I bought a 99 E.B. Expy about 2 months ago. I love it alot except for the fact that I can't turn off the 4WD. When I bought it it was getting like 13 - 14 MPG in combined highway and city driving. I started asking the question as to how to disengage the 4WD system like the 97 - 98's. No one really can tell me alot, so I've done some research. My friend has access to wiring diagrams and it seems 97 - 02's are very much alike pertaining to the 4WD system. I've started to buy the parts for the vacuum motor for the front diff, the 2WD and 4WD selonoids, and the switch for inside. The switch sends voltage to the GEM and with the different voltages tells the GEM which mode to run. I'm still in the process to find out if the Expy's run different GEMs, after that I can start my project...
Anyways, what I did for the time being, I unplugged the T-Case electrical plug, and I'm averaging 16MPG now in combined highway and city driving. The only things I don't like about my tempory fix are: the 4x4 light flasing on the dash, and the front diff is still engaged because of the absence of the vacuum assembly on the front diff to disengege the front axle. Why on earth did Ford take the 2HI option from the 99 and up?????

 

Just so I understand. You unplugged the transfer case for the AWD correct, and with this you see better gas mileage? I think maybe you have got some fuzzy math going on. The reason is that your truck is not a full time AWD. In other words the transfer case does not engage the clutch until there is a difference in the rotational speed of the rear drive shaft compared to the front. So by disconnecting the transfer case you did not reduce any driveline drag, unless you have a short or something in your wiring, or you are driving on slippery surfaces alot.

Your mileage difference could also be as little as running better fuel or anything else that would cause the computer to adjust to your driving habits. That does take a few tanks unless you reset it by disconnecting the battery and turning on the lights.

I would say plug it back in and do a mileage check over three tanks or at least over a trip and then compare.

 

Thanks for your reply, I have done several tests both with and without the t-case hooked up. As you may know, the 99 and up are rear wheel drive till there is slippage in which time the t-case transfers power up to the front axle. With mine, I could feel the front engaging alot with the t-case plug hooked up, especially when I go to get on the highway with more than half throttle. If you are dividing engine output to both axles at once, of course you will munch more gas. The way I'm driving it now is routing all of the engine's output to the rear wheels only, and the front end is just rolling along, engaged but has no engine power going to it. I went from an average of 13.5 (t-case plugged in) to 15.5 to 16 mpg in all of my tests without the t-case plugged in. I'm going to set this up to run like the 97-98 expy that had the 2HI option, just gotta see if the 99 GEM will run the 2wd and the 4wd selonids that the 97-98's had. I know for certain that the 98 Expy I had got better gas mileage than my 99, and I always ran the 98 in 2wd.

 

You must have a problem with your transfer case then. My understanding and I've owned both a '97 and a '00, is that they are the same setup except of the option to not just have 2hi. Everything else is the same. So if you can feel yours engaging on the highway then I would say that your have an electrical problem with your clutches.

FWIW I got virtually the same mileage with the two years, according to the display. Agian I think you are possibly finding a solution to your mileage but going about it possible the wrong way. Good luck.

 

Just to reiterate what he said, these vehicles are not full time Awd or 4wd. If the light ain't on on your dash, chances are the AWD isn't either. As a matter of a fact, you'd know if it was on. I drove mine in the snow last year and when it detected slippage, it sounded like the driveline was going to fall out as it engauged. (bang boom, clang bam!) Also, the power is not split 50/50 in all wheel drive. Thats 4 low, and you won't go more than 15MPH in 4 low, and at that you'll tear it up. The AWD feature is either 80/20 or 70/30 power split, which I can't remember.

If you are having some sort of AWD engaugement, and aren't getting a light, then you have an issue and by disconnecting it you are covering it up and sooner or later going to make it worse. I suggest you put it up into the air and see if you have four wheels turning, and I suspect you won't.