Four Wheel Drive Won't Engage

[LF_MN_71GMC]

Hey guys, I have a 71 GMC 4x4 half ton with a NP205. Here's the problem. I suspected the four wheel drive was not working so I put it on jack stands to test it. I can see both front drive shafts turning when in four wheel drive. However, neither wheel rotates.

I figured this was due to the locking hubs not making contact on the driveshaft splines so I took the hub off and manually slid the locking portion onto the splines and tried again. This was to ensure the shaft was locked onto the rotating assembly.

Result, now the drive shaft won't rotate when engaged. I remove the manual engagement locking portion and the drive shaft spins again. Thoughts?

Any help would be great.

[lowrollin70gmc]

So before you took the hubs off the front driveshaft from t-case to axle as well as both shafts in the axle were spinning but the wheels weren't? This leads me to think the hub doesn't engage when using the dial.

Hub engaged while disassembled the driveshaft won't rotate at all. Not sure on this one if the t-case is in N or 2Hi the front shaft should spin freely unless something internal to the t-case is preventing movement, but with the hubs disengaged it spins fine so internal damage makes no sense.

What is the t-case shifted into? If in 4Hi or 4Lo is the rear axle also off the ground?

Do the front brakes drag enough to cause the wheel to be difficult to turn? If so it may be hard to turn the driveshaft by hand while dragging the brakes.

[LF_MN_71GMC]

Right, both front axles are spinning when in 4H or 4L. Once you physically lock the axle by using the internals of the hub, the axle and wheel don't spin. I'm basically taking the lockout hub off and gutting it, then manually placing the lockout on the splines so I know its engaged.

I agree that the internals of the T-case are likely fine. Somethings not right about that shaft not rotating the wheel when I manually engage the splines with the lockout internals.

Any chance both sides have to be manually locked before they'll rotate? I'm wondering if the other wheel has the same issue of the dial not functioning. (Both sides during the test were put into the locked out position on the dial).

I thought the EXACT same thing about the front brakes. I put it in 4H and gave it just the slightest amount of gas to see if the brake drag (disc brakes) was enough to lock them. Didn't rotate the wheels (both when I manually engaged the splines and when I put the hub back together and turned the dial to locked out).

Rear axles are supported on jack stands as well.


Here's a link to the pictures of the internals. No idea if four wheel drive worked prior to wheel bearing service.

http://67-72chevytrucks.com/vboard/s...d.php?t=365134

[mknittle]

sounds like maybe a broken axle or stub shaft.
One time my dad had the rivits on a ring gear shear off. some big trhcks use rivits Pererbilt etc. any way it acted the same as yours.

[lowrollin70gmc]

It could be with only one wheel engaged that the differential is spinning the other axle shaft that's not connected to anything. Even if both are connected if the one you're watching has stiffer brakes the other side can spin faster to make up for it (open diff).

But I'm assuming it's also unable to be turned by hand, so maybe something in the hub is binding up.

[LF_MN_71GMC]

So what do you think? Maybe manually engage the other shaft as well to see if they both spin? I agree that if its an open diff the one with less load will continue to spin. So if I'm loading the drivers side shaft but not the passenger side it won't operate correctly.

I don't think the issue lies with the front diff.....(I REEEALLLY HOPE NOT), because the shafts roll when in 4H or 4L. It just seems to be a disconnection between the engagement.

However, if somebody can explain why, I'd be more than willing to tear into that front diff.

[lowrollin70gmc]

Yeah, I would engage the passenger side and have a helper watch that side while spinning it in gear (like you were doing before). Or idle it in gear and walk around the other side to see what is going on.

If it doesn't spin then apply some kind of braking force to the passenger side and see if that works. I think the problem is in the hubs if anywhere. The fact that the shafts roll when not hooked up to the wheel tells me that the ring and pinion is engaging. You can pop the cover off to check it out and change the diff oil at the same time. Only takes about 15 minutes with a few quarts of gear oil and some sealant.

Troubleshooting is the least fun, yet most fun part of these old trucks.

[LF_MN_71GMC]

Ha! Well put lowrollin. I'll do exactly that and we'll see what happens.

[LF_MN_71GMC]

I manually engaged the passenger side..........and we're ROLLING! After I took apart the passenger side hub I noticed the locking hub splines were clean half way to the fully engaged point.

Drivers side was engaging, passenger side had hung up some crap inside the channels for the splines. So the splines hung up on the passenger side and would't allow the driver side to roll either. All good now. Thanks for the help.

[lowrollin70gmc]

Glad it was something simple. Now to go play in 4WD!