Front wheels not engaging when in 4wd

Question

2010 Silverado 1500; 4wd; 525,000km; Ontario, Canada

4-low and 4-high appear on the surface to act normally when selected, but the front wheels do not engage.

---

Details:

4-low:

1. Put transmission into neutral.
2. Use the 4wd knob to select 4-low. The light on the knob indicates 4-low was successfully selected.
3. I can hear the expected loud click from under the truck, telling me something has happened mechanically to engage 4-low, which is good.
4. Put the transmission into Drive.
5. When driving, I can feel an hear that the truck is geared down, as expected.
6. However, the front wheels are not being engaged. I know this because I got stuck in a small amount of mud and observed that only one rear wheel was spinning in the mud. I had a spotter confirm what was happening. Normally, I think at least one of the front wheels would have engaged too, and I wouldn’t have gotten stuck.
7. I did an additional solo test by gunning the accelerator on a gravel/sand parking lot, and observed loose gravel only at the rear wheels, confirming the front wheels did not drive.
8. Furthermore, the truck makes sharp turns well, which would not be the case if 4wd was working properly.

4-high:

Behaves similar to 4-low—the front wheels don’t engage. Of course, the difference between 4-high and 4-low is that 4-high doesn’t require the neutral gear step, and doesn’t make the “usual loud click from under the truck”, not even when working properly.

---

Question:

Do these symptoms point to an obvious cause? As a novice, is there anything I can easily check without a hoist?

Answer 1

Your comment says that your Silverado has the Auto option, so this is the NP236 transfer case. First some background so you understand what's happening.

In 2WD the front wheels spin independently of each other in either direction. When you shift into Auto mode or 4WD, an actuator on the passenger side of the differential locks the right axle half to the differential so that both front wheels are connected through the front differential. (The left axle half is always locked to the differential.) In this mode, if both front wheels are off the ground, turning one forward makes the other turn backwards.

In addition, when you shift to Auto, the encoder motor (bolted to the transfer case) slightly engages a clutch pack that is internal to the transfer case, which allows it to send a small amount of power to the front drive shaft. This is enough to handle minor front axle traction needs, and is roughly equivalent to AWD with a limited slip transfer case.

In the event of major wheel slippage -- detected through wheel speed sensors showing unequal wheel speeds between front and rear -- the encoder motor applies greater force to the clutch pack to eliminate slip, which results in equal power to the front and rear drive shafts. With no clutch pack slip, you automatically get the equivalent of 4WD with a locked transfer case within a fraction of a second if you need it, but when you transition to hard pavement and wheel slippage ends, you won't get a crabbing effect in tight turns because the clutch pack is allowed to slip.

When you shift to 4WD from Auto, the encoder motor is commanded to apply maximum force to the clutch pack to give equal power to front and rear.

Because of the existence of the clutch pack in the NP236, the transfer case fluid will accumulate debris from clutch wear particles if the vehicle is ever driven in Auto mode, and for this reason, regular transfer case fluid changes are recommended for the NP236. Failure to change the fluid causes accelerated wear of the clutch pack, which consists of a stack of alternating rings of steel and fiber like a stack of washers.

You may have a worn clutch pack. If the clutch pack is worn beyond its service life, then even with the encoder motor calling for maximum pressure on the clutch pack, it will slip and fail to lock the transfer case. This gives you something like AWD: better than 2WD, not as good as 4WD in slippery conditions.

There's also a possibility that the front axle actuator is failing to lock. This would give you power to the driver side front wheel but not the passenger side. You can test this by placing the vehicle in 4WD, setting the parking brake and raising the right front wheel off the ground. You should not be able to spin that wheel at all. If you can, the front actuator is not engaging.

Finally, there is a remote possibility that the encoder motor or its separately replaceable sensor is defective. I say "remote" because this system is supposed to be self-monitoring and it should throw a fault code and flash the 4WD light if it fails to operate properly.

Answer 2

I took the truck to a GM dealer and they said the following:

I did a few tests and the 4wd is working now.

It's not clear to me if further work to the front differential is necessary.