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Refer to the Halfshaft Illustration.
The HUMMER diffs were designed for ease of use by military drivers, and it's my understanding from talking to many military HMMWV drivers that they have NEVER heard of "brake-throttle modulation", the most common comment being, "we just go till we get stuck, then put it in hi-lock and rock it back and forth till we get out".
As I understand it, BTM was discovered kind of by accident, and was not a design feature, but it WORKS.
I have a method I call "Constant-torque Modulation" which I find works even better than the standard BTM in the most extreme situations.
One of the things I found at Moab was that finesse is everything, and large power changes, rushing obstacles, jamming brakes and otherwise being ham-handed is detrimental to forward progress and wallet thickness, so I experimented with a modification of BTM.
When I approach an obstacle, I set up for the climb and when ready I apply FULL brakes. Then, I apply throttle and run the engine up into the best-torque range AND LEAVE IT THERE. In other words, I keep the throttle pressure the same throughout the climb. Then I use the BRAKES to adjust speed, NOT the throttle.
I have found that for the most extreme situations that this provides a much smoother climb with less slippage, bouncing, pounding or traction-breaking. I watched other drivers as they would approach, begin the climb, modulate, and then goose the gas part way up because they felt they were slipping. All this did was screw them up.
The constant-torque method offers you the most available torque for the whole climb at a steady power-state which does not fluctuate, while also ensuring that your Torsen's will be fully locked at all times so you don't spin a wheel and possibly snap a half-shaft through weak modulation pressure. The power input to the wheels is very smooth and I have been able to calmly walk up virtually every obstacle I have ever done at Moab with the sole exception of the Rockpile in Pritchett canyon, which required a slight run for inertial assistance due to my weight.
I most especially use constant-torque on DOWNHILLS, where it provides very, very precise control of your forward progress with little chance of accidentially "goosing" the pedal as you slide off the edge and bang down onto something on Nosedive, something I observed was a real problem. With the brakes already fully on, you just let off a hair to creep forward as needed.
Yes, it's hard on brakes and tranny, and should only be used on the severest of obstacles, using regular BTM on less demanding ones, but it's worth the effort, as it results in a smooth, finessed crawl rather than a banging, clanking gut-buster.
Try it sometime, you'll like it.
I broke a halfshaft when I hit the brakes while I still had the accelerator down. The wheel was spinning and the engine was up around 2000 rpms. That tiny little shaft did not like all of that torque slamming to a stop. The shaft broke at the spline inside the output flange at the differential.
I broke the right rear shaft...FYI, according to George Knight there are three different shafts on a Hummer. The rears are the same but the fronts are each different. I'm not sure why, maybe some physical size difference due to the steering mechanism.
Here's my take on how to replace a half shaft. Please feel free to add or detract if necessary. I'm sure that several of you will have better methods.
Tools you will need:
3/8" Socket Driver & Extension
It doesn't really matter where you break the shaft the replacement is the same. The replacement shaft is ready to install, no need to attach CV joints or grease any of the parts. The CV boots are even pre-installed.
First, jack up the vehicle. I use a Hi-Lift jack so I lift at the bumper shackles. A second jack will come in handy later but if you don't have two then blocks or jack stands will help.
After removing the tire you will see the access plug near the top of the geared hub. I used the end of the socket extension to remove the plug and washer. Inside the plug is the half shaft retaining bolt. Loosen the retaining bolt with the 9/16 socket and remove the bolt and lock washer. Remove that piece of the half shaft, it should be loose now.
Next, with the 15mm socket and wrench, loosen and remove the six bolts attaching the other end of the half shaft to output flange at the center of the rotor. Be careful not to lose any of the washers when you remove the bolts. You'll need to put the transfer case in neutral to turn the rotors in order to access all of the bolts. I was under the truck at this point but Scott's suggestion about using a long extension to get to the bolts is much better.
Clean all of the exposed areas, it will probably be pretty dirty. Take the new shaft, insert it into the geared hub. Secure the half shaft with the retaining bolt and lock washer. The service manual says to use Loctite 272 and torque to 37 lb-ft ( 50 N m).
The next part is a little tricky. The shaft is a little too long to move all the way up to the rotor and the brake cable is in the way. Remove the cotter pin and snap ring with the pliers & screwdriver, the brake cable will pop loose. This is where the other jack came in handy, to get the other end of the shaft up to the rotor I jacked up the A arm about 4". This created enough room for the shaft to move up all the way. Move the rotor until the holes line up, I think it will only line up one way. If you have new bolts and washers use them. If not, make sure to get as much of the old Loctite out of the threads as possible. If you have to use the old bolts, I'd suggest replacing them as soon as you can with new ones. The service manual says to use Loctite 272 and torque to 48 lb-ft ( 65 N m). Hook back up the brake cable and put the tire back on.
Everything should work just fine now.
Thinking about it now, it might be possible to attach the rotor side first, avoiding the need to remove the brake cable. But at that point, I'd already put the shaft into the geared hub.
BTW..a new half shaft is around $300.
Again, you experts please feel free to add your opinions, this replacement was my first shot. Better methods would be appreciated.
Two suggestions for field half-shaft replacement:
Carry a spare set of half-shaft bolts and the special Camlock washers or lots of RED locktite, the bolts should be replaced each time, not reused.
Buy 2 18" socket wrench extentions for your toobox. Put the two extentions together to remove the brake-side bolts. It's much easier than getting down under the truck, although you might have to lower the brake hose to get to the upper bolts if you can't turn the wheels a bit.
Blocks or jack stands are *absolutely mandatory*, especially if using a hi-lift, which is inherently less stable than the stock jack, but even with the stock jack, DO NOT work under an unblocked vehicle, ever. And be sure your jackstands are rated to hold the HUMMER!
Otherwise, very good directions! Thanks!
On the BTM training at the obstacle course (at AM General), I was shown essentially what I believe is ObeWanKenobe's (Scott's) CTM, and this was the only successful way I (we) got through the really slipery spots (mud and only two wheels on the ground). Two points I'd stress:
1. Brake pressure was much higher than I was orginally shown.
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