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Posi (Limited Slip Diff) for your RX ...


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Ot seems there was/is a mythical Unicorn among RX's.  In the early model years there was an optional Torsen posi-traction rear differential.  I'm very keen on finding the parts to make that happen in a 2009 RX 350.

So I went looking in the Toyota parts stream. I was searching the parts at a web site called Toyota Parts Overstock, or something similar. I was looking to see if some, or all, of the internals from an earlier rear diff would fit in a latter one? So I concentrated on the center section (the casting that carries the ring gear) P/N 41311J in the diagram. As I honed in on that part, I got a parts diagram with notes that that listed the years and models.

The internals seem to be an 8" 2-pinion (spider gear) "V-6" diff that was commonly used on most everything: TOYOTA: 7 models, 305 variants between 1982 and 2010. Fits CAMRY, CELICA, HIGHLANDER, PREVIA, RAV 4, SIENNA, VAN

And on page 1 at the bottom are some variation notes about the shape of the spider gears. And under that is a note about the model RX300 ... The rear end in a Lexus RX might be designated as an MCU15 ... [Nope - MCU15 is the designation for the Lexus Service Manual for the RX 300.  I can be found on-line by Googling in MCU15 and Manual]

Tthis is not conclusive proof of course, but aken all together, it seems to indicate that minus some variations in attachment, rubber isolation supports, and spider gears; this same rear end (internals) have been used in RX's from day one through some time in 2010. So the idea of finding a rare early RX 300 posi rear diff is possibly a good one. And with changing the external bits that make up the year model mounting variations, might actually fit a Gen II RX 350 ...

I have to do more research, and prolly digging at Pick-&-Pull looking at Highlander and RX models from various years. But if what I'm reading is true (?), we could use the RX 300 Torsen posi in later models ...

The trick will be to get a VIN number from an early vehicle with a Torsen factory installed. And use that VIN number to order the internal parts through Toyota's various web sites to build up a modern version (housing & mounts) to fit the vehicle of interest.  Or rebuild the one we have with selected new internals.

See attached for info:

Lexus RX Rear Axle Housing.pdf

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OK, I been digging and digging.  Been to all sorts of off-road after market and race after market vendors, and no Posi's available ...  It is believed that this is the Toyota 6.7" ring gear diff ... But Toyota made lots of OEM Posi's it seems. 

The RX uses a differential carrier and gear selection designated -02A as shown at the bottom of the build plate in the door jam.  This rear carrier and differential were available with a Posi (Torsen), all gear - no clutch, center section in RX's from 99 to 00.  Then Lexus dropped the option.

The same rear end was used in RAV4's, Highlanders, and RX's for many years.  Any independent suspended vehicle that shows -02A can prolly use this rear end ...

If you put this phrase into the search block in eBay, you will come up with a selection of used OEM LSD differential carriers.  The wrecking yard interchnage number is  440-58865A

Carrier Rear 2.928 Ratio Locking Fits 01-03 HIGHLANDER 

Build Plate 2002-b.jpg

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  • 1 month later...

Well I found one in Oregon with 158K on the ODO. Seller (B&D Auto Wrecking) confirmed that it was/is a locker. So after the previous three vendors failed to check well enough and either sent the wrong part, or just refunded my money after double checking, I pulled the trigger and got it. Yup, it's a Toyota OEM Limited Slip Differential carrier complete. Raining right now, so don't want to open it up, but will in a few days.

Looks like the right rear stub axle seal was weeping, so before it goes into my RX, I'll have the shop install all new seals and double check the lash. I don't want a noisy one replacing a dead silent existing one...

So I got a photo of the "Build Plate" off the drivers door jam (sticker) from the donor vehicle (Highlander) :
_highlander_lsd_build_plate_7fc77d8630dfdbf90958eafed47c6e4c57927a32.jpg
So you see the axle data in the lower right hand corner? It says -04A for this axle. My existing one is a std open diff (-02A). It's the thing you are looking for when out shopping. -04A is the axle code to look for in the field when hunting for one of these.

I have seen one door sticker with an -03A on it, but was not able to get any more info about what axle that might be ...

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  • 6 months later...

How do you like the locker? 

I drive an '01 Rx 300. I guess I just missed getting a locker if I'd bought a little older RX.

What little time I've spent off roading my RX it sure needs that locker!!

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OK, here is the long over due update.  I have an 03 and I contacted Jimmy Vasser Toyota (JVT) in Napa Calif to see whether they would do the swap for me?  Yes, they would.  So I found a Torsen rear diff at B&R Auto Wrecking in Oregon (excellent source of good parts: https://www.autowrecking.com/  ).  Bought it and had it shipped.  Around $240 by the time it got here.  I had already been shipped a couple of non-posi's by other vendors (advertised as posi's), and refunded my money when I pulled the rear covers and found them to be std open diffs.  eBay adverts can be misleading ...  But they all made it right, just annoying.

In case anyone is looking at door stickers and build codes, and trying to decode stuff, that is a gamble.  The only way to make sure is to put the VIN # into the Toyota "as delivered" database and see if it comes up with a posi as an installed option.  The donor vehicle for these parts was:  JT6HF10UXX0090348   I do not know which number in this sequence denotes posi ...

I took the car in to JVT to have some other service work done last month.  I needed the dash dissembled to fix a non-functioning blend door, an engine service, etc.  The car has less than 100K on the ODO, so I'm letting dealer do the work for now.  That way it's all in the Toyota/Lexus system.  This is planned to be our travel car, so I want any Toyota dealer to see what has happened so far, if I need a fix on the road ... I had them pull the rear cover and inspect (all good), install all new seals, etc.  In about a week, they called and said they had finish everything but ...

They could not get the axles to fasten up. The Torsen Posi has larger axle shafts and bigger differential flanges than a stock open diff.  So they "one bolted it" so it could be moved outside while I went on the hunt for used proper axles and hubs.  They quoted $4K if they had to order all new parts from Lexus.  That's a big ouch if I could not find what I needed ...

While this was going on, the Hennessy Lightning Fire blew up and became the LNU Complex Fire, and then parts of southern Oregon started burning too.  So B&R found the correct parts in one of their yards, but the yard was in a fire zone.  By the time we all slowed down from burning enough to resume business, weeks had gone by.  But JVT was kind enough to store the car and wait.  Another $180 for both rear axles, CV joints and hubs finally got drop shipped to JVT. 

While that was going on, the Glass Fire started.  We were in the fire zone and had to fight the fire to keep our house, so glad the car was safely away.  You can read about it here:  https://www.mercurynews.com/2020/09/29/as-famous-wineries-burn-around-them-not-so-typical-napa-valley-couple-fights-all-night-to-save-home/

After the fire was under control and travel could resume, we fetched the car.  JVT's charges were around $800 split over two bills due to the delay in parts sourcing.  I have since driven the vehicle a little less than 1,000 miles and it acts just like a regular RX 300 on the road.  No chirping, shudder, whine, or any other sign it has a posi.  But on the dirt or gravel it is different.  99% of the time you can not get it to turn a wheel from a standing start.  Granted I have not hit hard from dead stop, but I have a pretty steep part of my gravel drive (35* slope) and it'll go up that from a standing start on slope w/o so much as hearing one little rock roll over.  It is uncanny how grippy it is.

This car will be used to pull my 16' fiberglass runabout, hopefully to Lake Chelan next year, and launch it on some "primitive" ramps (gravel).  I expect it to perform perfectly, but I'll test that here on slippery tidal ramps in the next week or two. 

Anyway, anyone contemplating this should not be put off.  The parts are out there.  My stuff came off a nice clean wrecked 1999 RX 300.  But there were no fitment issues other than needing the correct axles and hubs.  There are no error codes.  The ABS seems to act normally.  I can't see why or how it is not a 100% improvement over stock open diff vehicle 🙂

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