Hello, I have an 03 GX470. Last night I got stuck for the first time in deep snow. I backed up in to a snow covered ditch, and bottomed the car with my drivers side sunk deep in to the snow. I put the car in Low gear and locked the Dif, but when I tried to climb out it didn't work as only two wheels were turning. I am very confused now, is my 4x4 broken? I had one turning on the front right and rear left. My understanding was that all 4 wheels should have been turning? I have a current problem where my trac, vsc, abs are lit on the dash board (speed sensor, and steering positioning sensor threw a code) could that be the cause of the problem?

Im sure it has been explained earlier how the 4x4 works on this car, but just cant find a definitive answer.

Thank u guys!

 

When you lock the diff both front / rear axles should be turning. When locking the center diff, the system automatically turns off the VSC. Maybe since yours was already off (warning lights on) it didn't lock the diff. Was the diff lock light on the gauge cluster flashing or solid? Flashing would mean it couldn't lock.

Also when you gave it a little gas and you being stuck you should've heard the A-TRAC engage. Did you hear any funky sounding noises from around the pedal area?

 

Does ATRAC automatically come on when you go into 4LO, or do you have to manually enable it via a push button? (In my 4runner it has to be manually enabled, but I don't remember seeing an ATRAC button in the GX)

 

A-TRAC will engage automatically anytime one of the tires start slipping.

 

On any vehicle, the differentials on the axles are designed to send more power to whichever wheel on the axle has the least amount of resistance.. Which allows the two sides to turn at different rates to compensate for making turns.

The drawback of this is that if you REALLY get one front and one back tire into zero traction situations, (such as bottoming out on snow and being high centered), then the differentials by their very design will send all the power to only those two wheels, which will render you stuck.

The electronic traction system on these should help alleviate that, but if all your dash lights are on like you were saying, there may just be an issue with the traction control system which is keeping it from operating

 

I thought if you have CELs on that the traction control is disabled by default. I bet if you resolve your dash light issues, the A-TRAC will start working.

Yoshi's explanation of open differentials is spot on and why traditional "four wheel drive" really only drives two wheels in low traction situations. Still better than RWD/FWD.

 

correct me if I am w rong, but the truck is an AWD in standard mode, ie it transfers power, as subare says, from the wheel that slips to a wheel that grips....lock the center diff and it is now a four wheel drive vehicle...locking the front diff, ie, both wheels spin at the same rate so driving on pavement causes the tires to scuff in a turn....moving the transfer case lever from high to low just gives you lower gearing, ie, more torque at very low speeds.

 

My understanding is that the front & rear differentials don't lock (only the center one does). It relies on A-TRAC to simulate a locking diff for the front & rears.

So yes, if you high-center the vehicle, and A-TRAC isn't working, then you're probably going to need a recovery...

Chip H.

 

I thought TRAC was on all of the time (which is a less aggressive form of ATRAC), and that TRAC gets disabled in 4LO, and you could then enable ATRAC. This is the case in my 2013 4runner, but maybe GX does this for you automatically.

 

We have A-TRAC and it works all the time whether you're in HI / LO, center locked or unlocked.

 

im so lost! So, what happens when i lock the centre diff? i thought im supposed to have all 4 wheels turning, with at least 25% of power going to the ones that have grip and up to 75% to those which have the least grip. Is the wheel slip controlled by traction control and it transfers the power to a non slipping wheel? Like i said my abs, trac, vsc are off as there is an issue with some sensors (btw if anyone has left front speed sensor and a steering positioning sensor id like to get those) when i was stuck i had only two wheels turning with the diff lock on...

 

Read this: http://www.lcool.org/technical/diffs/diffs.html

 

The GX has three differentials - one front, one rear, one center. Normally, they are all "open" meaning they'll spin as you go around corners as you drive.

One thing about all-open diffs is if one wheel loses traction (is lifted in the air, or is on ice) all the engine power will go to it. To reduce this effect, Toyota includes a system called A-TRAC, which is enabled all the time on the GX. What it does it look for wheel spin (via the ABS sensors), and applies the brake to only that wheel. This permits power to return to the other wheels and hopefully get you going again.

If you lock the center differential, that fixes the power ratio between the front & rear axles (sorry, don't know what it is on the GX - let's say it's 40:60 for now). If you lift a front wheel, that means that you lose 40% of your traction (since the front diff is open, it's going to let all that power go to the spinning wheel), up until A-TRAC kicks in and you'll get some of that power back. You'll still have 60% power going to the rear axle.

What if you have two wheels lift? One front, one rear? You'll lose 100% of your traction ... until A-TRAC kicks in. This is because the front & rear diffs are still seeing the percentages that the center diff is sending their way, but sending all that power to the spinning wheel on their axle. Once A-TRAC applies the brakes, you'll get some of your power back.

Mercedes M-Class runs with all open differentials and relies entirely on brake-applied traction aids (their 4-MATIC system) and for most situations is quite good.

Stock Jeeps are like the GX, but the owners will sometimes add locking systems to the front & rear differentials, so that not only is there a 40:60 rear split, but then at the axle, there's a further 50:50 left/right split (the front left wheel gets 50% of 40%, or 20% engine power, for example)

You should never lock a differential when you are on pavement or other high-traction surface. As you go around corners, the differential accounts for the different distances along the curve that the left & right wheels have to travel. If it is locked, that difference gets applied to the axles and other expensive drive-line components, up to something breaking, or the tires losing traction.

Hope this helps.

Chip H.

 

Thanks for explaining A-TRAC chiph9.

 

THANK YOU CHIP

Finally i understand what happened. considering i had no atrac working i lost traction to the spinning wheels...