My front driveshaft turns freely in RWD?

[scottm]

Lubing the front DS for the first time last night. The zerk fittings were almost lined up for my grease gun, thought I'd see if there's enough slop to turn it a bit. It turned, and turned, apparently not engaged on either end? I switch to AWD when its wet out, can tell it's engaged because it doesn't spin the rear wheels anymore, so everything seems to be working. It just seems odd that both ends of the front DS disengage in RWD.

 

[96r50]

I think your 4Runner has ADD

If it does have ADD (Automatic Disconnecting Differential?), when you switch to 2WD the transfer case disconnects from the front DS as you would expect and the front diff also disengages one of the CV's from the differential so that the front DS doesn't turn. Both CVs still spin since they're directly attached to the front hubs, but the one that is still attached to the diff is only spinning the spider gears inside the carrier, not the R&P and the front DS.

I'm going to follow that up with a big "I think" since I don't know as much about Toyota IFS 4WD systems as some of the other folks here

 

[scottm]

Seems like overkill, but kinda cool. It does have a center diff lock, but even unlocked I shouldn't be able to turn the DS like that, so it must somehow disconnect at the

 

[96r50]

Seems like overkill, but kinda cool. It does have a center diff lock, but even unlocked I shouldn't be able to turn the DS like that, so it must somehow disconnect at the

At the... the...?

I'm not sure how your transfer case works, but is it possible that 2WD disconnects the DS from the transfer, AWD connects it but leaves the centre diff open, and 4WD locks the centre diff? That would make sense given what you're describing, but it obviously requires more electronics.

The automatic FJC has a similar system I believe.

 

[scottm]

AWD connects the front axle to the system somehow, and there's a locking center diff. I don't know why they'd disconnect both ends of the front DS when disconnecting one end would do the job. The front DS might even rotate when it's not locked in, dunno. I have the FSM on a chip, if I really want to know.

 

[New80Owner]

I don't quite understand the necessity of the ADD system.

Sorry i usually roam the 80s forum, but my 4runner has led me here.......

Ok, i was just checking out the fsm transfer case section for the runner, and there are shift forks which engage the drive chain to transfer torque to front drive shaft. Now, in the ADD system, the axle shafts, spider gears, in 2wd continue to spin; while the carrier, R&P, front drive shaft don't.

When AWD is activated, a sleeve connects the left axle shaft to diff AND transfer case engages front driveshaft.

Why does this system require both actions? Can't the transfer just engage without the need for the sleeve in the left axle shaft?

My transfer is acting up, so i want to learn as much as possible. Actually i am thinking about buying a transfer from a junk yard and tearing it open.

Expert please chime in.......

 

[New80Owner]

Anyone?

 

[larsdennert]

The ADD is to keep the driveshaft from spinning to save gas. One half shaft gets disconnected and the spider gears spin in the diff from the other side. It is vacuum controlled. The other end of the drive shaft gets disconnected by the xfer in 2WD. I don't think it saves much gas. The V8 fulltime 4wd GX gets 19mpg at 65mph.

 

[New80Owner]

Why not just disconnect both half shafts while yota is at it? That way nothing in the 3rd member is in motion. The ADD to me is just more stuff to break, negating at fuel savings........

 

[larsdennert]

You mean locking hubs? From what ive seen on tacos with manual hubs vs live hubs with add, the add setup is stronger.

It takes 4x the friction to turn the driveshaft than the half shafts maybe.

 

[scottm]

When AWD is activated, a sleeve connects the left axle shaft to diff AND transfer case engages front driveshaft.

The ADD is to keep the driveshaft from spinning to save gas. One half shaft gets disconnected and the spider gears spin in the diff from the other side. It is vacuum controlled. The other end of the drive shaft gets disconnected by the xfer in 2WD. I don't think it saves much gas. The V8 fulltime 4wd GX gets 19mpg at 65mph.

It sounds like the front driveshaft should still be connected to the front diff, and the right half-shaft is still connected to the diff. For the driveshaft to be stationary, the internal diff gears have to be spinning at pretty good speed, something they rarely do when both half-shafts are connected to it. I'm not seeing how this is better than just letting the driveshaft spin...? A locking diff would've been a much better use of all this.

 

[larsdennert]

Yep. Like I said, I don't think it saves much gas but that is how it works. Btw the add is on the right side opposite the differential.