the reason you can shift "on the fly" is because if you moving along in a perfectly straight line and the front wheels are spinning at the same speed as the rear wheels, the transfer case simply has to mesh the two together and your in 4hi.
obviously if your rears were on ice and you were spinning away and you threw it in 4hi you would hear a loud bang.

 

If you actually were able to shift into 4Lo while moving your drive train would suffer the consequences of now turning at the rate of 2.64:1 VS 1:1. One of 2 things is going to happen. The engine will over rev and or the rear wheels will stop turning. Did this happen?

 

Thanks for all of the replies.
My wife was driving it at the time, so I'm going by her description of what happened. She said she was just driving along about 35 to 40 & the 4x4 & low range light came on at the same time it felt like she hit something & the truck slowed down immediately & her body was thrusted forward, like slamming on the brakes. She then could not go very fast, so she pulled into a gas station. After the engine was turned off, then restarted, the lights were off & everything was fine. Then she drove it home. I have driven it about 60 miles after that in various speeds & today using 4 wheel high due to the snowy roads. All seems fine. One thing I did notice is that the **** for the fan is right next to the 4 wheel drive selector **** & they do look similar. If she mistakenly turned the 4 wheel **** all the way to 4 wheel low, is it possible that it could shift to low when a certain speed is achieved?

Gary

 

I'm not sure what exactly happened, if was a mistake or not. I just want to make sure the truck is safe. I was hoping to get some suggestions of items to check, like wire shorts, or vacuum hoses or whatever.

 

i dont think your wife is telling you everything. haha.

go to a place where you know youll be safe and able to get help from and run through the procedures of going in 4hi and 4lo. drive around a little bit with them engaged. if everything does as its should then everything should be fine.

 

I've done it plenty of times, it is a PITA to reach down and switch the fan but hit the 4x4. I have had it go into 4x4 on paved roads and it will work fine. BUT if you flip it all the way over and leave it, the truck won't do anything. Just acts like 2wd.... I read the post and went and tried it. Even if I had the switch on 4x4 low, then stopped, pushed brakes, and switches to Neutral it wouldn't do anything. I had to move the **** back to 2wd, then to 4x4 low for it to do anything. Mine is a 98 btw. Hope this helps

 

Shifting into 4Lo at that speed will definitely produce what your wife described. The shift motor on the transfercase is controlled by the GEM. Are there any other strange things happening? Like the radio staying on with the key out or the blower fan still running when you have shut off the engine and have the keys in your hand. These would also be signs of a defective GEM. Water intrusion into the cab and dripping onto the GEM would account for this.
The GEM is what controls the electric shift motor. Depending on the input it will shift in and out of both 4x4 Hi and Lo. The fact that the LOW light cam on is a sign that the GEM sent a signal to the PCM for it to turn on the light. That means the the either the GEM or the 4x4 switch is defective. For the shift into 4Lo to occur the GEM needs to see the transmission in Neutral, the brake pedal being depressed and the switch turned to 4Lo. Even if the switch is defective or even if it was turned to 4Lo the other criteria needs to be met. Neutral and brake pedal input. Even some sort of a fluke short in the electric shift motor will not turn on the LOW light it will only shift the t-case. Find some one who can scan for GEM codes and see what that produces.

 

you should be completely fine. every four whell drive is set up to go into 4lo that way other than that it shouldnt hurt it at all

 

by never owning a 4wd and not knowing exactly how it works, and after reading what JMC wrote.. id def believe that...

 

If you move your selector **** to 4lo while not in neutral (and the brakes applied), all the transmission will do is switch to 4hi. It will switch to 4WD, but only 4hi. It WILL NOT shift into 4lo unless stopped, in neutral, and brake applied - assuming your truck is working properly.

So as far as you know, the truck was in 2hi and shifted to 4lo while driving? And then she pulled into the gas station, parked, shut it off, and restarted it and the 4lo light went off? just like that, she didnt move the switch or anything? Did she move the switch around at all after it happened? Did she check the 4x4 switch to see where it was at?

-sorry, I don't mean to be grilling you... I'm just tryin to help you figure it out. But if she didn't turn the **** or anything, then it probably is some sort of electrical problem, like what JMC had mentioned. I would definitely do what JMC suggested and take it to someone who can scan for GEM codes.

 

Hey illness, what year is that Seadoo? I have a '99 XPL and even though I am in my 50s and 225 lbs. it is still a blast ;-)

 

its a 97.. i reallllly want a 2000 or 2001.. got to love the power of the 951, im stuck with the 787 right now.. but i love it