I have just over 300,000 miles on my '86 and it started making this very loud dragging sound while in 2wd. The dealership mechanic said it was due to a rusty and dirty freewheeling hub. I cleaned them, greased them and tested them 20x in my hand and they are working fine now.

I tested the drivetrain on jackstands (stationary) for about 30 minutes and the hubs were locking and unlocking smoothly but, I noticed that when the truck was in 2wd, the front driveshaft from the A340F's auto transfer case was still spinning. Pretty sure this should not be happening. The truck did not make the dragging noise while it was stationary.

I tested the truck off-road today and the noise is either gone or extremely faint when in 2wd freewheeling. It still made the same loud noise sometimes in 4high or 4low, going forward or in reverse. I noticed it would make the noise more when I was driving on packed gravel and less when I was in grass.

Also, the fluid levels are all okay and the shifters are pretty solid. Any advice guys?

 

On the manual transmissions, the front shaft does not turn. I am guessing it shouldn't on an auto. I don't know the automatics to well.

 

That should not be happening. The transfer case uses a shift fork and hub sleeve similar to how a manual transmission engages gears. It is possible that this shift fork is bent or is misaligned on the shaft, or the shift gear head could have separated from the shaft. Or it could be just sticking a bit.

I find that these transfer cases seem to work best with Mobil 1 75w90. I would change the fluid, then exercise the 4wd lever a few times and see if it frees up. This would be the easiest and cheapest thing to do first. If that doesn't help, you probably have a mechanical problem inside the case.

 

The owners manual says to use Dexron II automatic transmission fluid, which I did change just after it started making the noise. I did try to shift between 4wd and 2wd a lot yesterday too. I think I'll do more shifting with it on jackstands and see if anything has changed since it tested it off-road.

 

Ok i was not aware the older models use atf. The later trucks with the a340f transmission use 75w90 gear oil in the transfer case.

I would not go against the manufacturer reccomended fluid.

 

Correction: I thought my truck had an A340F but it's actually an A340H.

 

That explains the ATF.