully in = 4LO

I think. With the actuator removed, you should be able to manually shift the rod just by pulling it in/out.

 

I remember trying that exact thing (moving it by hand and light smacks with a deadblow) and it wouldnt budge... but I was pretty ignorant of ALOT of things back then... like how all these moving parts worked together.
Knowing what I know now, I would block the whole truck up, put in nutral, and have someone roll a wheel to get things to mesh while I tried to push/pull the shift rod.
That's what I need to do first to determine if my TC even has the ability to get into 4lo. Like if it's a mechanical issue inside the case, or an actuator problem.
I think you are right on the positions.
Thx Fiat

 

Unless something is really wonky I'd be surprised if it was an issue inside the TC. Mine shifted into 4LO just fine after 10+ years of not being used. Not sure why it would work fine for 2Hi/4HI but not for 4LO.

 

Ahhh haaa... those are words I needed to hear.
Have read that i just don't have the inner wheel clocked right... has to be on the Very Last tooth on the shaft or something. Supposedly 15min fix

 

Yeah, I remember spending a lot of time getting the inner wheel and the pinion gear clocked correctly. I think you're on the right track- last tooth on the shaft sounds right. If fully out is 2WD and fully in is 4LO, then shouldn't the pinion gear start or end at one side of the shaft?

 

Yes. Mirror on a stick required to eyeball it.