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Rx400H Awd Rear Diff Problem - Help / Advise Please


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Hi,
I have a 2006 rx400h, with has covered 120,000 miles without a hitch until 2 weeks ago.


I got the check hybrid system light come on.
I had this checked the A.A and the fault code was - Drive Motor "B" position sensor circuit - POA 45.


This was message cleared by the AA service guy, the car was o.k for approximately 50miles then the same
Check Hybrid system indication came back on along with check vsc system and check awd system.


The car drove normally and was charging and using the Battery as per normal.


I took it to my nearest lexus dealer and the checked the fault log and got the same fault code - Drive Motor "B" position sensor circuit - POA 45.
They have told me that they have checked the circuit connections and all is o.k, but if it comes on again I will need the rear Differential replacing!!
Their diagnosis seems rather dramatic and very expensive £2300.


My questions are

1 - is their diagnosis reasonable?


doesn't seem very technically based to change out a full unit


2 - can an awd car be converted to be a fwd only vehicle?, lexus sell a front wheel drive only variant in the U.S.A.


the physical, I would Imagine would be disconnecting the power and monitoring to the rear diff, what would be required to convince the management system that the rear drive doesn't exist?


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Hello and welcome to the LOC.

With Lexus Hybrid technology being fairly new age stuff I doubt there will be many if any independent specialists out there who may be able to tackle a job like this. I've looked on the database for the RX400H and there is no mention of the hybrid traction motor system and in the parts list the sensor you mention is not even listed so may not be available as a spare part.

It would appear you are at the mercy of Lexus and even though it may be a simple sensor fault in the hybrid drive system they don't repair assemblies they just replace the whole unit hence the high prices.

It's a bit like buying a torch and then when the batteries go flat buying another instead of just replacing the batteries in the one you already have.

Converting to FWD only again is possible but will need specialist attention possibly a replacement ECU from a FWD model which would probably cost even more.

Sorry it's not good news.

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you should first disconnect the aux Battery (in engine bay)

open the rear near-side door, at the side under the seat is a plate which pulls up and reveals the service plug.

Using gloves, slide locking lever up and plug out the plug. Do not touch the terminals and wait 5 mins. before touching any HV cables.

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TO ramsascooby ,

I don't know what kind of problem you have with your lexus but i can help you by answering your second question,

You can disengage your RWD motor by turning your traction control off. but you'll have to do these steps every time you start your car ( at least until you figure out your problem ).

NOTE: i read about this method in a forum and was posted by a Toyota mechanic for CAMRY models, i tried it and now i can have FWD or AWD without traction control,

How to do that :

WARNING: The RX has open diff(s) on all wheels ! with that amount of power and torque wheel spin will occur on slippery tarmac & wet roads and your Lexus will understeer SO BE CAREFUL ( DO IT ON YOUR OWN RISK ) !!!!!!!

Method 1 ( FWD Maintenance Mode )

1- put your key in the ignition and switch to on BUT DON'T START THE CAR

2- press and hold the brake pedal ( until the end of the process )

3- GEAR IN PARK - fully press the throttle and release 2 times

4- GEAR IN NUTRAL - fully press the throttle and release 2 times

5- GEAR IN PARK AGAIN - fully press the throttle and release 2 times

6- DONE - the information display should show the following message ( FWD Maintenance Mode ).

7- you can start your car now and release the brake, only traction control is off now but in FWD only ... just turn the ignition key off and re-start your car and every thing is back to normal ( AWD with traction )

Method 2 ( FWD Certification Mode )

1- put your key in the ignition and switch to on BUT DON'T START THE CAR

2- press and hold the brake pedal ( until the end of the process )

3- GEAR IN PARK - fully press the throttle and release 3 times

4- GEAR IN NUTRAL - fully press the throttle and release 3 times

5- GEAR IN PARK AGAIN - fully press the throttle and release 3 times

6- DONE - the information display should show the following message ( FWD Certification Mode ).

7-you can start your car now and release the brake, only traction control is off now but in FWD only ... just turn the ignition key off and re-start your car and every thing is back to normal ( AWD with traction )

Let me know if it works,

Cheers,

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Hi, may i just say, this seems an awful lot of bother to go to. Presumably you bought a 4 wheel drive car, because you wanted 4 wheel drive, not 2.

Can't you just pay to have the repair? Anyway, the problem may never come up again.

j

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Your RX hybrid isn't AWD it's FWD with rear driven wheels assisted by Battery 'when needed' - it is in essence a FWD only 2T car.

Given how the hybrid system interfaces with the normal drivetrain, I think you will have to get this sorted: either a temp. fix and get rid or bite the bullet and get a proper repair completed, but for the latter I think only Lexus will be your only option.

I was never convinced by the robustness of this hybrid drivetrain, they're failing all too predictably at 6/7YRS.

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  • 4 years later...

I cleared the fault a week ago and it hasn't returned yet.  

I found two plugs on the side of the diff and tried to unplug them just to check connections.  One plug had two wires and came off quite easily.  The contacts inside were pristine - shiny copper. The other plug had more wires - possibly 5.  Unfortunately, I couldn't get this one apart as the clip seemed to be stuck.  

The inspection procedure also says to check the plugs on the MG ECU.  Where is the MG ECU?

P0A45 Diagnostics.pdf

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6 hours ago, ColinBarber said:

It's the ECU within the inverter assembly.

Thanks a lot Colin.  I think that's on the left hand side of the engine bay - am i correct?  Is there a guide to uncovering the ECU plugs somewhere?  (I haven't found one yet.  )

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It is the silver covered unit on the RHS of the engine - standing at the front of the vehicle looking into the engine bay.

The following is part way through the procedure for removing/replacing the inverter. If you just want to remove the cover and check the ECU connections then I don't know if you need to follow all the previous procedures of removing the wipers and motor etc. or whether the bolts are accessible and cover removable with everything else in place.

You will see the MG ECU connectors on step 26 on the next page.

http://workshop-manuals.com/lexus/rx_400h_awd/v6-3.3l_(3mz-fe)_hybrid/hybrid_drive_systems/power_control_system_hybrid_drive/voltage_inverter_hybrid_drive/component_information/technical_service_bulletins/page_2739/

Read the procedure from the beginning - you need to isolate the hybrid Battery before going near the inverter.

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