99 B3000 vibration

[mrdana]

Here's the symptoms: Truck will suddenly get a heavy vibration while driving, starting at about 15 mph, though the precise speed varies. Doesn't happen all the time, might drive for miles with no problem, might start up right after leaving the driveway. Once started, it continues until the vehicle is almost stopped, then it doesn't start again until 15 mph. Engaging 4WD stops it, and sometimes it returns when I switch it back to 2WD, sometimes it doesn't. The vibration continues even if the truck is clutched and/or shifted into neutral.

Thinking it was hubs or front axle, I disconnected the vacuum line between the manifold and the solenoid, so that the hub locks wouldn't engage, verifying that the front driveshafts spin free while the wheels don't move. Drove it for awhile with no vibration, then switched on 4WD (so the transfer case engages and spins the front differential even though the hubs are unlocked). After switching back to 2WD, the vibration started up, back to 4WD, stops again. This would seem to indicate that the problem is somewhere in the transfer case.

Thoughts?

 

[RonD]

I would jack up the front and check wheel bearings first, also bushings, give each wheel a good shake.

15mph vibration would almost have to be a wheel, other parts are too small and light, and not spinning fast enough to vibrate at that low a speed.

 

[mrdana]

It doesn't sound like a wheel bearing, and the way it comes and goes doesn't seem to fit, either. With the hub vacuum lines disconnected and engaging 4WD at the transfer case making it start and stop, that's why I'm thinking T-case. Unless the vibration is coming from a misalignment between the hub and front driveshafts when they're not locked together, but going away when they're spinning in sync (locked or not)?

 

[mrdana]

Apparently the problem was the PVH locking hub on the driver side, it's toast. Trying to ID the correct replacement part now...