We have an 07 Pacifica with the V-6 and 6 speed auto tranny. AWD of course. It has 70,000 miles on it and had an issue yesturday. Up to this point there has been no issues with anything in the drive train.

My wife was driving it and turning to get on to the highway and her passenger and herself felt the steering grap a bit and then there was a pop. She drove a little ways then called me and I went to investigate. There was no issues with the motor or the tranny itsself but there was fluid that smelled like gear lube all over the steering box. I decided to test drive it to see whats going on and the car ran fine and shifted with no problems, so i decided to take it the last couple miles home.

When I got home I jacked it up and crawled under to take a look. What I found was the power transfer unit in a couple pieces half cocked away from the tranny.

Now im thinking to myself, OH S***. what has happened here. Now as I was lying there looking at the situation, I realized that I don't truly understand how the AWD works. The reason for this is as I was messing with the driveshaft, looking at the PTU, I could spin the driveshaft around and around. This might not seem to weird but the rear tires are still on the ground and I'm spinning the driveshaft.

So what I'm getting to is first can someone explain how that system works (when it kicks in and out,etc)

Next would be, should I be able to spin the driveshaft with back wheels on the ground (logic tells me no,but not for sure with the AWD).

Last, I think is, What could have caused this to happen and what should I look for as far as damaged parts.

Please and thank you, I need as much help as possible.

 

Layman's explanation here: our AWD's are predominately FWD. Most of the time, our front driveline is carrying the load. The driveshaft connects to the rear carrier which is a combination over-running clutch and differential. If the conditions are right, the over-running clutch absorbs and transfers some of the torque from the driveshaft to the differential to "drive" the rear wheels. I believe you are able to turn the driveshaft because the over-running clutch has not begun to "lock up". That situation (you turning the driveshaft) I think approximates how things are during typical dry pavement constant speed travel.

The inner workings of the PTU and what likely broke go far beyond my pay grade. As you stated the "pop" occurred in a turn, it sounds the mechanism(s)/gearing in the PTU that allow the front wheels to move at different speeds locked up/bound and broke. I hope you can get away with just a replacement PTU. However I would wonder about CV shaft damage and about the gearing/coupling of the PTU to the tranny. Hopefully the innards of the PTU took the hit.

Good luck with it. I would be interested to hear what you find.

 

This is Not a Good Thing

But it does fit logically into an explanation.......

Just to be sure.....The driveshaft you are discussing is from PTU to rear diff???

To a certain extent, Busboy is correct, But if you say "AROUND AND AROUND"???? ...A small amount of "Free play is normal, but
With the wheels on the ground.....It shouldnt spin....But the fact that it does explains alot.....Your rear diff is damaged....the "Free spin " locked up the transfer gears in ptu, and sheared the case apart.

The first question I would ask you.....

"Have you Had ANYTHING done with the Tires Lately"?
This is Almost always the end result of mismatched Tire Diameter.

 

Thanks for the inputs and now i feel like a complete idiot. To answer questions:
Yes the drive shaft I'm refering to is from the PTU back to the rear diff. Like i said I can free spin it full rotations with the rear tires on the ground.

Yes i replaced the front tires last fall because they were bald and the rears had good tread on them. (we bought it in that condition) I replaced them with a set of used tires but I'm sure the diameters are not equal.

So I guess I've learned the hard way about AWD vehicles. Now what I understand from "Djinn-n-tonic" is that the rear diff is trashed too, because I can spin the drive shaft the way I can. Right???

I've looked for PTU's and what it takes to swap it and it's something I can handle doing, Just not as fast as a shop could do I'm sure. The ALLDATA program from my local NAPA dealer is helpful.

Now If the rear Diff is broke I guess I'll have to look for one of those too.

In replacing those are they all the same gear ratio or do i have to look for a certain one?

Thanks again and keep the info coming. I'll let everyone know how it comes out and what I find as I tear it apart.

 

Hey "Team",

I am glad Djinn weighed in, although it makes things sound a bit worse.

In re-studying the manual I see that the overrunning clutch is there to allow the rear end to "over run" or spin faster than the front (heavy braking) without allowing the rear driveline to "drive" the front. In other words power/torque should only flow front to rear, never the other way. Makes sense.

So Djinn a question: are you saying that you think the failure path was rear to front? In other words, failure in rear carrier occurred first allowing "rear to front" torque that led to PTU failure? And you said you think the rear diff is damaged. Do you think it actual damage to the differential gearing or a overrunning clutch/viscous coupling failure? Or both?

Thanks

 

After thinking about it, It could concievably happen either way. What confuses me, is since the PACS use that rubber coupling instead of a U-Joint, it should have ripped apart before cracking a PTU off of its mounts, and spitting it out.

I would be more inclined to think the Viscous assembly Ground itself up, but we dont service the assemblies seperately, so I'm Yet to see what a Blown up Viscous Looks like internally.

As for replacement...If you still have the tag on your diff, it will make life easier, as all you need to do is match the letter Codes, (Most of them just fall off During the winter, Never to be seen Again). If youre Flying Blind....Same year with the same Motor trans Combo should work.

 

I was told by my local chrysler mechanic that the driveshaft will turn even with the rear wheels on the ground because the diff has an electric unit that engages the clutch assy for the diff when needed.

So now I'm getting confused on weather it's bad or not. Is there a way of checking the diff to be positive that it's bad before buying one?

 

Ok.....This is new to me....

According to the 07 wiring diagrams.....Yes there is a Rear Diff solenoid...... But I have removed Countless differentials from Pacs, for axle and seal service, and the occassional Mismatched tire issue....And For the life of me...I DONT EVER REMEMBER "UNPLUGGING" A DIFF?????

Easy two step process.....1 Crawl under car...2 Look and see if there is a Wiring Harness on the Diff.

 

Thanks Djinn for your insight.

I am more familiar with the '05 and have that manual. Different engine and tranny on the '07 so likely it could be different.

 

Well I've started thr tear down and looked at the rear diff and there is wiring going to the unit like the dealer mech said.

So far everything is coming apart pretty easy, just down to unbolting the rack and pinion assy from the front frame and lowering it down. Then pull the PTU out. Replacement is on the way. It would be nice to have a lift rather be lying on the garage floor.

Thanks everyone. I'll let you know when I'm done on how hard it was overall to do.