Thank goodness we finally have an answer to go with where to buy them and how much they are.

 

I thought the a4wd used sensors on the t-case? My abs fault light is on yet my a4wd still works as always. Doesn't seem like they rely on the same sensors.

 

Depends on why the ABS light is on. If it's not on because of a bad wheel sensor, then the sensors are intact and working on behalf of the A4WD system.

 

There are speed sensors on the t-case. It was my understanding that these were used to engage the a4wd not abs wheel speed sensors.

I did some searching trying to find the answer this morning but didn't find anything conclusive.

Does the a4wd work off of abs sensors, or sensors in the t-case that measure relative propshaft speeds?

 

I thought it was the wheel sensors but I could be wrong and confusing the system with another manufacturer. I'll have to check my FSM to see what it says.

 

Thats a good question because I have the lovely abs light on and when you turn slowly into a parking lot my pedal will start pulsing, and the last time it rained I puposely nailed the gas and only the rear wheels were spinning, I have felt the awd kick in before, and I know 4wd does work, but I would like to know if the abs sensors also control the 4awd functionality as well.

 

I could be wrong but I feel that the ABS sensors has nothing to do with the 4x4 stystem including the A4WD mode. It just seems by knowing what the ABS is for, what is there on the vehicles operation that would link it the the 4x4 system. I would enjoy to know this in depth after reading some of the past posts on here, especially if i have an issue in the future that any info related to this would help.

 

I found an old press release form BW that described the operation very well. I can not for the life of me find it right now though.

 

According to my (1997) FSM, the front and rear driveshaft speeds are monitored by a Hall Effect Sensor mounted on the transfer case for use by the A4WD unit. There is also a Vehicle Speed Sensor (VSS) on the rear of the transfer case for reporting vehicle speed. The impression I got is that the VSS is not the A4WD Hall Effect sensor, but it wasn't overly clear in the description of the system.

 

That sounds correct. I have full speedo and a4wd function but not abs. I think the sensor on the axle is abs only.

While I was out searching around I swear I ran across ohm readings for the sensors. They should be able to be tested.

 

Expedition 4x4s use an automatic full-time four-wheel drive system designed by BorgWarner for Ford Motor Company. BorgWarner calls it Torque-On-Demand (TOD), while you all know it as ControlTrac, which is Ford’s registered trade name for the 4x4 system.

The 4x4 system does not shift into and out of four-wheel drive, as most misinformed people will tell you. Literally shifting from 2-wheel to 4-wheel drive is pure nonsense. It would create too much wear and take to long to get torque to the front differential. Furthermore, that’s not how BorgWarner designed the system to work.

In A4WD mode (currently called 4A mode in newer Expeditions), the entire four-wheel drive system is continuously engaged. The front wheel hubs are always engaged, so the front wheels are always locked to the front axle shafts & front differential, which in turn, is always engaged with the front propeller (drive) shaft. Last, but not least, the front propeller shaft is always engaged with the electronically-controlled multi-plate clutch pack (center differential) pioneered by BorgWarner, which sits inside the two-speed (dual range) BorgWarner transfer case.

How the system works ...

BorgWarner uses front & rear propeller shaft, steering, throttle and brake input sensors to relay information (data) to the on-board computer control unit that controls the automatic full-time four-wheel drive system.

2H mode – The electronically-controlled multi-plate clutch pack (center differential) is told to always route torque to the rear wheels, regardless of traction conditions, imitating rear-wheel drive.

4A mode (A4WD mode in older Expeditions) – The electronically-controlled multi-plate clutch pack (center differential) is told to route torque to the rear wheels, and to reroute a certain percentage of torque to the front wheels when traction loss is detected. How much torque (in 10 percent increments) that goes to the front wheels depends on the severity of the traction loss.

Torque is transferred from back to front or front to back in secrecy and faster than the human eye can blink.

While transferring torque to all four wheels, the electronically-controlled multi-plate clutch pack runs through cycles to allow just enough slipping between the front & rear propeller shafts to completely eliminate any driveline binding or torque windup that could typically occur when driving or making turns on dry pavement. This is what allows the full-time operation of the automatic full-time ControlTrac four-wheel drive system.

The front to rear torque split is infinitely variable in 4A (A4WD) mode.

4H and 4L modes tell the electronically-controlled multi-plate clutch pack to lock-up, mechanically locking the front & rear propeller shafts together, giving a permanent 50/50 front to rear torque split. Expedition 4x4s in 4H mode and 4L mode should NEVER be driven on pavement, regardless of whether its dry, wet or covered in snow.

 

You make this statement, then procede to explain the 2wd and 4wd modes. The transfercase most certainly DOES shift between 2wd and 4wd.

Are you aware that most 4wd vehicles have the front driveline engaged at all times?

This is a modified part time transfer case. Trying to portrait it as a full time system is incorrect. Full time 4wd systems use a center differential. The argument that the clutch pack serves as the differential is a little far fetched. The end function is somewhat similar but, the simple fact that a clutch pack suddenly engages the front drive upon the detection of slip makes it part time.

The clutch pack clearly does wear out as we have recently had a contributor have this happen. So you are correct about the part where shifting between 2wd and 4wd creates too much wear. It does so because the design is still based on a part time transfer case. With a differential in the center there is no response time from the sensors or engagement motor.

I am pretty happy with the function of the system as long as you understand its limitations and anticipate the response lag. I have owned several true full time 4wd systems and lots of part time. The BW TOD is a nice bridge but it is not a full time 4wd transfer case.

 

I think he meant when in A4WD there isn't a constant mechanical shifting in and out of 4WD going on. When in A4WD the TC is in 4WD but the front driveshaft is engaged/disengaged via an electric clutch system.

A4WD systems do, 4WD systems with the 2H option do not have their 4WD engaged at all times. If they did there would be no need for a shift motor other than to go between 4H and 4L.

 

The definition of full-time four-wheel drive is being able to operate on or off road (which the Expedition’s system can do) so the Expedition’s four-wheel drive is most certainly full-time. Control Trac - Wikipedia, the free encyclopedia

Part-time four-wheel drive systems are recommended for off road use only.


Indeed. The only thing that is "shifted" is torque, from back to front. From that point, the center multi-plate clutch controls the front-to-rear torque split. It will return to routing torque to the rear wheels when the computer control system is satisfied that the Expedition is moving along nicely.

 

There is though. You can try to call that a differential torque transfer but it is awfully rough. I can feel it engage and disengage all the time going from unplowed side streets to plowed main roads.

First you say it isn't shifting in and out but now you say it is engaged/disengaged? Isn't that the same thing?

If there was a true center differential as found in the front and rear, it would be full time. There is no engage/disengage just perfectly even power distribution full time. As proof of the part time nature of the case, let's see it run a full time 50/50 torque split on the road. Can the clutch pack handle that duty cycle? A full time transfer case with a center diff can.


Both GM and Dodge have had the only 4wd disconnect point as the t-case for many years. Axles are splined to the hubs, no lock outs, and the front driveshaft turns all the time. On the early IFS GMs they tried a disconnect at the axle for a while. It was a junk system and they abondoned it. Ford was pretty much alone for years having the front axle disconnect, or offering lock out hubs on HD trucks.