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post #1 of 2 (permalink) Old 07-08-07, 05:06 PM Thread Starter
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Loud Thumping Upon Acceleration

For approx. 2 years i've had a slight thump underneath the floorboards on our 99 expedition. It always happens upon acceleration and evens out when above 45 mph. Occasionally when going uphill and passing the rig wanted to pull to the left or right. Time and time again i climbed underneath to check things out. Started with checking brake calipers. Have check bearings, tie rods, sway bars, springs, cv joints, shocks, ujoints and anything else i couldthink of. Everything seemed good. Recently took the rig to the shop for abs problems and the shop noticed the loud thumping noise too and came to the conclusion it was in the transfer case or rear end.
To isolate to the transfer case i wanted to drop both drivelines and start the rig and put in gear to see if noise persits. Never made it that far and I only dropped the front driveline. Been driving with the front driveline gone and thumping has disappeared. My ujoints look fine. I would imagine if the problems were in either the front or rear differential the thumping would still continue being the front axles will still turn with automatic hubs. I've came to the conclusion that the driveline could be out of balance or the transfer case is trying to engage full time 4 wheel drive when it is not supposed to. It is my understanding that the torque on demand transfer case has a magnetic clutch to engage the front wheels when it senses tire spin in the front while in all wheel drive mode. Does anyone have suggestions on this problem? Oh yeah, it doesn't matter if in 4 wheel or all wheel drive.

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post #2 of 2 (permalink) Old 07-12-07, 08:04 PM
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Re: Loud Thumping Upon Acceleration

Sounds like 4X4 is stuck on. Look on the front axle in the middle and you will see the front axle engagement thingy (sorry, I am a little drunk and can't remember) and you may find it is seized keeping it in 4X4 mode.
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