Teach me about part time AWD systems?

Hello everyone, I've been thinking about all these PT AWD systems in newer vehicles (GM, Ford, Toyota, etc.), and just curious how they function. I read about all these electronic limited slip systems and rear differential failures, elsd pumps failing. I have a vehicle here that was offered with AWD/FWD, found a totaled awd one nearby and wondering if I could actually get away with a stronger all mechanical awd system using a standard rear differential? Couple questions which mainly pertain to ratio's and rear diffs before I start buying things for a project that may not work...I mostly just like to learn things and these systems are surprising the way they can transfer torque front to rear, wheel to wheel. the vehicle wouldn't have any of the electronics to control the awd system, hence the want for fully mechanical.

As far as I know, the rear drive shaft is constantly spinning along with the front axles they are locked together, then the rear differential uses some sort of viscous coupling to transfer the torque to the rear wheels to help once the front is slipping/spinning faster that the rear. The front ptu is the same part number whether its an auto/manual/v6 /4 cylinder on this particular one, so It leads me to think it can be bolted on in place of the passenger side jackshaft and just spins the same speed as the front axles. The mounting points all look to be there on the vehicle for the different rear subframe, fuel tank etc.

Do these rear differentials and front PTU's just use matching gear ratios? Is there even a normal gear ratio in the rear differentials? Would it ever be possible to just swap out the electronic assisted rear differential with a full mechanical unit in the back? Are there other older haldex differentials that are fully mechanical?

It seems there is quite a few cars now coming out with FWD/AWD configurations, would be neat if they could be converted with junkyard parts!

Thank you all for all the help and things I've learned from this site!

The worst part of some of these PTU type systems is that they drive the rear from one output of the front diff, which if it's got an open front diff is just awful. Put the left front wheel on ice and suddenly, you're stuck.

the pre-97 bravada's, astro's, and typhoon/syclone's all used the 4472 transfer case on the back of a 4l60 that was a true AWD system. Im not 100% sure on this but i THINK it should be able to bolt to the back of any 4wd/PTawd 4l60/4l60e to get a true AWD. I think they also make adapters to put it on the back of 4wd 4l80's as well.

That is kind of where I'm questioning the part time system, it uses an f40 6 speed manual trans and there IS an lsd that can be installed in the trans which would alleviate that issue of spinning one wheel and being stuck....but that still leaves the rear end issue. I'm looking to have a functioning awd system without buying a subaru, which seems like the only option...

rslifkin wrote: The worst part of some of these PTU type systems is that they drive the rear from one output of the front diff, which if it's got an open front diff is just awful. Put the left front wheel on ice and suddenly, you're stuck.

Not stuck, just incovenienced. Apply some light brake pressure to stop the spinning wheel, and the others will go. Takes some finesse, but it works.

Most FWD part time systems are computer controlled. Front wheels spin and the computer engages the rear differential . Then there are the old manual system in rwd systems. Normally in two wheel drive and then you shift it into 4wd On some the transfer case has a differential. This is called full time.

Speaking of front wheel drive based systems, there are two basic systems- most of them drive the angle drive (or power take off) using a spline from the outside of the front diff case. There might be some that just drive off the axle, but I've never run into one. The angle drive will have a 1:3 ratio, or thereabouts, which spins the driveshaft to the rear. The diff will then have a 3:1 ratio to bring it back down. As far as applying the torque, older ones often have a viscous or plate clutch, usually in the diff,(pre 03 Volvo, older CRV) but sometimes in the angle drive (Hyundai). Subaru had a clutch on the transmission output. Newer generally use a small hydraulic pump, computer and wheel speed sensors to apply the clutch. Rav4, 03 and newer Volvo) All I've seen of those are in the diff.

The worst part of some of these PTU type systems is that they drive the rear from one output of the front diff, which if it's got an open front diff is just awful. Put the left front wheel on ice and suddenly, you're stuck.

As streetwiseguy says, the vast majority if not ALL of these systems actually spline to the diff carrier and aren't just driven off one of the differential side gears. As a consequence of that, you usually can't just bolt a Tcase to the side of a FWD trans without opening it up and at least changing the diff carrier.

I really enjoyed learning how the Honda dual pump awd system works. I'd like to learn about more of the other systems if someone has some links floating around to post.

In reply to Vigo:

Honda's SH-AWD is pretty neat.

http://blogs.youwheel.com/2015/03/31/acura-sh-awd-comprehensive-analysis/

<img src="#//s1012.photobucket.com/user/sassyness77/media/Mobile Uploads/Screenshot_20151212-230846_zpseghpsj7m.png.html][/URL]" />

posting on phone...technical issues This is the ptu in question bolted to the f40 transmission. It looks to me that it just uses the axle stub but I could be wrong.

There have definitely been a few that use the axle stub, AFAIK. I'm pretty sure they're the minority though, fortunately.

If you look close, there is the splined shaft coming out the middle, and a larger splined tube surrounding it. The larger spline will be whats driving the pto. The small one is just a free running shaft to drive the right axle.

Streetwiseguy wrote: If you look close, there is the splined shaft coming out the middle, and a larger splined tube surrounding it. The larger spline will be whats driving the pto. The small one is just a free running shaft to drive the right axle.

Ah I see now! Well that just means id need the whole trans which isn't an issue. I think I'll just need to find one of these cars to lift and spin the wheels and try and calculate the ratios.

Different app but it seems to be similar design. The Chrysler AWD system used in the 90s-00s minivans uses a PTU splined into the front differential. The PTU is a ring and pinion where the ring is driven by the input and the pinion drives the output. The rear diff has a ratio opposite that of the PTU which makes the rear wheel speed match the front wheel speed. IE the PTU has a ratio of 1:3.5 and the rear diff is 3.5:1.

The old Chrysler van system is very simple and it's pretty cool and unique in that you can 'gear down' the whole system without having to modify any part of the AWD system. I'm having to think fairly hard for the stuff Spoolpigeon posted (thanks for that!!), and that's coming from someone who has actually been into and rebuilt planetary-based wet clutch driveline pieces before. Which means i'm enjoying this reading a lot.