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Discussion Starter #1
I have a 2007 FJ. The rear started clacking. It was exactly one tire revolution. I put the truck on 4 jack stands and the right rear tire didn't move. It could spin freely though. I pulled both axles and they were fine. The truck is 4WD and has a elocker on the rear.
 

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You are only supposed to get power to 1 wheel in the back.... 4wd = power to 2 wheels.... 2wd = power to 1 wheel

When does it clack?
 

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IF you can restrain/hold the wheel that turns the opposite wheel should turn.
 

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If it’s sat on stands and in neutral then if you rotate one wheel the one the other side should rotate in the opposite direction.

If there is a ’clack’ when you drive and you are sure it is rear end it could be teeth on the crown wheel and/or pinion being chewed. If that’s the case it will jam up pretty quick. If you snap a shaft you usually just hear a click and loose drive. A front CV can clack too as it fails....
 

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Just to make sure there is no misunderstanding:
1. Automatic transmission?
2. When you had it up on jackstands and were spinning the rear wheels,
a) the transmission was in PARK,
b) the transfer case was NOT in neutral,
c) the differential lock was NOT engaged,
d) the parking brake was completely released?

Normally, on a conventional open diff when both wheels are in the air and the driveshaft is locked, spinning one wheel will cause the other wheel to spin in the opposite direction.

When you pulled the axle shafts, you confirmed that the splined section was present on the very ends of both driveshafts? Its not unheard of for an axle shaft to fracture right at the end of the splines, leaving the splined part in a side gear. A quick look might lead you to believe that the shaft is OK, but its actually missing the last 1.5" long splined section.

If the parking brake was severely dragging on the opposite wheel, and the transfer case was in neutral, it might be possible to spin one wheel and have the driveshaft turn, instead of having the opposite wheel spin. But you would certainly feel a lot of drag, as you would would be turning the driveshaft and some transfer case internals at a "geared-up" ratio.

More likely, you broke a spider gear; less likely, a side gear.

Here is a very clear animation describing exactly how a differential works, and the function of the side and spider gears.

 

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Discussion Starter #7
Yeah I took the axles out and the diff. Sent the diff to a shop and they said it was fine. I think I was missing some basic understanding of how diffs work. I knew the power moves from wheel to wheel depending on what has less resistance like the water flow analogy but I don't think I realized that on jack stands, even though both tires still have little resistance, one tire will have less resistance and spin w power and the other won't. That confused me. Automatic in neutral on jack stands the rear tires spun in opposite directions as they should. In drive and being ultra careful, the right rear didn't spin w power (ie just idling on the stands). I thought open diffs would at least spin the tire a little bit but not none. My mechanic friend told me to take the diff apart bc something must have been wrong. It didn't make sense to me bc the diff gears are all smaller and would have clacked faster. The diff oil looked fine w no particles or chunks. Learning experience for me. Since the clacking comes w every tire revolution, It'd be the tire, brakes, ebrake, whatever. This is my buddy's truck. I talked him into getting a FJ. It didn't take much really, we're going to the FJ Summit this year. He said he put wheel spacers on and then took them off bc they caused body rub. So likely there's something going on there. Thanks everyone. I'll keep ya posted. But to confirm, w the truck on stands, ebrake off, in drive, letting it idle w no brakes applied, it's normal for only one rear tire to spin?
 

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But to confirm, w the truck on stands, ebrake off, in drive, letting it idle w no brakes applied, it's normal for only one rear tire to spin?
Yes.

If the total friction on BOTH sides of the rear axle assembly (including drag from wheel bearings, axle seals, service brake drag, parking brake drag, etc.) were EXACTLY the same on both sides of the rear axle assembly, theoretically both wheels would spin at the same rate. However, total friction is almost never exactly the same, and if the rear axle is supported on jack stands, with the engine idling, only one rear wheel will be turning. Add some drag to the spinning wheel, and the opposite wheel will start to turn.
 
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