I had something wierd happen today that I hope someone can help me with. I had to unload some stuff out of my truck, and since I cannot turn my '04 CC SD around in my back yard, I simply backed it through the rear gate. To keep from tearing up my wet lawn, I put it in 4 Low before going through the gate, then backed up about 60 feet or so to my garage door. After unloading everything, I tried to drive out, but the rear spun (open rear axle). I didn't know what was going on for a second, so I gave it a bit more gas, then the transfer case engage with a loud CLUNK ( I thought I broke it) before the truck shot forward. I realize the truck has to travel a bit before 4 Low enages, but does it have to be forward? I thought it didn't matter which direction the truck moved. Obviously I was wrong, but I'd like to know for sure. Thanks.

 

Lever or switch on the dash?

Foot on the brake, tranny in neutral, shift to 4LO...it'll clunk.

If it didn't clunk until you moved, maybe you forgot something in the 3-step method.

 

Never heard of it before,might be the only way too find out is more trial and error gently of course. Possibly had a vacum leak so one of the hubs didn't lock up right away. Maybe try it with turning hubs in manually.

 

I heard the "clunk" of Lo-range engaging when I first turned the switch, and it did everything normal (definitely was in low range) as I backed up so I know low was engaged. As for manually engaging the hubs, my aftermarket wheels have covers preventing that unless I unscrew them. I haven't driven it since, but am going to try it again this afternoon and see how it works. Thanks guys.

 

If you shut off the motor while unloading it could have disengaged, and then re-engaged when trying to pull forward, don't know why it didn't work , I know mine will engage when in reverse. Maybe internals were not warmed up enough, did you give it alot of gas when in reverse before doing the same in Drive?

 

on my 06 4x4 esof i always lock the hubs when plowing even though they are auto . when i shift from reverse to foward ,if i dont give it a couple of seconds, the front wont engage and then clunks as it engages.i dont know if some things wrong or if for some reason that when you switch from reverse to drive ,the transfer case will disengage and then re-engage.

 

I have parked in low traction areas and left it in 4lo so the truck wouldn't slide. Fired it up and took off and it was still in 4lo.

It takes a vacuum pulse to lock/un-lock the hubs and your hubs shouldn't unlock unless there's a vacuum pulse to do that. They should stay locked if you shut your rig down.

 

It did it again today. I got a load of firewood and again, put it in 4low to keep from tearing up the lawn. Thsi time I did engaged 4 low, drove forward about 50 feet, then backed through the gate, into the back yard. As we unloaded the wood, I needed to move the truck ahead, so my wife moved it with no problem, no wheelsin at all. Then about an hour later, I finshed up and got in it to drive out of the yard and again, rear-wheelspin only. I was more gentle with it and got it in 4 low by rocking it back and forth, being vey gentle with the throttle and finally it went into 4 low and I drove off with no drama. I'll have to remove the covers off the front wheels to check the position of the hubs, but I don't see how that could be the problem since it goes into 4 low easily and quickly as long as I do it while the truck can still move under it's own power. If it gets stuck first (when you really need it to be in 4wd) then I'm screwed. I'll check it out and report back.

 

I assume you had shut off the truck for the hour of unloading, so I would say that is the normal for auto hubs,the loss of vacum with the key off allows the hub to disengauge. Might remove the hub covers for the winter so you can engage them manually.. I think that will solve your problem. I have the open hub covers for the front so I can still turn the wheel locks if needed.

 

The lights on the dash are tied to switches in the shift motor assembly, so if the 4x4 lights are illuminated then the shift motor has completed the shift into the indicated range. The only way it won't be in the range indicated is if there is a mechanical issue with the shift linkage inside the case.

In order to shift into 4-low, two shifts must be accomplished inside the transfer case. First, the front drive is engaged as the shift mechanism moves through the 4-high position, which it must do to reach 4-low. This involves engaging an electromagnetic clutch that connects the front drive gear to the rear output shaft to bring it up to speed, then moving a sliding, locking collar that connects the front drive gear to the rear output shaft. Next, to shift to 4-low another collar is moved that changes the way power flows through the planetary gear set to achieve gear reduction.

At the same time you move the selector switch out of the 2-wd position, the control module activates the pulse hub lock system by sending a vacuum pulse to the front hubs to lock them.

So, in the OP's case, if he has noticeably achieved 4-low gearing but the front wheels are not being powered then either one or both of the front hubs are not locking, or the shift collar that engages the front drive system inside the transfer case is not engaging.

The easiest thing to do is the next time the concern happens, see if the front driveshaft is turning while the rear wheels are slipping. If the shaft spins, then the transfer case is doing its job and the problem is somewhere in the hublock system.

As far as the hubs losing vacuum, the hubs DO NOT have constant vacuum applied to them to keep them in the locked position. Once the pulse is sent to lock them, they mechanically lock and remain locked until another pulse is sent to unlock them.


Jay