I have a 2008 RX400h with 185,000 miles. I recently fixed the frozen shift cable connection to the lever on the transmission and replaced the range sensor neutral switch. Car starts now and goes into gear but the rear wheels are locked…I think due to the differential. When I jack up the car and put the car in neutral, neither rear wheel will turn but there is about 1/2 inch of play in them. With the front jacked up, the engine turns the front wheels when in drive or reverse. I wanted to get some ideas before I take to the mechanic. BTW, it has nothing to due with the parking brake as I ruled that out.

 

I,m slightly puzzled over this,
my rx ( 06)only has electric drive to the rear wheels?
there is no actual propshaft to the rear wheels is the 08 a different chassis then?
I ask because my first thought was its just because you are trying to turn the electric motors in the differential
which is actually very hard to do by hand and it does appear its locked

 

I don't have a RX400 but have torn apart my 2013 Tesla Model S rear drive unit 3x. While my motor is a lot bigger than RX400 rear drive motor. Basic design is quite similar. Here is a RX400 rear drive unit tear down video


The basic design are as follows

• An electric motor consist of stator and rotor. RX400 one is quite common that rotor use perm magnets and you can see it's not easy to pull out at 2m14s of the video due to the inherent magnetic force. There is quite common design. Stator gets hot and need to be cooled. This motor is bathed in transmission fluid for cooling like all other Toyota hybrids.
• Then there is a ~10:1 gear reduction ratio with 3 shafts. A primary connected to the electric motor, an intermediate, and finally the differential with its large crown (more like a ring in EVs due to gearbox layout) gear on the outside for final reduction. Basically ~400V motor needs ~10:1 ratio to get to correct torque and roadsteads. So all the hybrids and EV gearbox use this 3 shaft design with diff integrated into the final shaft. Only Toyota's front wheel drive hybrid (All Prius and I think RX400 has a front electric motor too) uses eCVT which is a planetary gear ratio reduction that is different than the typical 3 shaft 10:1 gear reduction design.
• RX400 rear differential is an open differential and not limited slip design from looking at the video (1m14s) and google results. Here is a video explaining both
  • link
• No parking pawl is visible in the tear down video. 08 RX400 has parking brake integrated into rear brakes (disc for drive, hidden drum section inside for parking) So in theory, when turning one wheel will just cause the other wheel to spin the other way.

So in theory, there is nothing that prevents the wheel from turning other than the parking brakes. You can turn one wheel independent of the other even if the other is on the ground. Furthermore, even if anything seized before the finally shaft with the differential, wheels can turn independently.

So when you have both rears off the ground and either will turn means some some diff element might have seized up. But this seems quite unlikel as they would need to be completely destroyed. ATF coolant is also not a corrosive agent so no rust issues (My Tesla motor is cooled by coolant so it better not leak into the motor cavity!) Did the rear end roll properly prior to your recent fix?

Maybe the easiest test is to pull the axle from the hub but this only eliminates a locked parking brake and/or both hubs locked which seems quite unlikely. Anyhow, prior to repair effort history might be useful.

 

RX 400 has single front electric and 2 rear axle electric motors? other than that you explanation is really clear thank you!

 

Looks like 1 front which is the typical Toyota MG1/MG2 with planetary eCVT design. And 1 rear with differential as noted above driving both tires (can google how a differential works to split power to left and right wheels which need to turn at different rates going around a corner) Here is the same guy tearing down the typical Toyota hybrid front end hybrid system (in all Prius and Toyota hybrids I think)


This professor breaks down the entire front end hybrid system if want to get a sense of how it works.


Planetary gear system for gear ratio change is Toyota hybrids bread and button (everyone else do failure prone CVT belts or clutches) Here is a good explanation of planetary gear system. Ratio is changed by rotating the SUN gear (middle one)

 

Thanks for all the guidance. After my original post I figured out that the rear wheels are powered by the rear electric motor. Since that is seemingly seized, is there a possibility that the ECU need to send a signal to unlock it?

Also, just prior to this episode, the car was driving uphill after a a big snow storm, so the rear electric engine probably kicked in and may not have been engaged for a long time.

And no codes from my OBD2 scanner.

 

Thanks, prior info is helpful. So it was working properly prior to the repairs.

From the teardown video in post #3, I see no locking mechanism of any kind for the gearbox or the electric motor. Therefore It shouldn't matter if any command is send to this unit. It should spin as long as axles are not locked by parking or driving brakes or seized wheel hubs. But I'm just going by the teardown video (06 RX400) and the drive unit is removed from the car.

Exactly how are you spinning the rear wheel? Rear up on stands and front on the ground? and trying to spin the rear wheel with the car in N? I wonder if hitting some kind of traction control. If spin the rear wheels while front wheel is not moving. Traction control will think rear wheel is slipping and may command the motor to counter that spin. Although I wouldn't think traction control is operating in N. But I don't know the specific traction control logic of this car.

There are 3 speed sensors in the rear as far as I can tell. 1x on each wheel and 1x inside the actual motor.

As for OBD2, thats not going to be sufficient. You will need Techstream to dig into anything on the electric side. Here is an example of techstream snooping my 11 Prius HV battery cell voltages during a short EV mode only drive ( link ) Generic OBD2 can't see any of this. I'm sure generic OBD2 is not going to give you either front or rear electric motor info, HV battery info, probably not even wheel speed sensors...

Attached are PDFs pulled from my local library's Chilton subscription for all library users for reference.

DEADLY HIGH VOLTAGE WARNING Its quite dangerous snoop around the deadly high voltage (HV) system including this motor without proper knowledge. Any orange cable is carrying HV and is deadly (will kill you) so there are lots of precautious like Personal Protection Equipment (PPE... 1000V gloves with additional outer glove for anti cut/puncture) and disabling HV circuit from the HV battery. So becareful and familiarize these if go poking around there.

 

Thanks. This is great info … particularly the PDFs on how to remove and install the rear electric motor as I could not find and videos on how to do that job.

 

Can’t just start removing parts when HV is involved. Enough voltage and amps to kill a person.

You need to go find factory service info on disabling HV and HV safety requirements.