I have a 94 6.0 V12 and I have been trying to track down a noise in the rear end. It's a cyclic whump, whump sort of noise, and I thought a cyclic noise would be U joints, since a bearing noise should be more of a constant noise.

So to diagnose, I have removed the halfshafts from the differential and started the engine on the hoist. It was in neutral when I started it and I was surprised to see the speedometer showing about 20 km/h. I got out and looked at the diff, and sure enough the flanges were spinning. So I put it in drive, and the speedo went up to 40 km/h!

Shouldn't neutral be completely disconnected from the driveshaft? I would have though it should be like putting the clutch in on a manual car, no power is transmitted to the rear wheels. Obviously not in this case!

I'll start a new thread for the diff noise.

Craig

 

What you are most likely experiencing here is parasitic drag inside the transmission causing the unloaded driveshaft to spin.

Put it in Park.

 

Very common.

I was told years ago that it's caused by 'fluid shear' in the transmission

Cheers
DD

 

The noise could be loads of things, driveshaft UJs, propshaft UJs, actual propshaft internal rubber coupling going home, gearbox rear bearing, gearbox mounting, rear wheel bearing, diff mounting bolts, bottom wishbone diff mounting bolts, diff input or output bearings, hub location pivot, diff itself (but I agree a whine is more usual). Sorry!
Greg

 

YUP, all the way back. My first car, a 23 Ford T had a planetary transmission. The percussor of many automatic transmissions.


The transmission and engine shared a common sump. In cold weather,
the fluid drag was considerable. Too much for an electric crank.
And hard to fire by hand.


My system was to jack up the left rear. Hand crank, fire it, push it of the jack and jump in, in quick jump in quick succession.


Carl