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The Hummer Knowledge Base

BTM - General:

>"Donald Calebaugh,MD"
>A basic, show one's ignorance type of question?
>Does brake/throttle modulation occur only in HI/LOW lock or can it be
>initiated anytime ?
>Thanks in advance,

Anytime, but if using it without lock --which engages the front driveshaft, you will be able to apply traction to both rear wheels. Putting the brakes on --(also to the front which occurs in Throttle Brake Modulation) without having any pulling from a front wheel drive situation-may actually hold you back.

So my answer is it may or may not work depending on the terrain situation and how much brakes are applied.

Personally I waiting to get Electro-Mechanical lockers for Front and Rear diffs, so I never have to use TBM ever again (puts undue load on the drivetrain) Have a completely open diff for the street (better than a torque biasing for street) and full spooling at a button control-(rear only, rear and front if needed).


** BTM's ability to function is not limited to transfer case selection. However, because of added gear reduction of "Low-lock", it is easier to modulate the system. Lower gear ratios, (2.73:1 X 1.92:1) in high or high lock are more difficult. Too, remember BTM's basic application is when wheel spin begins to occur, release pressure on accelerator, apply hard pressure on brake, re- apply pressure to accelerator and when you need to, allow some relief on the brake pedal to "feel" the BTM working. I use BTM in situations in low-lock when I know I will need it. I keep light pressure on the brake, ready to apply more when I need the four wheel engagement.

Dennis Kulp

The front drive shaft is not "engaged" by locking the transfer case. This is a full-time "four-wheel drive" system. That means that there is a differential in the transfer case that supplies power to both drive shafts, similar to the way that the regular diff supplies power to both halves of the axle.

This means that in Hi (i.e., not Hi-Lock), if any one wheel is free to spin, drive power is lost to all of the other wheels. Locking the transfer case means that one wheel on each axle must spin for problems to occur. When using BTM or a locking diff, each wheel is driven, regardless of whether any of the others are spinning or not.

Here is an overview of what happens. The gearing inside the Torsen diffs (the stock Hummer diffs, also available as after-market for many other vehicles) is much different than normal differentials. As long as the two output shafts (axle halves) are turning at nearly the same speed, power is provided to both. When one begins to spin faster than the other, only the spinning one is given any power. This operation is very similar to other diffs, and is what allows Hummers to go around corners without the tires scuffing.

What BTM does is to restrict the speed of the "slipping" tire to about the same speed as the other tire, thus forcing the diffs to power both sides of the axle. This very closely approximates the operation of a true locked differential (i.e. like a solid axle).

The differential in the transfer case is not a Torsen, and is not a "torque sensing" diff. This means that BTM has no effect on it. If it is locked (i.e. in Hi-Lock or Low), it is disabled, and power is sent to both drive shafts where the differentials there determine how to power the tires. When it is not locked (i.e. Hi), only one drive shaft will get the power (the one that is turning faster - i.e. slipping). Because of this, and the higher gearing of Hi range, BTM without locking the transfer case diff is of limited value.

Dave Breggin
'95 Diesel Wagon

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