'99 Won't lock front in 4wd

Hi everyone, new member here.

I have a problem that I can't seem to find an answer to. I have a '99 Silverado with the NP246 TC and the problem is that the front wheels won't "lock" when in 4wd hi or low. When in snow just the back wheels will spin unless I choose Auto mode - then they work fine. It's been this way since I got it a year ago. I had everything checked out by a GM mechanic and he couldn't pin point the problem. Said I would probably have to replace the encoder motor or the transfer case. I don't feel like spending that kind of money without more research. There are no error codes from the transfer case or the main computer. When I press the 4wd hi button I hear a soft whining coming from the encoder motor like something is turning but no sounds like something locked into place. Is that all I'm supposed to hear? I do get the clunk when going to 4wd lo when the other gear is engaged.

The thing that gets me is that the "Auto" mode works perfectly - no problem at all - front wheels pull just fine - so the clutch pack is working. I've read about the problems with the clutches and they seem to work fine. The switch works fine too. When it was up on the lift he put it in 4wd hi and the front wheels were turning. I'm guessing this is from the slight friction that is in the clutch plates even if they're not locked. When one wheel was stopped the other took over so the front diff switch was working. He couldn't stop both front wheels at the same time but maybe the viscosity of the fluid in the clutches will always send a little power to the front. But when a real load is applied, like in the snow or mud, the front wheels just sit there. The fluid is the newer blue stuff by the way. To me it seems as if whatever part is responsible for locking the clutch pack together so they don't slip in 4wd hi / lo is not working. Again, the clutches work fine in Auto mode so I don't believe that is the problem.

Any ideas would be greatly appreciated.

Thanks
Chad

Hi madchad and welcome to the best auto. forum site available.

It seems there are a lot of posts on here about the push-button switch assembly going bad. You may try entering "push button 4WD switch", or a similiar phrase in the search box and see what you come up with. It justs sounds like the problem may originate there, since the other components are working as they should in auto-mode. Maybe you'll get some more replys here from others who have had a similiar problem. Good Luck.

MadChad, I just brought my 99 suburban back from the Transmision shop. The Chevy Dealership diagnosed that the Encoder moter was bad. We put a new motor on the Xfercase and did not fix the problem. Apparently there is no signal coming from the computer to the transfer case. The Mechanic said it might be a bad computer that is located somewhere on the external firewall area. He said that water gets in them apparently from time to time? It is supposed to go back in on Monday for another look/see. Ken

Thanks for the reply!
Can you let me know what you find out when they finally fix it?

That problem would make sense. I'm curious, did everything else like the 4wd lo gear change and auto mode work on yours?

Thanks for the reply!
Can you let me know what you find out when they finally fix it?

That problem would make sense. I'm curious, did everything else like the 4wd lo gear change and auto mode work on yours?

Does anyone have anyupdates on this one? Thanks

Well we tried all kinds of things to fix it but it is still broke. Now when I say we tried all kinds of things. We pulled and replaced the Encoder motor and traced all of the wiring as best we could but it still only fails when it gets wet. Some where there is a moisture related problem. I am assuming it causes an out of tolerance resistance reading. It definitly is not a shorted Encoder like the Chevy dealer tried to sell me!!!!!!! So I am still looking for the problem !??!? Ken

Well we tried all kinds of things to fix it but it is still broke. Now when I say we tried all kinds of things. We pulled and replaced the Encoder motor and traced all of the wiring as best we could but it still only fails when it gets wet. Some where there is a moisture related problem. I am assuming it causes an out of tolerance resistance reading. It definitly is not a shorted Encoder like the Chevy dealer tried to sell me!!!!!!! So I am still looking for the problem !??!? Ken

these motors fail many ways the one way that could be your problem is the motor or wires to motor has excessive leakage of current to ground. do you know what that is? the motor being shorted is a hardware guy type of desciption as to whats going on. this may not be shorted or open or seized etc.......