Checking the Hummer Drive Train

Geared Hub Article

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Hummers have freeplay in the drivetrain. That's why they rock when you come to a stop. Like anything else too much freeplay isn't a good thing. The symptom that got me checking the drivetrain was a clunk noise every time I would push the accelerator down while driving around 30 mph on a flat road. I 'd get up to 30 mph, take my foot all the way off the accelerator and then push it down again. Every time I put my foot up and down I'd hear a clunk coming from the bottom of the truck. In this case it was just the automatic transmission clutch activating. The transmission through various sensors will sense when and how much pressure it needs when engaging it's internal clutches.

Checking the Drivetrain

On a flat surface I jack up one corner of the truck under the A-arm to get the tire just off the ground.
I go under the truck and look at the half shaft as it comes out of the geared hub. I move the tire back and forth a bit and watch the half shaft. If I can move the tire and the half shaft doesn't immediately move I know I have free play in the geared hub.

Next do the same thing looking at the half shaft as it comes out of the geared hub. Watch the brake rotor. When I rotate the tire back and forth and the input side of the half shaft moves I see if the brake rotor is moving with the half shaft. If it isn't the half shaft has free play You might want to grab the rotor and move it while watching the half shaft at the hub.

Next I put a screw driver in the U joint on the input side of the diff and try to turn the diff. If the U joint is bad you will get free play/ wiggle in the joint. You can check all the U joints this way.

You can check your transfercase by moving the tire back and forth. Watch where the front drive shaft enters the transfer case. The front drive shaft will rotate back and forth taking up slack in the chain and planetary gears in the case. It's not unusual for the drive shaft to rotate back and forth 3/8" on a good case. If you hear the chain hitting the inside of the case you definitely need a new chain.

Then I chock the wheels that are still on the ground and put the transfer case in Neutral. If you don't chock the wheels the truck will roll. I rotate the drive shaft watching for free play in the U joint as it enters the diff. As you turn the drive shaft watch the brake rotors for movement. If you can turn the drive shaft back and forth a bit and don't see movement in the output / rotors then you have free play in the diff. You can grab the rotor and drive shaft and move both to detect free play

It's not unusual to have some free play Only experience will tell you if there is too much. If you do all 4 corners you should find differences. If you find one diff much looser then the other then you may have a problem. If one half shaft is tight and another has 1/4" of free play then you need a new half shaft

I've found that new half shafts don't have any discernable free play. Half shafts on high mileage trucks may have a little bit of movement. U joints shouldn't have any free play. Geared hubs have a little detectable free play Differentials seem to be a big variable. In my opinion the tolerance of the diff's from the factory is pretty loose.

Another important item to check is the brake caliper brackets. These brackets attach to the differential with 2 large bolts. If they work lose your brakes will literally fall off. Before that happens the bolt head will back out and contact moving parts which will let you know fast. I've seen this in a number of trucks. When I do a brake job I always check for this. It first showed up as a medium pitched clunk when I would back out of my garage first thing in the morning. After awhile it became frequent until I found it. The noise is the bracket moving back and forth as the brakes are applied. You can stick a screwdriver and try to pry on it to see if it's lose.

Another thing I do to check the drive train is to make sure the calipers are lose by prying the brake pads back. Stick a screwdriver in the end of the rotor and move it right to left. If it moves around you may have worn diff output bearings.