A buddy of mine has a 98(?) Explorer Limited (I'm pretty sure that's what it is). We got into a discussion about how the 4WD/AWD works on his vehicle. I assume it is the same as my 4x4 Expedition when my Expedition is in "A4WD" (not 4HI or 4LO). He has no A4WD/4HI/4LO selector.

My understanding of the "A4WD" mode in my Expedition is that when it senses slip in the rear end, it will engage the T-Case and send power to the front axle. However, the front end is only an open diff. so the front wheel that slips the easiest will get the power. Since I have LS in the rear, I could have at most 3 wheels driving.

My previous Land Rover had full-time 4WD in which power could be transferred to any wheel via open(?) diffs by modulating the brakes thru ABS. Under various conditions I could get different combinations of 2 wheels driving. I question whether it had open diffs since I could get both front wheels to drive at the same time.

So, what's going on with his Explorer? How does it manage power distribution?

 

My Chilton manual describes it as using a series of viscous clutches to transfer power to both the front and rear drivelines. There is no low range, and I wouldn't be surprised if it doesn't even have a way of "locking" that center "differential." I've heard of people who were dissappointed when they learned this. The salesman had led them to believe that they were buying some rugged 4WD machine to learn that they had really bought something much less.

 

Your friend's AWD is different from your Expedition. Your friends AWD uses a viscous clutch system that can transfer up to 99% torque to the front or rear differential. When no slip is detected, it supplies about 35% of its torque to the front, 65% to the rear. It doesn't rely on slip to get power to the front wheels. It is smoother reacting than your Expeditions is however it does not offer the ability to lock in a 50/50 split of the torque like you have when you lock it in high. As you mentioned, it also doesn't have a low range either. Your Expedition is a lot closer to my '97 Control Trac equipped Explorer. Mine has the Auto 4x4, High & Low. I do not have a 2WD mode and never needed it. Ford even dropped the 2WD mode from the later Expeditions as well. The Auto 4x4 can transfer up to about 90% of its torque to the front or rear but it relys on slipage before the transfer of torque happens. It uses a mechanical system and can be felt easier than the AWD system. Press the gas pedal down quickly in the rain and you will feel the rear tires slip a bit and then a lurch as the front wheels get power. On the AWD you will not feel it, it will just take off.

The AWD is better suited for bad weather roads and mild off-roading. The Control Trac is a better choice for when the road gets rough.

 

Thanks guys! Both replies are great.

So am I correct in understanding that the transfer of torque in the Explorer AWD is simply from one axle to the other, and that the front & rear diffs are open (possibly LS in the rear)? And if so, does the Explorer AWD use ABS to 'control' the open diffs?

Thanks again.

'01 350 PSD CC 4X4 LARIAT
'02 Expedition XLT 4x4 5.4L

 

I also have a '97 EB with Control Trac (5.0 V8) however, it doesn't have any switches, buttons, or controls. I wrote to Ford because the owners manual indicated that if you had Control Trac then you should have a switch or Hi/Lo, etc. This was their reply:
-------- (sorry for the reply length)----
"According to our resources your vehicle is equipped with All Wheel Drive (AWD) (4x4 with Control Trac). Here is some information regarding your vehicle's Control Trac system:
Explorer's Control Trac" four-wheel-drive system has been refined to make it more "transparent" to the driver, while improving its abilities in limited traction situations. For the first time, the optional, two-speed four-wheel drive system has its own electronic controller, which can be likened to the latest high-speed personal computer. The extra computing power gave Explorer's engineers the ability to fine-tune the system to a wide variety of demands.
For example, sensors measure and compare front and rear drive shaft speeds, as well as throttle position - to figure the driver's intent into the mathematical formula that determines how much torque to send to each axle.
These measurements are taken 50 times per second, for an almost seamless adjustment of the clutch that controls the transfer case output in the 4x4 system's "automatic" setting.
If the rear wheels lose traction, the optimal amount of torque for the situation is transferred to the front - up to 500 foot-pounds. But the system doesn't have to wait to sense slip before it takes action. For example, if the Explorer is sitting at rest in a four-wheel-drive setting and the driver presses the accelerator, the
controller sends torque to both the front and back wheels within milliseconds and then samples the results. This heads off wheel spin in low-traction situations.
The Control Trac" system includes a pushbutton switch - versus the former rotary **** - on the instrument panel that allows the operator to select "4x4 Auto," "4x4 High" or "4x4 Low" modes.
In 4x4 Auto - recommended for most driving situations - the system
continuously monitors conditions and adjusts front-wheel torque,
combining transparent all-surface operation with highly capable four-wheel drive. When in automatic mode, the Control Trac" drivetrain
normally delivers all engine torque to the rear wheels. It uses an
electronically controlled electromagnetic clutch to redirect up to 100
percent of engine torque to the front wheels when it detects rear-wheel slippage or heavy throttle application.
In 4x4 High - recommended for severe winter weather and off-road conditions - the system effectively locks the front and rear driveshafts together to provide optimal traction of four wheels.
In 4x4 Low - recommended for low-speed off-road driving or high-torque situations, such as towing a boat out of the water - the system locks the front and rear driveshafts together but adds a 2.48:1 gear reduction, which provides extra pulling power. Neither 4x4 high nor 4x4 Low is recommended for dry road operation.
---------------
When I wrote back again and told them I had no switches or buttons, they replied:

"Your vehicle is equipped with an all wheel drive (AWD) transfer case.
Power is constantly supplied to all four wheels through the transfer
case. The AWD system is full time and has no external controls. Like
all utility type vehicles, your AWD vehicle has special design and
equipment features that make it capable of performing in a variety of
off-road applications. Specific design characteristics give these
vehicles higher centers of gravity than ordinary passenger cars."