I thought this was an interesting behind-the-scenes view of Toyota's response to determine why the front differential gave out on this new Tacoma. In short, they claim everything was manufactured and assembled properly but that a combination of factors including the low-end engine torque, being in 4-Low, front-rear torque distribution, the very slippery surface, and ultimately traction control braking was sufficient to exceed the limits they placed on the front differential. The fix will be in the software that was already developed for hybrid models.
2024 Tacoma Front Differential Failure
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AZjeff
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In other words the front diff was designed too light for the max available power and Toyota counted on programming to protect it.
That is true, but in that situation, it shouldn't have broken at all IMO. Plus aren't Toyota's supposed to be overbuilt?
Yeah right.
I'd rather have that issue then a weak built 4WD system.
The lead engineer claimed it would be a rare instance that would break it as is (and TFL just happened to stumble upon that instance) but that the programming should do the trick even in the rare cases. What I didn't like is I think he said all the above factors exceeded maximum spec limit by something like 1.45% and that doesn't seem like a lot of leeway there. This doesn't make me feel better about them removing the transmission cooler from my Tundra if that's how little room they leave between maximum spec and failure.
IDK...don't make up like they used to?
Slippery surface should not be factor in this failure.
Indeed.
The Tacoma in question was all-stock, on the as-delivered OEM tires.
As a long time Toyota fan who gained an appreciation for them when I was a Toyota Master Tech, I am disappointed. This has the same drivetrain as the upcoming 2025 6th gen 4Runner, which I had planned to consider buying to replace our 5th gen.
If the failure limit of the A.D.D. is so easily reachable and exceeded on a bone stock truck, imagine how fragile it will be on bigger tires when on a tough 4wd trail. One of the appealing things historically about Toyota 4WD trucks and SUV's, is how durable and reliable they were, even in modified form.
Without strengthening actual metal parts, and band-aiding with a 'Fix' via the software to make the traction control system less effective, I suspect this scenario will be a recurring theme.
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A slippery surface should be less stress on the drive train. Sounds undersized like the rears on the early Nissan titan. If you bounce, all bets are off.
Put a heavy dose torque management on it like they do on the class 2 and larger.
Put a heavy dose torque management on it like they do on the class 2 and larger.
I appreciate Toyota’s root cause analysis and response here. It is very telling.
The investigation into the root cause (of the broken differential) has absolved their manufacturing process of having made a defect (i.e. they made the differential correctly). This means the design itself has failed, (Toyota designed the thing wrong, but manufactured it correctly per current design specifications and manufacturing process).
If everything met specification, yet the product failed (destroyed itself, not just missed target performance) in the field against foreseeable noise (use environments) factors, then the design and marketing teams failed to understand customer requirements at the design input phase; and the team created an insufficient product design accordingly.
The differential needs to be stronger, not just limited by software.
One of Toyota’s past strengths in engineering has always been controlling costs by designing products to be as strong as needed, but no more. To do that solid and clear customer requirements as design inputs were researched, and specifications designed to reliably meet the customer requirements (desired target performance, not just meet “specifications”). Frills, doodads and gewgaws, were always secondary to robustness of the design against all foreseeable requirements. Not anymore, apparently. Toyota seems to me to have failed at the very start of their product design project for this truck. I suspect to meet cost targets in a highly competitive market, something had to give and here we are.
I think Genichi Taguchi would be disappointed.
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It wasn't the "Differential" (Ie: the ring and pinion or spider gears) that failed. It was the sleeve that engages and disengages drive (which IS designed as a fusible link) in the A.D.D. (Automatic Disconnecting Differential) portion of the assembly. That part (and maybe more) are under-designed for the torque capable of being produced.
The Engineer basically said that the traction control system applied the brake to the individual spinning wheels so well, that the amount of torque sent to one wheel through the A.D.D. engagement sleeve, exceeded it's torque capacity, and the sleeve failed. So the fix is to limit how hard each brake can be applied, thus limiting the potential of excessive torque going to one wheel. Which also limits how effective the traction control will be.
I'd personally like to see them strengthen the parts to comfortably handle the torque load in the drivetrain. Of course that would add weight, and potentially negatively effect fuel economy...
The Engineer basically said that the traction control system applied the brake to the individual spinning wheels so well, that the amount of torque sent to one wheel through the A.D.D. engagement sleeve, exceeded it's torque capacity, and the sleeve failed. So the fix is to limit how hard each brake can be applied, thus limiting the potential of excessive torque going to one wheel. Which also limits how effective the traction control will be.
I'd personally like to see them strengthen the parts to comfortably handle the torque load in the drivetrain. Of course that would add weight, and potentially negatively effect fuel economy...
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The Toyota TRAC and ATRAC brake the slipping wheel on both axles for a suedo LSD.
On my Tacoma, a 23, it is actually quite effective. The Tacoma will spin all 4 tires, Being a manual, i just let the tires spin continuously and slowly by not disengaging the clutch. It will walk through anything so far. You have these guys that drive an automatic that are on-off-on-off-on the throttle and thats not how the system works. Slow steady throttle and you're out. THIS, being the ADD, I think it was at the torque rating with the on-off-on throttle action. My duallies will only spin 3 tires and ONLY because I got the LSD through the FX4 packages. Most standard "4x4" Fords are open front and open rear.
On my Tacoma, a 23, it is actually quite effective. The Tacoma will spin all 4 tires, Being a manual, i just let the tires spin continuously and slowly by not disengaging the clutch. It will walk through anything so far. You have these guys that drive an automatic that are on-off-on-off-on the throttle and thats not how the system works. Slow steady throttle and you're out. THIS, being the ADD, I think it was at the torque rating with the on-off-on throttle action. My duallies will only spin 3 tires and ONLY because I got the LSD through the FX4 packages. Most standard "4x4" Fords are open front and open rear.
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Very disappointing for a truck known for its legendary durability.
Great post.
Thanks for correcting me and highlighting the A.D.D. purpose, function and design.
Appreciate the fuse-able link analogy. Back the days of manual transmissions and early Audi Quattro, the clutch was the fuse, sparing the rest of the drivetrain hard parts. It makes sense the A.D.D. would serve a similar purpose here, by design. I agree with your assessment it should strengthened.
Your mention of fuel economy highlights that regulators cause design constraints beyond those of the customer/end user.
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