I have a question, I recently bought a 98 5.4 Expedition XLT ,I saw the selector switch had 4 positions, 2WD, a4wd ?, 4hi,4lo. I'm now picking up another Expedition, a 99 5.4, and the gear selector only has 3 positions, the 2WD option is eliminated, its either a4wd ? ,4hi,4lo. and it seems to always be in 4wd. Am I to believe that in 99 they eliminated a 2wd option?? I put question marks next to (a4wd) because I might have that abbreviation spelled wrong, I dont exactly know what it means(that position), I dont have any manuals.
Please share some info.
Thanks
Lowflyingbird

 

Your 1998 Expedition has the A4WD option, which is Automatic 4WD. You can still keep it in 2WD, or switch between 4hi and 4lo.

If you leave it on A4WD, your truck will automatically switch between 4WD and 2WD depending on the terrain you encounter. This is a useful feature for times when you hydroplane in the rain, for example. If you do not want it to switch to 4x4 without you doing it, then leave it in 2WD.

Your 1999 Expedition is apparentlly an All-wheel drive model. 2WD was not eliminated in 1999, you just have a model where full-time 2WD is not an option. The A4WD works the same as I explained above. You can still switch to 4WD full-time and 4WD low-range. It will remain in 2WD most of the time, but I would personally prefer the ability to control it myself.

 

My 2000 XLT did not have the 2WD option, but I see on our 2004 EB that the 2WD selection is back!

I recall reading somewhere that when you put the truck in 2WD it also disengages the front drive shaft from the transfer case forward, allowing you to gain better fuel economy; in A4WD, the only things not engaged are the electronic locking hubs. Is there a truth to this?

 

As you've seen, the '97 & '98 Expeditions had a 2WD selection for the transfer case. This 2WD selection was eliminated in the later models and did not re-appear until the 2003s came out.

The A4WD selection is "automatic" 4-wheel drive. In this mode, the vehicle detects rear wheel spin and automatically engages the front wheels until the slippage is no longer detected.

For the '97/'98 models, A4WD uses vacuum to "engage" the front axle then, via viscous coupling, continuously sends ~20% of the power to the front wheels. When rear-wheel slippage is detected, the driveline sends up to 50% of the power to the front.

When Ford eliminated the 2WD option from the transfer case, they also changed the way that the A4WD system works. We never had one of those later models, so I didn't care how to find out how those systems worked. I'm sure you could find out with some aggressive searches.

 

Half-right... The A4wd description is right on. Another way to describe it is 'rear-wheel-drive-until-the-rear-wheels-slip.' The main difference between the 97/98 and the 99-02s was the center axle disconnect. In theory, the vacuum disconnect motor would break the passenger side axle shaft, allowing the diff carrier and pinion (and, therefore the front driveshaft) to stop turning. In reality, it still moved due to the design of the guts of the t-case.

The system does not, however, use a viscous coupling. It's an electromechanical unit controlled jointly by the GEM and/or the Torque On Demand Relay (which itself is controlled by the GEM). As far as I know, the only true all wheel drive transfer cases Ford ever used were the 44-06 cases used in the 2nd-gen Explorers equipped with a V-8. (and those did, in fact, utilize a viscous coupling, but were not selectable and had no low range) The A4WD system used the TOD relay to vary the duty cycle of the transfer case, effectively varying the torque split, but the ramp-up from the minimum 13% duty cycle to the full 100% duty cycle was almost instantaneous, so for all intents and purposes, the system was either engaged or disengaged.

The t-case on the 97/98 works exactly the same as the t-case for the 99-02s. It's an automatically engaged system. The 99s ditched the center-axle disconnect front axle for a live front axle, and ditched the 2wd position since the front drivetrain was turning anyways. Yeah, you lose some of the control to a mediocre (at best) program, but it did increase reliability by eliminating the trouble-prone electro-vacuum-mechanical CAD system.

FWIW, you can wrestle control back from the system on the 99-02s by performing the 'brown wire mod'. The entire system is virtually identical (albeit super-sized) to that found in an Explorer. One snip of a wire, a simple toggle switch, and you get true 2wd, as wella s 2wdlow when you want it, and full auto (i.e. stock) function when you don't with the flip of a switch.

-Joe

 

I love the setup, but after doing my future blower setup on the motor, I want to be able to spin some rubber sometime!!
Thanks
Lowflyingbird

 

You won't want to do that with your current transfer case... it will be short for this world if you do the brown wire mod and insist on roasting your tires. The system does not tolerate extended periods of large differences in axle speeds. Even when disconnected electronically, the mechanical system just isn;t designed for it.

If you want to perform tire-roasting burnouts, you'll need to at least remove the front driveshaft, if not perform a full 2wd conversion.

-Joe

 

Hi,
Well its just tough TeeTee's for me. I made my last 4wd into a 2wd before, I just wouldnt do it to this truck, its too nice. Stock form will be just fine.

 

Hi everyone I am new to this site. To all of you having problems with your fords, I would be glad to help you fix it. I own an Auto repair shop and have almost any tool you would ever need. I have all data and mitchell on demand. the wiring schematics are but kicking. so if you need schematics we can hook you up. For the real head aches I have a ford oem NGS SCANNER. I have no problems loaning it out. ( Deposit Required ) I have the new Snap-on Veras paid $ 14000.00 wont even do neer the stuf OEM SCANNER CAN DO. I have a 1999 Expedition 269000 orig miles on engine and trans. started having 4x4 isues, thank god for NGS scanner it checks everything out. NO GUESS WORK. IT SIMULATES THE 4X4 SWITCH CHECKS ALL THE RELAYS, GEM MODULE . i PROGRAM A LOT OF KEYS. YOU CAN TURN ON AND OFF LOT OF OPTIONS. I COULD GO ON AND ON . IF NEED TO CONTACT ME MY EMAIL IS aaffordableauto@gmail.com If I can save you money by helping you do the repairs Im all for it. I am a ASE MASTER IN AUTOS AND LIGHT TRUCKS, SCHOOL BUS,AND BIG TRUCKS. WOULD GET MORE BUT HAVE TO TRAVEL ALL OVER THE STATE TO GET RE CERTIFIED. CAN ONLY TAKE 8 TESTS AT A TIME x 3 = 24 TEST PLUS THE ADVANCED TESTS FOR SMOG. damn. THANK YOU JON