Transfer Case Question

zaac · March 21, 2011

How does a manual shifting TC work with auto locking front axle? I have a truck with manual shift TC and manual hubs and if I try to shift to 4x4 on the fly with the hubs unlocked it'll grind. I understand why because the front drive shaft isn't turning while the rear one is. But wouldn't that be the same with manual TC and auto locking front axle? The front axle isn't locked when the TC is shifted to 4x4 so why doesn't it grind? I also have a Chevy with NP246 TC and I believe the TC locks in before the front axle but this TC uses clutches so i can see how that works. Hope someone can explain cause I can't find any info on the internet. Thanks.

zaac · April 5, 2011

Anyone know how the system works?

Ezrider_92356 · April 6, 2011

im not positive on your particular truck, but a lot of times transfer cases that came for auto hubs had syncro's in them.

Shatterz · April 6, 2011

In a gm independent front susepnsion 4x4 system, the hubs are never "unlocked". The cv shafts are permanently locked to the wheels and spin all the time with no way to disconnect. When you shift into 4x4 on a manual transfer case, the entire opertion of actually getting into 4x4 mode is not completely manual. The only "manual" opertion you are performing is shifting the t-case. As you complete this shift, the transfer case sends a signal to what I believe is called the front axle actuator and encoder motor which syncs the front axle and engages power to the cv shafts.

Gm has used several different designs over the years for the front axle acutator, I know they have used a vacuum actuated system, and thermally acuated system, but I believe now all the actuators are fully electronic.

Please anyone feel free to expand or correct me as I'm not 100% sure this is exactly how it works, haven't dug into my shop manuals in a while for a description of operation.

zaac · April 8, 2011

im not positive on your particular truck, but a lot of times transfer cases that came for auto hubs had syncro's in them.

Thanks Ezra, I didn't know they had a syncro in them. I looked at an exploded view of a NP241 like what came in the '88-'98 Chevy's with auto locking front diff and they do have a syncro. Makes sense now. The transfer case can engage the front driveshaft via the syncro which then gets both axles turning the same direction and speed so that the front diff actuator can engage the right axle. Then you're in 4x4.

In a gm independent front susepnsion 4x4 system, the hubs are never "unlocked". The cv shafts are permanently locked to the wheels and spin all the time with no way to disconnect.

Yes, you are correct, there are no hubs to unlock but there is a sleeve that the actuator operates that unlocks the right hand axle. I don't think it's that great of an idea because the left hand axle will constantly be turning the spider gears in the diff. Not only that, the 2000-2002 NP 246 like in my truck has the clutches pre-loaded so the front driveshaft also turns all the time even in 2x4. Thanks for the help.