Hi, gang. I put the '00 Tahoe Z71 (so a '99 drivetrain) that I bought a few months back into 4Hi for the first time since my test drives, and it's all messed up -- a bumpiness and halting when turning, and a big clunk when I go back into 2WD. Driving straight ahead it seems OK, but I haven't done it fast or for long -- scared to! Any idea what's likely to be wrong, and what it's likely to cost? Thanks!
4WD Problem - Help!
- Thread starter Scoobey
- Start date
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blk87brd
Full Access Member
If you are on bare pavement, any 4wd will load up and fight you. Try it on gravel or at least on wet pavement. Tere may very well be nothing wrong with your truck.
Really? Man, I hope you're right! But we had an AWD van that didn't do that, but this is my first 4WD. I thought the 4Hi was supposed to drive OK even on dry pavement -- as when you're in and out of patches of snow, that kind of thing. Can you explain more why it would "fight me" (great phrase!) like this on dry, and how I might tell whether it's broken or not?
blk87brd
Full Access Member
I had a 99 Tahoe with the manual shift on the floor. On dry pavement in fwd it would do exactly what you describe. I now have a 99 Yukon with the electronic shift on the dash. In 4wd Auto it runs beautifully, but in 4wd Hi it acts just like the 99 Tahoe. I don't think there is any differential in the front end so slow turning wheels try to "fight" the opposite wheel. My Chev pickup 4wd did and so does my CJ. In awd they have some form of slip mechanism built into the diff. Take it out on some grass or gravel and it should act noramlly.
OK, we need a little refresher course from “Four Wheel Drive 101”
If you were to drive in a circle on fresh snow, you would see 4 circles. All four tires run in their own circle.
In a left hand turn, the inside would be the left rear, then the left front, then the right rear, then the right front on the outside.
The front axle has a differential to allow the different speeds of the right wheel and the left wheel. So to does the rear axle. A differential for speed difference from left to right. Now for THE problem. You don’t have a differential between the Front drive shaft and the Rear drive shaft.
Average the speed of the front wheels and the rear wheels. They are different. That difference when locked together with four wheel drive and nice rubber tires with six thousand pounds sitting on them, will increase the pressure until one of the wheels “bucks” and slips back to release the pressure building from the different axle speeds.
If you zooming through mud or snow, it easily slips a little in the turns without bucking and you never notice anything.
The longer the wheel base, the more extreme the problem. They have tried differentials between the front and rear drive shafts, and the net result was the wheel with the lightest load, of the four would spin.
All wheel drive vehicles have a clutch device of one sort or another between drive shafts, to keep the trauma to a minimum.
When driving straight the axles are turning the same speed, it is only in a turn when the axles must turn different speeds.
If you had four wheel steering, so the front and back wheels followed in the exact same track, you would not have this problem. Then again, four wheel steering has it’s own problems.
Find some nice soft gravel and lean out the window and watch your left front wheel while in a tight left turn, in four wheel drive. You will see it kick back every now and then, giving up the pressure building between the axles. :yesnod:
