Shuddering low gear acceleration when towing heavy?
 

Hello. 2002 F350 7.3, MANUAL transmission ZF6.

Bought this truck used for hauling an excavator, and after fixing damn-near everything engine/turbo/and-cooling-system-related ---- it's finally seeing some use.

Just recently started hauling 12,000lb payloads.

When I accelerate hard in Low gear, and then shift into 1st and accelerate hard again, I feel the whole truck shuddering.

It's like a shimmy-shaking vibration all through the vehicle.

It doesn't happen in higher gears, only when the following conditions are met...
1) Starting from a dead stop
2) 12,000lb payload (heavy)
3) Accelerating hard (Low, First gear)

I would also like to add that I experienced this shuddering vibration very intensely when I put the transfer case in 4wd LOW (front manual hubs left unlocked) and towed 14,000lb up a steep hill in 1st, 2nd, and then 3rd transmission gears.

^^^ By leaving front hubs unlocked and transfer case in 4wd LOW, I was effectively lowering my final drive ratio even more for extra pulling power in 2wd hill-climbing. I don't do this often, only when absolutely necessary. ^^^

I must also add that I do have slightly taller rims/tires than OEM ford WITH the original ford gearing (3.73) so I'm more like 3.51 final drive ratio.

Thanks!

02 F350 7.3l ZF6

Check the Bell housing of your transmission for cracks. I had a 99 that was doing the same thing and it was cracked. It only did it when pulling heavy loads.

Luke

Oh dear. Now I'm freaking mortified. If that is the problem I can't even begin to start crying fast enough. I'm going to crawl under the truck right now. Where should I look for cracks?

I'm a bit of a truck hypochondriac, the shudder is probably nothing serious. It's like the truck vibrates when accelerating in low gear with a heavy load.

Perhaps the rear leafs are wrapping. It's as if I can feel each combustion of the engine bouncing against the rear suspension.

Accelerating hard also torques the whole chassis down in the rear left corner. If I put a level on the rear tailgate and accelerate hard with a heavy load, the passenger side rear corner rises up and the driver side rear corner drops down. Suspecting this is normal, due to all the torque going through the driveline.:humm: