Shudder in Reverse up Steep Hills

Benjamin Service Manager New Jersey Posted   Latest  
Resolved
Drivetrain
2011 Lexus RX350 3.5L (2GRFE) 6-spd (U660E)
C1298
Shudder In Reverse Up Steep Hill

So we've got this great customer in an older but well-maintained car, 287K on the clock. Previously, 08/31/21, we replaced rear differential magnetic coupler and the problem seemed to be resolved. (I was not involved in that diagnosis or repair.)

Customer returned 05/02/22 with "same" condition, and we pulled DTC C1298 “Linear Solenoid Circuit,” from the system, referring to the rear magnetic coupler. Diagnosis concluded magnetic coupler was faulty, no other circuit faults noted, and thus replaced under 12-month parts warranty. (I agree this may have been a knee-jerk reaction and I should be punished.)

The DTC has not returned, but the customer returned a few days later stating the problem is even less intermittent. Still no DTCs. We are suspecting the transmission center differential or torque converter, but we're not sure why the noise and vibration occurs in the rear and only on hills under light acceleration. We cannot reproduce it moving forwards or power-braking or using the parking brake.

We've gone as far as electrically disconnecting the coupler, no change, removing the rear driveshaft section, no change, swapping driveshafts from a known-good vehicle, no change. The driveshaft seems to be sitting too low in the rearmost driveshaft carrier, but there's nothing we can see pushing it down. I thought maybe engine mounts but engine orientation is similar to a known-good vehicle next to it. We tried shimming and flipping front and rear driveshaft carriers to reduce the tension, but no change. Disassembled rear brakes to check for parking brake dragging or similar, but nothing abnormal noted.

Is there another test that can absolve or condemn the transmission? I have a feeling it's something so simple and stupid I've overlooked it. Thank you for your assistance.

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Dan Owner
North Carolina
Dan
 

Graph the transaxle input speed sensor (NT) PID with your scanner while the complaint is present. If you see wavy lines (slip grab slip grab) during reverse movement, then it’s inside the transaxle. Normal operation should be zero in gear while not moving and a steady linear increase with increasing speed. Nice and smooth. You can also brake torque in reverse to see if it will slip with…

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Benjamin Diagnostician
New Jersey
Benjamin
 

Definitely a great idea to check NT speed. I'm wondering if Techstream is fast enough to catch this or do I need to put my scope on it? Would be good practice for scope, in any case.

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Rolando Diagnostician
Texas
Rolando
 

By chance, have you checked the condition of ALL drivetrain fluids? I'm talking transmission, front diff. and transfer case, (if applicable), rear diff, etc. By the symptoms you are describing, almost sounds like clutch chatter. Odd that it only happens at certain times though. Unrelated to your problem but on the early GM 8 speed trans, the torque converter clutch used to chatter only once…

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Benjamin Diagnostician
New Jersey
Benjamin
 

You're right, that was a stupid question. I can't believe I haven't taken any fluid samples. Monday time to get dirty, thanks.

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David Owner/Technician
Alberta
David
 

I am with Dan Hogan on this one. One more thought. You likely would see input and output speed sensors doing something naughty in reverse if it is transaxle related? You could graph them or maybe there is a PID for ratio you might graph?

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Benjamin Diagnostician
New Jersey
Benjamin
 

Agreed, definitely will try this, Monday.

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Mark Diagnostician
Ohio
Mark
   

That many miles you may be dealing with trans sprag chatter.

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Timothy Owner/Technician
Wisconsin
Timothy
 

Almost bound to be the transmission torque converter or hydraulic fault internally. Have seen the issue on several of these Toyo AWD trucks. My sister even has the issue on her RAV4 (essentially the same truck). It took both an RDU and a transmission converter to clear the issue…these transmissions are not the transmissions of Toyo's past. "lifetime' fluid may be the cause of this. People just…

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Benjamin Diagnostician
New Jersey
Benjamin
 

Yeah, “lifetime” fluid is accurate, but not precisely what customers want. Customers hate to spend money on fluid services, but then they're surprised when the equipment that fluid protects fails and costs them even more money. $300 every 30,000 miles or $8,000 every 200,000 miles, if it makes it that long, just doesn't add up.

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Timothy Owner/Technician
Wisconsin
Timothy
 

Another way to look at ‘lifetime’ warranties: what lifetime? Your life, my life, the life of the fluid, the vehicle, what?! Essentially a marketing term, that means absolutely nothing. If the part lasts five minutes, that was its ‘life’!

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Benjamin Diagnostician
New Jersey
Benjamin Resolution
   

So I went with Dan's and David's recommendation to check the input shaft speed. The car was blocked in so I didn't take it for a test drive, but a stall test snapshot was enough to confirm an internal transmission fault. In the screenshot, I wrote in where I did the stall tests, and you can see the slightest 50 rpm bump in the “D” test, and then pretty much freewheeling in the last “R”…

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