[AbbyNormal]

Maybe working too good is the wrong description, the issue is that my van is in 4WD all the time. It acts like the front and back drive shafts are locked together and no slip is being allowed between them. Years ago, I owned a 4WD Jeep and the van is acting like the Jeep if you left the transfer case engaged all the time. During a turn, when one wheel hits a slippery spot, the van jerks as the built up drive train tension is released.
My new-to-me 2015 XLE or limited (107K miles) has no dash error lights and the AWD light comes on briefly during driving on very slippery roads. The van has acted this way since I purchased it 7K miles ago.
I attempted to describe the issue to 2 Toyota dealers but they had no clue what I was talking about.
I have run across the terms viscous coupling and electro magnetic coupling but don’t know if the van has one or both or what their function is. I’m hoping someone can give me an intro to the Sienna drivetrain and suggestions on how to test for, or track down this issue. Thanks.

 

[datsa noydb]

Well, your suspicion is correct on one point, at least. Your van IS in AWD all the time.

I had an '05 Sienna AWD for about 11 years, then traded it for my current '18 AWD. I think I know what you are talking about when you describe it as being locked up. When turning sharply in my driveway when the pavement is wet or snow-covered, the tires do slip a bit, giving the impression that something is not giving like it should. However, that is the only time that I notice anything even slightly negative about the system. My '05 did not do that.

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[AbbyNormal]

Well, your suspicion is correct on one point, at least. Your van IS in AWD all the time.
.

Thanks for the confirmation. One major concern I have is the excess wear this causes on the tires. I wonder how common this problem is since short tire life is a common theme I read on this van. I'm still hoping for some insight on how the drive train works.

 

[2018 XLE]

Our Siennas have an electro magnetic coupling in the rear axle. I wonder if the unit has locked up internally or if there is power on all the time. I would look for a fuse or go under and see if there is a connector to unplug for a test. A more difficult test would be to remove the driveshaft if it disconnects from the transfer case without leaving an opening for a fluid leak. It will then be front wheel drive only with no binding. I have seen posts here on how to replace the magnetic particle assembly.

 

[AbbyNormal]

Thanks for the info 2018 XLE. Any under van test might have to wait a bit since I don't have an indoor work space. When the coupling is engaged, does that positively lock the front and back driveshafts together or does come other component allow some slip? A related question: is some power transmitted to the rear differential when the electro magnetic coupling is not engaged?

 

[2018 XLE]

My other Toyota is a 4Runner with part time 4 wheel drive. That one will bind and be difficult to turn when engaged on dry pavement. Yours appears to be acting like that.

We have a 2018, there is a drive shaft from the front transaxle to the rear diff. It turns all the time when moving. The magnetic coupling engages when electric power is applied but it's not a full lock up. It couples loosely or more tight depending on the power sent by a control system. Ours has an AWD Monitor display with bars showing power being sent to the front and rear axles. I'm not sure if it really shows an accurate percentage of power or if it is just a nice display.

Any acceleration will show some power to the rear. Never as much as the front but close at times with hard acceleration. It does this even at highway speed. Usually only 1 or 2 bars for the rear, then it quickly drops to zero.

I have had no problems with the front feeling like it's connected and causing wheel skip or binding like when turning sharply into a parking space.