Today my AWD troubles ended. About a year ago, the 4hi in my AWD 99 Expedition kicked in while i was making a slight right hand turn into our neighborhood. I went through hell trying to figure out the cause.

Heres the thread i created while the truck was as the dealer. The GEM ended up getting replaced (free of charge), but it didnt fix the problem with the 4x4.
https://www.ford-trucks.com/forums/5...questions.html

I then took the truck to a mechanic around the block who builds 4x4 trucks often. I had him do the ball joints, front halfshafts, front and rear diff fluid, and transfer case diff fluid...yeah yeah all with proper amount of modifier. The problem still existed, so i just had him remove the front driveshaft because i no longer had any money to put into the truck.

The year went by, ive driven to Northern California, Denver, Utah, and Vegas several times with the front driveshaft disconnected with no problems what so ever...until i went to my mothers in Denver for Christmas this year. Her driveway had a layer of ice under 4 feet of snow. I couldnt get my truck up her uphill driveway and i ended up ruining her lawn and some Christmas decorations. I was set on getting to the bottom of this 4x4 crap as soon as i got home.

Home again, i went to AZ and bought a transfer case shifter motor. Ive always thought the transfer case motor was the culprit, but i have a bad habit of going to extremes to start. The Ford dealer should have caught the bad motor, shame on them. A couple of weeks ago, i changed the motor and havent worked on the truck until today. I put the front driveshaft in, took it for a drive, and it works perfect! When the switch is in AWD, it rolls freely when steering wheel is locked to the left. I switch into 4hi, and the front end binds and the truck stops moving. I switch back into AWD, and the front end lets go and the truck starts rolling under its own power again.

Im so damn happy this is over. Later tonight, im going to run a wire into my cab from the 4x4 fuse so i can turn the whole system off until i want to take the family up to the snow or the desert. I never want to go through this again. Its disheartening when this is your first truck and the dealer shrugs their shoulders and an awesome 4x4 builder cant figure it out either.

I just wanted to make sure you all change your transfer case shifter motor before you come here posting about your AWD system kicking in and not going back out of 4x4. It really clutters the search function here on the site. Ive wasted more time posting about my problems, then it takes for me to fix them!

Take a clue!

 

Great info.

My A4WD system is a bit out of whack, so I did the in-cab switch a while back. I've contemplated changing out the shift motor, but honestly didn't think that was the problem. Now I'm thinking a $70 investment would be prudent before taking to the dealership for diagnosis.

 

I will be adding the A4WD switch this spring.
~Gage~

 

I took the truck to Calico this weekend and did a ton of dirt road driving with a small trailer. Those roads are VERY rough. I drove out on Friday night, put the fuse in and had no problems with the 4x4 the entire weekend. I felt confident enough that i fixed the AWD problem, so i left the fuse in its place and drove all the way home to the LA area without any problems.

 

I thoght the shift motor only engages low range and some electromagnetic thingy did the 4hi?

 

The automatic 4-wheel drive A4WD system is a full time 4-wheel drive system with an electronic shift 4x4 system that allows the operator to choose between three different 4x4 modes. The operator can switch between A4WD, and 4WD high mode at any speed, and 4WD low mode. To engage or disengage low range, the vehicle speed must be less than 5 kph (3 mph), 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 A4WD, 4WD high, and low range positions. There is no A4WD lamp, and the 4x4 lamp is controlled by the GEM and tells the owner that the vehicle is in 4x4 high mode. The LOW RANGE lamp is controlled by the PCM.

The vehicle speed signal is hardwired between the powertrain control module (PCM) and the GEM, speed control, overhead message center, air suspension, and EVO steering. The instrument cluster also receives the speed signal, but receives it over the standard corporate protocol (SCP) communication network. The PCM determines vehicle speed from the output shaft speed (OSS) sensor which is hardwired directly to the PCM.

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

The shift motor sense plate, an integral part of the electric shift motor, is used to inform the GEM what mode the transfer case is actually in.

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

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 three 4WD modes.

The torque on demand (TOD) relay is used to activate the A4WD clutch within the transfer case by the GEM. This relay controls the torque split between the front and rear axles.

The throttle position signal is hardwired from the PCM to the GEM. The GEM uses the throttle position to anticipate a rear wheel slip condition when the vehicle is in A4WD.

 

Well, I almost understand. So does the shift motor engage (apply power to) the front drive shaft or the A4WD clutch?

 

The shift motor switches between driver selections (A4WD, 4Hi, 4Lo). The clutch distrubutes power to the front wheels. Mine was stuck in 4hi, due to the transfer case shifter motor not shifting out of 4Hi.

 

We found ourselves a couple times this winter, getting the 4Hi stuck on (not the light, but the actual system) and having to put in park, let it sit, and then back in drive again...this is due to icy road, then dry, then icy, ....you get the idea. The A4WD system in ours reacts too slow in these conditions, so we put it in 4Hi frequently. Obviously, this is bad for the 4x4 system and all, but better than getting put in a tree! It always comes back out of 4Hi, just seems to sometimes require the tires to stop moving a few seconds before it can unbind, and disengage.