I would not rule that out at this time as the problem may be in some way tied to that and the ABS system. The ABS can be a real headache with all the sensors, wires and the module(s). I hope it's not there, for your sake.

 

just disconnected the 5A 4WABS fuse. the abs light came on but the problem still exists.

 

There is a sensor on each front wheel/hub assembly and the rear sensor is on rthe top of the differential.

 

I forgot to mention the speed sensor on the T/case.

Here is some more information for you;
SECTION 308-07A: Transfer Case — General Information 1998 Expedition/Navigator Workshop Manual

DESCRIPTION AND OPERATION

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Transfer Case —Automatic Four-Wheel Drive (A4WD)
The automatic four-wheel drive A4WD system is an electronic shift 4x4 system that allows the operator to choose between three different 4x4 modes as well as two-wheel drive (2WD). The operator can switch between 2WD, A4WD, and 4WD HIGH modes at any speed. To engage or disengage LOW range, the vehicle speed must be less than 5 kph, the brake depressed, and the transmission must be in NEUTRAL.

In A4WD, the generic electronic module (GEM) varies the torque split between front and rear drivelines by controlling the transfer case clutch. The transfer case clutch allows for slight speed differences between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. When the rear wheels are overpowered, the GEM detects this slip condition, and the duty cycle to the transfer case clutch is increased until the speed difference between driveshafts is reduced.

The 4WD mode switch is a rotary switch which allows the vehicle operator to choose between 2WD, A4WD, 4WD HIGH, and LOW range positions.

The vehicle speed sensor (VSS), mounted on the rear of the transfer case, informs the GEM of vehicle speed.

The Hall effect sensors, mounted to the transfer case, are used to detect the speed of the front and rear driveshafts.

The electric shift motor, mounted externally at the rear of the transfer case, drives a rotary cam which moves the mode fork and range fork within the transfer case to select between the 2H/A4WD, 4WD HIGH, and LOW range positions.

The shift motor sense plate, an integral part of the electric shift motor, informs the GEM to the position of the transfer case.

The transmission range (TR) sensor, located on the left side of the transmission, informs the GEM of the transmission gear position.

The Expedition/Navigator is equipped with 2WD and 4WD vacuum solenoids attached to the upper RH side of the engine compartment dash panel, they alternately route vacuum to the vacuum motor which engages/disengages the center axle disconnect collar in the front axle assembly.

The 4x4 shift motor relay is a module containing two relays which, under the control of the GEM, shift the transfer case shift motor between the 2WD and the three 4WD modes.

The solid state clutch relay is used to activate the A4WD clutch within the transfer case.

 

ok, so the gem could be causing a problem. how bout this fact tho: before i installed the front driveshaft i could engage 4L and none of this bucking/jerking and pulling to the right would occur.
if the speed sensors up front were faulty, wouldnt i have the same effect while driving without the shaft attached? and pulling fuse 104 makes it drive perfect again.
my dad said he thought the front differential might be bad. i dont know, could be a number of things. im going to try and get a shop to do a free diagnosis. hell, its worth a shot.
also, if i had a shop hook up a real time data tool, would that tell me anything useful about the speed sensors or diffs or t-case? had a shop do that on my 01 camaro a while back and it told them alot.


couple more things: unplugged rear sensor on rear diff. no effect. speedo still worked too?? found hall effect sensors on tcase. did not disconnect those yet. checked front and rear diff fluid, both topped off and not dirty at all for the age.
when in 4h or 4l it acts the same. when i punch it in gravel the rear pass and front driver wheels are the only ones to spin out. does that say anything or is that normal?

 

I think the problem could be the sensor(s) on the T/case because you said the lurching and jerking went from one wheel to the other and I assume that includes the front wheels. If you can get a quality diagnosis from a shop with the tester for driveability by all means get it as that should stop all the guess work.
Let us know how it comes out.

 

mel, gotta bug u once more lol. btw, i owe ya and appreciate the help bro. ive been youtubing how differnetials work. very simple, i love it. so after examining both front and rears i found no wires or vac lines or hydraulic lines anywhere. i have a feeling that i have an abs controlled LSD. how can i fully disengage the abs system? if i can do that and stop any abs pulses from reaching the wheels i should have a kind pf "raw 4wd system" lol . much like what i saw on youtube called an "open differential" this would help troubleshooting immensely. i could rule out the tcase and focus on the abs.

 

The ABS does not control the differential, it only controls the brakes. The LSD or TractionLok is basicly the same as a Positraction rear, there are no electronics involved.
The ABS gets signals from the speed sensors by way of the ECM and the GEM which cause it to pulsate the brakes when it senses a brake applied skid.
The speed sensor is located on the top of the rear diff. and there are wires going to it. There is a speed sensor on each front hub and there are wires coming from each of them. There is also a couple or 3 speed sensors on the trans and T/case. I strongly suggest that you find each sensor and verify that the wires are not cut or broken and thet they are plugged into the correct plug. During the inspection you should also verify that the "tone rings" and magnets are clean of any magnetic debris and dirt.
These trucks do not like having their electrics tampered with and it might open up a can of whup *** on you if you get it pissed. Be careful, be very careful.

 

lol, k bud. im gonna pop it up on all 4s tomorrow with some heavy duty jacks/stands and really do some troublshooting. btw, this haynes manual really sucks. its not in-depth enough

 

Both Haynes and Clymer manuals are a joke and not a funny one either. I was once told that they were and may still be owned by Snap-On Tools who also own Motors and All Data. Kinda smells like a monopoly don't it?

 

Double post. Moderators, please delete.

 

The front wheel speed sensors are only used by the ABS System.

The instrument cluster uses the VSS on the back of the transfer case to determine vehicle speed.

If you have an open rear diff the tendancy is for the right rear to spin. Do you have an open diff?

With an open front diff the wheel with the least amount of traction will try to spin. Maybe you right front axle is not being driven. That would make the vehicle want to turn to the right when the left front is driving. Has the front axle assembly appear to have been replaced? If so, does it have the correct gearing? DOes it have a vacuum motor to control the sleve collar on the right axle?

 

pdqford: no it does no have a vac motor anywhere on the front diff or the "sleeve collar" on the pass side. not that i was able to see or feel anyway.
i had thought the same thing about it pulling to the right. that very well could be the reason. when i have all 4 wheels off the ground i will know more.

one more thing, yes, in 2wd(fuse 104 pulled) i peel out and only the pass.rear spins.

 

We have gotten sidetracked with the vacuum business which may be my fault and if it is I apologize.
There are no vacuum motors, lines or other vacuum connections involved in the operation of the 1999 Expedition A4WD system that I know of. Forget all that you have read in that regard as it will just be confusing.
If you want you can PM your email address to me and I'll send some pages from my Ford service manual. The files are so large I don't want to try and post them here.

 

ok, so lifted off the ground all 4wheels turn just fine and have the same amount of resistance. 4HI is a different story. things sound like theyre binding up. i found some sensors on the tcase but they didnt come out easily so i didnt want to risk breaking one and having a downed vehicle. itll be at the shop as soon as i get time