A Dr. Bob sized post...
from an engineering perspective, based on reading dozens of cases here and Honda service manual and TSB's:
Once upon a time Honda engineers initially designed a conservative, controlled, low RPM CONTINUOUS slippage system to mitigate rough torque pulses when VCM was engaged (3 cy mode), instead of exclusively relying on temperature fussy and short lived fluid-filled, electronically controlled engine mounts, like previous Gen. And I'm guessing under low torque situations (cruising, light loads...) they were right that MM-prompted ~30k fluid change intervals, and the TC clutches would last for the entire service life (150k mi?). Somewhere in design and endurance testing, someone thought they could further mitigate the roughness VCM caused under higher torque
demands--and someone else wanted to eke out a few more tenths in MPG-- where the 3 cyl mode torque pulses were more vigorous and annoying. They (guessing) decided that they could allow the TC slip mode to extend to these higher torque scenarios, Heavier loads, higher cruising speeds,moderate hills, and moderate
have fit in that criteria. Theses changes were made (guessing) late in development, and with a manufacturing deadline looming they punted the rigorous endurance testing for the extended scenarios, and instead relied on cursory testing and computer modeling predictions.
And then they went to production.....There weren't statistically significant numbers of failures immediately in production years 2016, 2017, and by the time the TC failures became obvious through service data, several hundred thousand Pilots were in the wild, and Tesla-like over-air updates weren't available.
So Honda issues a manually applied software update to (presumably) narrow the torque range TC slippage is allowed to occur, at the expense of some NVH. The update may also may have narrowed the VCM operational range, but that would impact published MPG. Honda of course recommended to replace the abused fluid. Their hope of course is that the update is applied to enough Pilots to keep them running without error codes past drivetrain warranty. Or, more generously, that they caught enough Pilots earlier in their service life, to have addressed the issue before significant TC clutch damage had occurred.
It's also un-clear whether the late 2017 TSB for the update totally
solved the TC issue for subsequent model years. Seems to have helped--only Honda would know through service data-- but still some reported TC issues through 2020 MY.
My '20 EX-L didn't appear to have abused it's DW-1 in 11k miles with a previous owner, and as I indicated elsewhere, has been perfect since then, by just disabling VCM.
In Noxema's case, 50k running with VCM enabled slowly beat the TC to death, it seems. Towing with the TC slippage strategy accelerated the kill. Fluid frequency, fluid type, an external cooler, I am convinced, would not have, did not, individually or collectively save it. The software update might have prolonged death, but likely the TC fatal damage was already done long before 50k.
The transmission cooler made no difference, I think, because the entire fluid volume (base oil) was not getting overheated
, but rather the friction modifiers were being destroyed, which may have fatally change how the PCM makes adjustments to TC clutch pressure to maintain a target slip RPM, which exacerbates the fluid deterioration, until the PCM can no longer achieve target slip (i.e. the 200 rpm bobbing of the tach people observe).
Looking a bit deeper into role the friction modifiers used in trans fluid--the initial hints from my use of Lubeguard years ago-- the modifiers are (mostly) designed for a specific transmission and PCM, to control the transition of torque in the last few percent (50 rpm or less differential ) of clutch engagement, and with an PCM commanded torque reduction at that point, to yield smooth, seamless shifts.
Also it dawned on me that VML, with its empirically mentioned lower friction modifiers, might
actually lead to ACCELERATED failure in the 6-speed torque converter
as it might make that designed
low rpm differential clutch slippage condition generate more localized clutch face shear damage (removal of material) and destruction of the critical friction modifiers.
VML, with its lower friction modifiers, of course would make for firmer shifts in the transmission proper
, as those interior clutches are designed to spend their time either disengaged or rapidly transitioning towards 100% engaged, with a fraction of second spent in controlled slippage/shear. In other words, firmer shifts are the only consequence of VML for the interior parts.
Of course no one (that I am aware of) has done a single variable test
on the 6-speed, by ONLY changing out DW-1 for VML and measuring fluid condition, shift performance and TC performance for a statistically significant mileage (say 15k miles or more). Even Grease (as I recall) documents somewhere in an older thread, that his Crosstour TC slip w/ VCM enabled was less but still excessive, with VML alone
? How many miles before he disabled VCM...and added a cooler and flushed in fresh VML?
I harp on this topic because we CAN offer to the casual DIY an apparently very successful, and cost effective solution for these 6-speeds, just by disabling VCM and refreshing abused DW-1, without violating their conscious or the unknown long term results of using non-OEM fluids (pick yer product). Instead, we burden the casual DIY'er with highly anecdotal benefits of VML and the complication and expense of (probably paying someone else to install) an external cooler.