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The Hummer Knowledge Base

BTM with TT4-Equipped Hummers:

> Since I have torque biasing differentials, I could still use the BTM method on
> my 2k wagon.  Hitting the brakes disables TT4, so I can have it either way... is
> that right?

The short answer is "yes". The better answer is "kind-of". When AMG started testing ABS they originally had the torsen 1 diff's that have been in the truck since they first started making them. Unfortunately they discovered some very unpredictable handling characteristics when testing the ABS when on surfaces where there was significant differences in traction between one side of the truck and the other. When the factory guy told me this, I had visions of the truck rapidly swapping ends and spinning out. So.... They switched to the torsen 2 differential. The T2 has a much lower torque bias. The T1's were biased about 2:1, the T2's are about 7:1.... So it takes significantly more force on the brake pedal when trying to use BTM with the TT4/ABS trucks to get torque to transfer and BTM to work. When it does, the braking at the other three wheels and the lower biasing prevents it from being as effective.

The nice thing about computer controlled TT4 is that the brake is only applied to the one (or two) wheels that don't have traction... So your not eating all that torque up at the other four wheels which helps significantly and is why it works despite the lower bias of the T2 diff.

You CAN use BTM on a TT4/ABS truck. You won't be happy with it, compared to how it works on the pre '99 trucks. I've just switched up to a 2000 year truck from my 97.5 and while I'm going to give TT4 more time, I really don't like it compared to BTM on the earlier truck. Most people I've talked to who have experience with both BTM and TT4 really dislike TT4. There are two "good" things about TT4 IMHO: 1) is that it only applies the brake to the wheel without traction and 2) it lets anyone make forward progress without having to know anything about BTM and torque modulation.

You will find TT4 to be much more effective if you hold the throttle constant, right at the peak of the torque range, and let the computer do the work. If you keep changing the throttle position (instinct is to "give it more gas" when the truck stops moving) you will confuse the ABS computer and it will have a rougher time figuring out what to do.

Oops, someone pointed out that my bias ratio's were wrong. That's what I get for not checking my figures.. Someone who would know pointed out the correct ratios: "T1 bias ratio is 3.2 and the t2 is half of that or 1.6" and that those ratio's are only valid when the diff is new - at it get's broken in, the ratio's go down.


> Ok, I'll ask:  Why does the ratio go down?  And does that mean that it makes
> BTM harder to do?

The ratio went down because they put a different torsen design into the differential. The gear sets are obviously different.

What I was told by the factory tech was, that when they were testing the new ABS system where one side of the truck (say the left) had excellent traction and the other side of the truck (the right) was on a slippery surface like ice - extremes of traction - that the truck's behavior was "unpredictable". What I took that to mean is that the truck's torsen 1 biased too quickly under ABS and the truck rapidly started swapping ends going down the road.

So they put the T2 diffs in with the lower bias ratio - which takes more effort to get the diff to transfer torque to the wheel with traction - but the behavior on the road under ABS braking with extremes of traction was much more acceptable (i.e. it didn't go spinning down the road).

After all, what happens in ABS? The truck detects that a wheel has no traction because it has stopped rotating - so the ABS computer rapidly releases brake pressure to that wheel so that it starts to turn - and then modulates the brake pressure to keep the wheel turning, but still slowing the truck as much as possible. What is comes down to was a requirement for a less sensitive differential that doesn't react as fast when the ABS computer is doing it's thing - which prevents the torsen from transferring braking torque and and spinning out the truck.


Here's more discussion of this on the forum

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