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-   -   A-TRAC/TRAC/AUTO LSD Explained (Easy to understand, I hope!) (http://www.tacomaworld.com/forum/technical-chat/70551-trac-trac-auto-lsd-explained-easy-understand-i-hope.html)

David K 01-19-2010 11:37 AM

A-TRAC/TRAC/AUTO LSD Explained (Easy to understand, I hope!)
Have you wondered about your 2009 or newer Tacoma TRD Off Road 4WD's (USA) with its many drive options or switches/ buttons (VSC Off, A-TRAC) and what they do???

I sure did... and this is my THIRD 4WD Off Road Tacoma (2001 extra cab, 2005 double cab, 2010 double cab)...

The first two had standard 4WD with OPEN differentials (means when you are in stuck or almost in a stuck condition (mud, sand, snow) ONE tire in front and ONE tire in back get power and spins... often helping to bury you deeper! However, the first two also had the LOCKING REAR DIFFERNTIAL... which gave BOTH back tires equal power... This may not help get unstuck unless one of them has traction (on dry or solid ground). Mostly the Rear Locker greatly improved steep hill climbing on rocky roads... in my experience.

Limited Slip Differential (LSD) action means the normal spinning of a tire without traction is controlled or stopped so that the OTHER tire on that axle can get power... This is a great bonus to us off roaders because we need the tires that have traction to work and don't want the other tires (in loose sand/ snow/ mud) to spin and bury us.

LSD (limited slip differential) action can be done inside the differential (with clutches) or at the tires (with brakes). Brakes can be easily replaced as they wear out, but re-building a differential is a bigger issue! Toyota wisely went the brake route for their system.

The system is called 'TRAC' in 2WD* and 4WD High Range. It is called 'A-TRAC' in 4WD Low Range. In 2WD, both brakes and power regulation is utilized. In 4WD, engine output is not regulated.

*Also, in 2WD you can have 'enhanced' LSD with even heavier braking action to the spinning tire with a quick push of the VSC Off switch, which turns on 'AUTO LSD'.
Only in 2WD can you go into AUTO LSD. Since we have 4WD, I would think using it would be even better than AUTO LSD? Perhaps helpful for those with only 2WD.

Toyota also allowed us the CHOICE of having LSD or OPEN differentials... Just in case you want the tires to spin freely (rocking back and forth out of a stuck, or doing doughnuts in a wet field)!

To have open differential(s):

2WD: Normal is TRAC on... Hold the VSC Off switch (the one with the truck and wavey tire lines) down for 3+ seconds, while stopped. VSC, TRAC and AUTO LSD are all off.

H4 (4WD High Range): Normal mode is TRAC on... just like in 2WD. To open the differentials: Hold the VSC Off switch down while stopped for 3+ seconds and TRAC and VSC are turned off.

L4 (4WD Low Range): Normal mode is open differntials.*

*Many of us find it odd that L4 is normally in the open diff. mode... as you would typically only use L4 for the most severe traction situations where LSD would be a benefit.

To activate LSD in L4, push the A-TRAC switch next to the cigerette lighter. Once pushed, A-TRAC will always be on in L4, unless the switch is pushed again.

The Rear Differential Lock is also available in L4 with or without A-TRAC (LSD on the front axle) turned on. With A-TRAC on, one would have maximum traction, however when the rear is locked, A-TRAC only works when the truck is going less than 3 mph. Obviously, Toyota wants us only to use the rear locker for getting unstuck or very slow crawling. VSC, TRAC and ABS are all off in L4, as well.

For typical driving, H2 (TRAC on) and H4 (TRAC on) will do almost everything you would want. In L4, I think you would do better with A-TRAC on... would give you the same traction benefit as when in H4... So, that would be the only switch to activate when four wheeling slow in low range... unless you did get stuck (if that was possible now). In that case, you have the Rear Diff. Locker to activate in L4.

I had the opportunity to test the new truck in deep sand and hill climbs shortly after getting it and was amazed at the improvement over the previous Tacomas' open differentials. The truck in H4 would not get stuck in sand with the tires fully inflated. In the past, lowering pressure to 10-15 psi would have been required. After driving around in H4 (TRAC on), I opened the differentials (TRAC off) and the truck bogged down in the sand (got stuck). I then went into L4, A-TRAC and Locked the Rear Diff. I applied the gas and the truck pulled itself out of the sand! All without getting out of the cab to remove sand from in front of the tires or letting any air out. Once 'on top' of the sand, I switched back into H4 (TRAC on) and drove about easily.

Because the owner's manual was lacking clarity and nobody at my dealership could explain the new systems well, I needed to do this... I did get a lot of help from the people here on Tacoma World helping me understand what I had... and I thank you guys a lot for it!

The list, below itemizes the drive modes... we have nine!

1) 2WD: VSC and TRAC active, engine regulated ('Limited Slip Lite').

2) 2WD: 'AUTO LSD' ('TRAC OFF MODE'). No engine regulation for 'strong' limited slip.

3) 2WD: 'VSC OFF MODE' (TRAC is off for 'open differential').

4) 4WD-Hi: VSC and TRAC active. No engine regulation for 'strong' limited slip.

5) 4WD-Hi: 'VSC OFF MODE' (TRAC is off for 'open differentials').

6) 4WD-Lo: VSC and TRAC are both off in L4 ('open differentials').

7) 4WD-Lo: 'A-TRAC' on gives a 'locker-like, super strong' limited slip, front and rear.

8) 4WD-Lo: Rear Differential Locked and open front differential (A-TRAC off).

9) 4WD-Lo: RR DIFF LOCK + A-TRAC on, 3 MPH limit.

Thanks everyone, and I hope this helps other newbies to understand '09 and '10 Tacomas with TRAC, A-TRAC, AUTO LSD...

Here I am zipping along a deep sand beach in Baja over New Years 2010... in H4 (at 32 psi in the tires)!


1) 2WD: VSC and TRAC active, engine regulated ('Limited Slip Lite').
2) 2WD: 'AUTO LSD' ('TRAC OFF MODE'). No engine regulation for 'strong' limited slip.
3) 2WD: 'VSC OFF MODE' (TRAC is off for 'open differential').
4) 4WD-Hi: VSC and TRAC active. No engine regulation for 'strong' limited slip.
5) 4WD-Hi: 'VSC OFF MODE' (TRAC is off for 'open differentials').
6) 4WD-Lo: VSC and TRAC are both off in L4 ('open differentials').
7) 4WD-Lo: 'A-TRAC' on gives a 'locker-like, super strong' limited slip, front and rear.
8) 4WD-Lo: Rear Differential Locked and open front differential (A-TRAC off).
9) 4WD-Lo: RR DIFF LOCK + A-TRAC on, 3 MPH limit.

David K 01-19-2010 12:41 PM

These two charts (orig. posted elsewhere on TW) may also be of some help:


David K 01-20-2010 07:09 PM

Another way to word the drive mode selections:
Normal Mode (no pressing of switches):

2WD (H2): TRAC & VSC on. TRAC (Traction Control) uses brakes and regulates engine output to achieve LSD (Limited Slip Differential action). VSC (Vehicle Stability Control) keeps truck moving where desired, on slippery roads

4WD (H4): TRAC* & VSC on (*engine output not regulated)

4WD (L4): (no trac, no vsc)


Short press of VSC OFF switch:

2WD (H2): AUTO LSD on (no trac, no vsc)

4WD: no change, from above


Long press (hold 3 seconds) of VSC OFF switch (when stopped) for OPEN Differential(s):

2WD (H2): All off (no trac, no vsc, no auto lsd)

4WD (H4): All off (no trac, no vsc)

4WD (L4): no change (no trac, no vsc)


Press A-TRAC (Active Traction Control) switch:

4WD (L4): A-TRAC on (creates the traction abilities of front and rear lockers while in Low Range 4WD). Unless pressed again, A-TRAC will always come on when shifted into L4, from now on.


Press RR DIFF LOCK switch:

4WD (L4): Locks the rear differential. Can be used with or without A-TRAC activated. A-TRAC only active to 3 mph with the rear differential locked.

David K 02-04-2010 04:31 PM

From the Internet...
As I find well worded tech. info. pages on the traction modes, I will add them...

This from: http://www.sae.org/technical/papers/2000-01-1636

Active-TRAC (A-TRAC) is the system for off-road 4WD vehicles. This system consists of independent four wheel brake control system and engine torque control system.

This system applies the brake to any spinning wheel, and sends torque to the other wheels with grip. Therefore, the vehicle gets strong LSD(Limited Slip Differential) effect, and it has the same traction performance as a center and rear differential locked vehicle.

Because the vehicle with A- TRAC does not have a differential locking mechanism, it no longer has the phenomenon of tight corner braking, and it frees a driver from operating the differential locking system. Therefore anyone can easily enjoy off-road driving with A-TRAC.

David K 02-04-2010 04:35 PM

From the Internet
Another... from: http://behindthewheelnews.toyota.com/

Over the past several years, driver assistance features have become commonplace. The functionality and convenience of these systems continue to help develop vehicle collision avoidance capabilities. By their very nature, driver assistance systems are designed to help detect, judge and react to anticipated collision scenarios.
Anti-lock braking system (ABS) is probably the best known driver assistance feature. ABS has evolved from simple lockup control only during braking to more advanced systems that control traction during acceleration, braking and cornering by braking wheels individually while integrating other vehicle operating factors such as engine power output.
Most new vehicles are equipped with ABS these days, but ABS is not a one-size-fits-all system. ABS can include a number of additional features, which are found on a number of Toyota vehicles:
  • Electronic Brake-force Distribution (EBD) -- modifies braking force at individual wheels to compensate both for changes in vehicle load (occupants and cargo) and to help increase braking efficiency during cornering.
  • Brake Assist (BA)1 -- monitors the force with which the driver depresses the brake pedal and provides additional brake fluid pressure when the system detects an emergency stop.
  • Traction Control (TRAC) -- helps reduce drive wheel spin during acceleration using ABS and control of engine power output.
  • Vehicle Stability Control (VSC)2 -- helps prevent skidding or spinning while cornering by controlling ABS and engine power output to help keep the vehicle traveling in the steered direction, even if the driver enters a turn too fast or steers the vehicle too sharply for road conditions.
  • Vehicle Dynamics Integration Management (VDIM)2 -- combines a range of vehicle stability control functions into a single, seamless process to help provide overall vehicle dynamic drivability.
SUVs and trucks, such as the Toyota Tundra and 4Runner, may be equipped with specialized ABS-based functions such as:
  • Active TRAC (A-TRAC) -- During 4-wheel-drive operation on a snow-covered road or in rugged off-road conditions, A-TRAC controls engine output and brake fluid pressure so that the drive force is distributed to the wheels that have traction. This enhances drivability in extreme road conditions, an effect that is similar to a Limited Slip Differential (LSD). Off-road drivability is equivalent to having the center differential locked and a Limited Slip Differential on both front and rear axles.
  • Auto Limited Slip Differential (Auto LSD) -- Auto LSD uses the TRAC system to achieve the capability of a Limited Slip Differential (LSD) when driving in 2WD mode. (On a 4WD vehicle, Auto LSD only operates when the vehicle is being driven in 2WD mode.) Because Auto LSD and Rear Differential Lock perform similar functions, vehicles may have one system or the other, but not both. While Auto LSD components are similar to those in the TRAC system, there are important differences.
  • Downhill Assist Control (DAC)3 -- DAC allows the vehicle to descend a steep hill in a stable manner without the wheels locking. It does this by controlling hydraulic brake pressure at all four wheels, helping to maintain a constant, low vehicle speed.
  • Hill-start Assist Control (HAC)3 -- The basic function of the HAC system is to help increase control on steep upgrades and stopping and starting on slippery surfaces. The HAC system is designed to help prevent the vehicle from rolling backward or slipping sideways during transition from a stopped position to climbing an upgrade.

David K 02-04-2010 04:55 PM

I like these lines about A-TRAC from the above two Internet finds:

"Off-road drivability is equivalent to having the center differential locked and a Limited Slip Differential on both front and rear axles."

"... has the same traction performance as a center and rear differential locked vehicle."

4x4FishTaco 02-05-2010 11:19 AM

Thanks a bunch for the writeup! I re-read the manual a few times and was still confused a bit on the new systems. This thread is exactly what I needed. :mudding:

David K 02-05-2010 02:20 PM

Exactly how I felt! The good guys here on TW were a great help and provided many of the basics... then, I took my new Tacoma to Ocotillo Wells with a follow up to San Felipe, Baja... The new drive systems were the best! This is my third Tacoma 4WD, but the first with front and rear LSD (TRAC and A-TRAC).

Anyway, I know this has been discussed numerous times, but a little bit in different threads... NAYo and Isthatahemi were very helpful, as were others... I just needed to have ALL the data on one page (or one thread).

What I am still seeking is a side-by-side comparison of the 'mechanical' action of TRAC (H2 & H4), AUTO LSD (H2), and A-TRAC (L4). Maybe it is already in front of me, but not in an easily recognizable/ user friendly table.

TRAC is 'soft' LSD compared to AUTO LSD and A-TRAC in that braking the loose tire is more gradual. Works using vacuum, not hydraulic pressure... per what hemi has said.

I can tell you that TRAC in H4 was great, even though it was 'lite' LSD... So, don't underestimate TRAC even though it doesn't brake as strong as A-TRAC. I also have learned that the 'yellow wire' mod. for the Tacomas (without A-TRAC) provides TRAC in L4, not the same as A-TRAC. TRAC in L4 is still far superior to open differentials and that is why the yellow wire mod is so popular.

If the weather cooperates, I am taking the truck back to Baja in a week to drive a very tough trail that is like California's Rubicon with plenty of boulder climbing... that my last two Tacomas needed the locking rear differential to climb (Mision Santa Maria road). I want to see if I can climb the same grades in A-TRAC, without the rear locker. Last trip: http://vivabaja.com/msm

Anyway, keep 'testing' your new Tacomas and report here what you have found with the new drive modes.

1) 2WD: VSC and TRAC active, engine regulated ('Limited Slip Lite').
2) 2WD: 'AUTO LSD' ('TRAC OFF MODE'). No engine regulation for 'strong' limited slip.
3) 2WD: 'VSC OFF MODE' (TRAC is off for 'open differential').
4) 4WD-Hi: VSC and TRAC active. No engine regulation for 'strong' limited slip.
5) 4WD-Hi: 'VSC OFF MODE' (TRAC is off for 'open differentials').
6) 4WD-Lo: VSC and TRAC are both off in L4 ('open differentials').
7) 4WD-Lo: 'A-TRAC' on gives a 'locker-like, super strong' limited slip, front and rear.
8) 4WD-Lo: Rear Differential Locked and open front differential (A-TRAC off).
9) 4WD-Lo: RR DIFF LOCK + A-TRAC on, 3 MPH limit.

>>> An update: 'TRAC' in H4 is mechanically the same "strong" LSD function as 'AUTO LSD' is in H2, but on all 4. 'TRAC' in H2 is different as it uses engine output regulation.
It is some of our opinions that TRAC in H4 should have been called a different name to distinguish it from TRAC in H2.

Wing 02-10-2010 12:07 AM

Solid write up on the systems. Maybe it's late or maybe I'm just really brain dead atm but I'm having trouble understanding all nine different drive modes. Perhaps it's all the acronyms as well...

Just a suggestion but maybe if you were to give an example of scenarios in which all nine drive modes would be most effective in, might help common folk such as myself understand their many uses.

Anyway, rep going out!

ilg 02-10-2010 12:48 AM

Wow! Great write up. Sometimes hearing about someone else's personal experiences really helps to understand these new systems. The manual helps, but can almost make it more confusing.

David K 02-10-2010 07:35 AM

I am happy to put stuff down in a way we can understand... Here is another way that may help to see the nine modes as being different:

Basically, we have:
1) 2WD with 'mild' LSD (Limited Slip Differntial called 'TRAC'),
2) 2WD with 'strong' LSD called "AUTO LSD", in the TRAC Off mode), and
3) 2WD with open differentials (VSC Off mode, for both TRAC and VSC off).

We have:
4) H4 with 'medium' LSD (TRAC with no engine output regulation), and
5) H4 with open differentials (VSC Off mode)

We have:
6) L4 with open differentials,
7) L4 with 'strong' LSD (A-TRAC)
8) L4 with rear locker and open front differential, and
9) L4 with rear locker and strong front LSD (A-TRAC).

As for when you use those 9 modes... well, whenever you need to! Seriously, the standard setting in H2 and H4 will work. I have never needed AUTO LSD because I have 4WD, so why put your truck through any stress?

H4 with TRAC is fantastic for most 4WD applications... I was amazed on how it took me over deep beach and arroyo sand without deflating the tires (as always necessary before).

L4... well this is the one mode that I say to push a button for, the A-TRAC button, and leave it that way. L4 is for slow crawling up and down grades or through deep snow/ mud... or if you do get stuck in sand. Amazing technology makes our Tacomas tractors (almost)!

Locking rear differential: Well, with A-TRAC I just don't know if this is important anymore. Better to have it and not need it, then to need it and not have it! Before A-TRAC, I used the locker many times for steep hill climbing... and it worked. Try a steep climb and if you are halted before you get to the top... try the locker and see if you get any further. My testing at Ocotillo Wells, didn't do anything more than wher A-TRAC got me... There is more 'testing' to do, however!

OPEN DIFFERENTALS? We have the option of making the differntial(s) open in all three drives (H2, H4, L4)... if you just gotta play in the snow or mud with one tire per drive axle spinning away, and the other doing nothing... like on the pre-2009 Tacomas.

So, you have traction opens galore! However, the standard mode in H2 and H4 will take care of you... and push the A-TRAC button once in L4 and leave it that way... making it the 'normal' mode in L4, will be the way to go IMO!

Have Fun Everyone!

darenjpdx 02-12-2010 08:47 AM

I am curious if anyone else has experience the following. I have a new 2010 Double Cab 4wd 6 speed manual with Off Road Package. I recently drove it in the snow and was curious to see how the traction systems operated. I have owned 6 previous vehicles with traction and stability control and have been an advocate for them and would not purchase a vehicle without them. I have driven these previous vehicles extensively in snow-filled winters (during my years in North Dakota)

My problem is that my new Tacoma doesn't seem to have a functioning TRAC (traction control) system in 2wd mode. I push no buttons, so this is in normal mode. There is most definitely NO throttle intervention when the rear wheels spin. I can spin the wheels wildly revving the RPM's to 4000+ with little forward movement on snow. If the road is flat, I can do this forever. The only time throttle intervention occurs is when the vehicle starts to slide sideways and I presume then the VSC is kicking in. If you spin the rear tires wildly, the crown of the road will eventually get you sliding a bit sideways at the rear and then and only then does throttle get restricted. I did this just to test the systems and was very disappointed about the lack of traction control (VSC or stability control worked well like in all my previous vehicles to keep the vehicle from fish-tailing or plowing.) I run snow tires (studless Michelin Alpins) and with careful use of the throttle I got around fine, but I'm just not used to NO throttle intervention when the tires spin. This goes against what is in my owners manual and what is posted above. I tried the same in 4HI and had the same response (though of course it's a bigger challenge to get wild wheelspin in 4 hi--but I see that the tech info above says I should not see throttle intervention in 4 hi, but I should in 2wd.) Am I missing something? Can anyone else with a 6 sp TRD get the rears to spin out of control with no apparent intervention when in normal 2wd mode? Thanks for any responses!

David K 02-12-2010 08:56 AM

Interesting... have you tried the AUTO LSD to see any difference in traction? How about turning the VSC and TRAC off (open differential) to make sure that is the same as what TRAC 'on' is doing? In other words try all three 2WD modes and see if there is differences.

I would take to Toyota service department and tell them the TRAC in H2 doesn't work.

My personal answer is use 4WD (H4)... you have it just for those conditions that require better traction. No sense in thrashing your truck in 2WD, when you have 4WD.

David K 02-18-2010 09:14 AM

Wondering if Darenjpdx ever got it fixed or figured out?

David K 02-20-2010 12:27 PM

We may never know?

It did sound like his TRAC and VSC were both off, however...

David K 02-21-2010 03:58 PM

Today, took the new, stock truck (with the Rugged Trails) up to Panda Jerk's "Drug Airport" 4X4 area... (see: http://www.tacomaworld.com/forum/sou...port-pics.html ) off the north end of San Marcos' Twin Oaks Valley.

After going up, around, and back down from there... and never using the rear locker, I can say that Active Traction (A-TRAC) works like GOLD! The rear locker may never be needed!?

Just the long rutted out grade alone was a good test... hairy stuff doing it alone (just in case),,, and even dropped the rear tire into a super rut where the turck came to rest on the spare and trailer hitch, when I was backing down from a questionable 'dead-end'... The A-TRAC with maybe just two tires having traction, pulled me out. A rear 4X4 mud flap got pinched off... as I knew would happen sooner or later!

Nice to find a good 'proving ground' for low range 4WD closer to my home than Ocotillo Wells!

David K 02-23-2010 07:35 AM


Originally Posted by New Mexico Taco (Post 1461566)
Great write up about the Auto Lsd and Trac system but what about those of us that do not have A-Trac and a locking Diff. I have an 2010 SR5 V6 auto 4x4 that is one step below the TRD package I think.
I have the TRAC and Auto LSD. When I press the TRAC button and the LSD light comes on red does that mean that it is off ?
Confusing as hell though I have never been stuck in snow and sand YET letting it work on its own. Would like some options

Since I don't have your owner's manual or truck to look at, I am going to take a stab at what you have...

Press the button in 2WD and that turns ON the AUTO LSD, which is a stronger LSD than TRAC in 2WD. Hold the button down for 3+ seconds, while stopped, in H2 or H4 and you turn off everything for open differential(s).

In H4, you are normally in TRAC mode. In L4, you are normally in open differential mode.

SR5, but not a TRD 4WD...

Right, the Active Traction Control (A-TRAC) is only on the Off Road TRDs... which means you should have enhanced traction control with AUTO LSD in H2 (quick press of switch) and with TRAC in H4 (nothing to press).

To have open differentials in H2 and H4 you hold down the switch for 3+ seconds while stopped. In L4 you only have open differentials and no other modes from Toyota.

H4 with TRAC will get you most anywhere... and L4 is for crawling up grades or through mud. Toyota should have made TRAC standard in L4 for enhanced traction while crawling. It isn't as strong as A-TRAC, but having TRAC in L4 is a great improvement, and there is a mod to give you TRAC in L4 ... see this thread: http://www.tacomaworld.com/forum/tec...4low-trac.html

KenpachiZaraki 02-23-2010 07:56 AM

Great write up, makes want a 2009+ now :(

David K 02-23-2010 10:31 AM

I traded my 2005 for this one, a couple months ago... with Toyota's problems, maybe now is a good time... ads here in SoCal are offering deals on Tacomas. Maybe you will have the same in Texas or New Mexico?

David K 03-03-2010 05:03 PM

A-TRAC Video
Here is a 3 minute You Tube of A-TRAC in action in deep sand... basically showing that you have the same 4 wheel drive action as lockers front and rear... as the video shows all four tiress with torque...


A-Trac demo using a Tundra:


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