How does tod work?

[DannyDigital]

I spent 6 hours stuck in some mud on Friday. I have no 4x4 experience at all. I've owned a CRX and 2 GTI's so my understanding of these drivetrains is rudimentary at best.

Here is why I'm confused...



The rear driver side wasn't touching any ground and the driver front was, but didn't appear to be spinning. The front passenger side was also spinning freely but the passenger rear wasn't.

The two wheels with decent traction areas weren't moving and the two that had no traction were spinning freely.

Why?

[4X4 UFO]

As I understand it, the TOD computer compares front and rear driveshaft speed. If the rear is moving faster than the front, indicating some wheel spin, the progressive clutch pack keeps sending torque to the front driveshaft, until the split is 50-50. VX's have a limited slip differential in the rear, but not in the front, so both rears should turn. Since the front differential is not a limited slip type, as soon as the first front tire starts spinning, the other front tire will no longer try to turn. In trail/muddy conditions, you can select 4L on the transfer case shifter. This locks both front and rear driveshafts together, and eliminates the progressive rear/front torque split. In 4L, it stays at 50-50. Still, if you lose traction on one front wheel, you'll get no help from the whole front end. Both rear wheels should always be pulling.
If I've missed anything, let me know, folks!

Randy

[SlowPro48]

Why?

The TOD varies the torque split between front and rear via an electromagnetically activated clutch pack. If you're in 4L this clutch is bypassed and the transfer is handled by a mechanical lock so torque split is 50/50. The reason you got stuck is the open differential in the front and limited slip in the rear. Torque sent to the front will follow the path of least resistance - if one wheel spins, all the torque gets delivered to the spinning wheel with no traction. On the rear, because of the LSD, there is some torque delivered to the non-spinning wheel but obviously not enough to get you out of trouble. Did you try feathering the brakes? Sometimes that will stop the spinning wheels and divert torque to the ones with traction.

If you're going to do this on a regular basis you might want to invest in an air locker. Don't know if it's better to have a locker on the front or the back though - will leave that bit of advice to the off-roaders on here.

I hope you weren't by yourself!

[DannyDigital]

Ah that makes sense. I was with my brother, but I'd been through that trail a couple times without problems so I wasn't too worried. It was a little muddier than usual though.

Had to wait for a friend of mine to get out of work to pull me out... Now I see why someone said they keep a deck of cards in their truck at all times.

[deermagnet]

This is a nice little Isuzu tech video I have about TOD from 1998. I believe everything is the same for our VX except we have no TOD switch. Ours is on all the time. The method shown for reading and erasing TOD codes is the same as in the VX manual. The quality is not the best, but it's very watchable.

11:39 and 13.9 mb, WMV file only-
http://www.drivehq.com/file/df.aspx/publish/mgpa/techvids/tod.wmv

Mark

[Triathlete]

If you were only going to lock one end or the other the front would be your best choice since it is an open diff. The rear limited slip can be manipulated with a slight application of the E brake. Of course optimally lockers front and rear is the ideal set up for off road.

I had an old military instructer that when confronted with a question on how something worked that he had no answer would always answer "AM/FM"...
"Amazing Miracles and F'n Magic"!

[VXorado]

The rear limited slip can be manipulated with a slight application of the E brake.

I've never heard that before, could you explain?

[rowhard]

I've never heard that before, could you explain?

Was a trick that Todd Adams taught us, or me anyway. As your starting a climb or whatever, you apply some emergency brake like in a hand brake turn and that helps lock up the rear diff. It is a feel thing,.....

AM/FM

[AlaskaVX]

Can't understand why the LSD wasn't turning both rear wheels together ?

Limited slips still slip quite a bit it only transfers "X" amount of power to the other side by using a sping pack and clutches.

Quoted from rom Howstuffworks.com

The spring pack pushes the side gears against the clutches, which are attached to the cage. Both side gears spin with the cage when both wheels are moving at the same speed, and the clutches aren't really needed -- the only time the clutches step in is when something happens to make one wheel spin faster than the other, as in a turn. The clutches fight this behavior, wanting both wheels to go the same speed. If one wheel wants to spin faster than the other, it must first overpower the clutch. The stiffness of the springs combined with the friction of the clutch determine how much torque it takes to overpower it.

Getting back to the situation in which one drive wheel is on the ice and the other one has good traction: With this limited slip differential, even though the wheel on the ice is not able to transmit much torque to the ground, the other wheel will still get the torque it needs to move. The torque supplied to the wheel not on the ice is equal to the amount of torque it takes to overpower the clutches. The result is that you can move forward, although still not with the full power of your car.

In this case there wasn't enough torque created to turn the other rear tire with our limited slip system.

The rear limited slip can be manipulated with a slight application of the E brake.

I've never heard that before, could you explain?

Now with an e-brake you can apply more restriction to the spinning wheel enhancing the torque tranfered to the other side. Since the e-brake is only hooked to one rear wheel this only works when trying to transfer it one way. And since I think it is the rear left it could have helped you in this situation, but if you chose other side through the puddle it wouldn't help at all.

And you need some mud tires.... that alone could have got you through this IMO

[blacksambo]

Remember, unlike alot of SUV's, with Isuzu's full-time TOD on, the VX is truly rear-wheel drive biased, meaning the torque goes to the back first, where the LSD resides. This tendency is obviated when the 4L is engaged, then it mechanically equals things up, front to back, torque-wise. The automatic full-time TOD system was originally meant to allow smooth transfers of power across varying terrain, at relatively high cornering speeds for a tall vehicle. Superior, changing-terrain, handling was the primary purpose of the full-time VX TOD system. Swamp crawliing calls for the 4L mode, exclusively, and the addition of locked front hubs would help immensely to compensate for the lack of LSD on the front axel.

[samneil2000]

I agree with everything that's been said here. But it looks like you could use some tires with a bit more bite as well, especially if you will be doing a good bit of that sort of riding.

[vt_maverick]

I agree with everything that's been said here. But it looks like you could use some tires with a bit more bite as well, especially if you will be doing a good bit of that sort of riding.

+1 - Perhaps the wisest (and cheapest to take) piece of advice in this entire thread.

[JAMAS]

I agree with everything that's been said here. But it looks like you could use some tires with a bit more bite as well, especially if you will be doing a good bit of that sort of riding.

Took the words out of my mouth.

[BigMeatVX]

Back to the topic...here are some pics of the TOD internals...

[Greasemonkey]

Can't understand why the LSD wasn't turning both rear wheels together ?
Agree with the comment about being in 4lo and having better tyres
cheers
Steve