Trucks brake better in 4WD.Some trucks will disable the anti-lock,and some change the programming or strategy. Even without ABS they stop better in 4WD.
Heres why.......The 4WD tends to prevent the fronts or rears from locking up by themselves, as all 4 wheels are tied together. You also get effective engine braking at all 4 wheels too. 4WD also helps the vehicle track straighter, so your less likely to spin out.
Once the truck is in 4X4, the whole theory of the front axel being 70% braking and the rear being 30% braking goes out the window.
Every wheel would have 100% braking. Plus, either all wheels would lock or not lock when braking. Of course it all goes out the window when your already sliding out of control.
ABS does not work with part time 4WD systems
ABS works as designed as long as a part time 4WD vehicle stays in 2WD mode. In 2WD all 4 wheels are able to rotate at different speeds (made possible by the differentials) and more importantly in 2WD both axles are allowed to turn at different speeds because the drive shafts feeding power to the axles are not connected.
As soon as you shift into part time 4WD (find out which kind of 4WD you have) the two drive shafts become connected inside the transfer case and both axles would be forced to turn at the same speed. More importantly, when stopping both axles will be slowed down at the same rate (any brake proportioning will be out of force) and the ABS system would get confused. After all, the ABS's job is to detect variations in wheel speeds and act accordingly. ABS will not function properly on part time 4WD systems!
Because part time 4WD and ABS do not mix, most manufacturers offered only rear wheel ABS on these vehicles since the rear axle needs ABS the most, due to the tendency to lock up first.
Some manufacturers saved the money and did not offer ABS at all on part time 4WD. On some vehicles a warning will flash that ABS is not working properly when in part time 4WD (Mercedes G500) - others offer no warning and the familiar pulsating of the brake pedal indicates that ABS is working . Well, its not.
However, on dirt roads and other off-road situations ABS could be bad for you and it should be "off" anyway. ABS was designed to keep wheels from locking up. ABS was designed to make driving safer (in most circumstances).
Locked up wheels can no longer steer the vehicle. For example, you would not be able to steer around an obstacle with locked wheels - with wheels locked you would slide straight into it. So, even though you are able to turn your steering wheel - the vehicle can not follow this command when the weels are not rotating. ABS keeps wheels rotating.
Again, ABS makes driving safer.
Not so on a dirt road!
Sand and gravel under the tires act like tiny ball bearings when you step on the brakes. As a result there will be very little friction/traction between tires and ground.
Therefore all four wheels want to lock up immediately.
ABS would keep them from doing so.
So you keep rolling and rolling, and rolling, and rolling. You might be rolling for too long and your stopping distance is going to be that of a container ship. Too long to be safe. You might even drive off a cliff.
Here is what you get with ABS off: When you hit the brake good and hard the wheels will lock up - the tires will immediately begin to dig in pushing against sand and gravel - this builds little berms in front of each tire, which very effectively slows down the vehicle. As you see, on dirt locking wheels can be good for you! Picture a cartoon horse stemming all 4 into the dirt after a fast run.
In fact, stopping without ABS on sand or gravel can be so brutal that you better have your seatbelt on - otherwise you will end up with your face in the steering wheel. Been there.
To deal with the negative effects of ABS on dirt, some manufacturers had installed a kill switch for ABS in their full time four wheel drive vehicles (for example early 90's Mercedes-Benz G-Class) but recent changes in the law prevents them from providing this option any longer. Seasoned four wheelers have either installed their own kill switch or simply pull the ABS fuse before hitting long stretches of dirt road.
Vehicles with part time 4WD don't really have a problem with ABS - ABS does not work in combination with part time 4WD anyway.
Also, on some vehicles you have an option of using full time 4WD/AWD or part time 4WD. Its up to you to chose the right setting.
ABS works really well with full time 4WD, AWD and automatic AWD.
However, it works only as long as the center differential of a full time 4WD is not locked. Locking the center differential turns full time 4WD into part time 4WD.
A word of caution. Some vehicles combine 4WD systems (could be full time plus part time - could be automatic AWD plus part time, etc.) On these vehicles it is your responsibility to know which setting to use (full time 4WD or AWD should is best for every day driving) and to know that once you select part time, ABS is compromised. Unfortunately, most do not have in dash warning about the non functioning ABS.
Jeep is a prominent example. The newer Grand Cherokees have automatic AWD. Both ABS and ESC work perfectly. In high range that is. Once you shift into low range the systems turns into a part time 4WD. In this setting ABS and ESC are out of force. And no warning is flashing to remind you of that fact.
It is possible that Jeep assumes that you know what you are doing. Know that ABS is no longer there for you. Since low range is a dedicated off-road setting.
