Trans “disengages” from engine on highway

Truck has been at the dealership for almost 7 weeks now (returned to me “fixed” 3 times with same major issue happening the first time I drive it, 2 different dealerships now) with zero fixes or answers, hopefully someone else has had this happen and can shed some light.

2017 Tacoma TRD, 3.5, auto, 100k miles

75-78 mph with cruise on, drive for almost 10 miles when the cruise turns off, check engine light comes on, and engine disengages from trans. Push gas pedal and rpms increase, but speed continues to decreases until it coasts to a stop. Shift to park, turn engine off, back on, CEL turns off and truck drives normal for a random amount of time until same issue happens. While slowing to a stop, I’ve tried shifting it into neutral and back into gear with no change.

Any constructive help is appreciated!

 

First I have ever heard of this. Tuning in to see what comes of it.

 

Sounds similar to what I experienced twice in friends trucks (Ford and ram) I've been in... your trans is slipping once it gets up to temp.

 

Has either dealer reported checking the trans fluid level? Or its condition?

Not saying that's it, just to start simple.

Next question. If you do not use cruise, but replicate driving otherwise, does it still happen?

Have they provided the codes that were set? Even though the light goes out, the code history should be there.

@ 100k I assume you are out of warranty? You need to be looking for a 1st rate independent trans shop in your area. Not a franchise like Aamco, but a true independent.

Please let us know the resolution.

 

Welcome to the forums.
Grab all your paperwork and call coorporate and open up a ticket.
That sounds very dangerous.

 

Actually this is an electronic/valve type issue. Not "slipping" just because its warm. There is something keeping fluid pressure from getting where it needs to. I can't believe dealership can't figure it out.

Simple question, has the fluid ever been changed? Mine is brand new i haven't looked at maintenance schedule yet.

Next, need a scan tool for this, but need to be able to read all codes when the light comes on. Once you shut the truck off the computers "reset" and thats why you can drive again. This doesn't always store the codes. So you need the codes while the light is on, preferably right as the mess is happening. That will tell what system is going faulty, toyota service manuals would have extensive information on causes, how to sort out and how to fix. Sounds more like whoever at your dealership is working on it is lazy or dumb.

 

What's the code for?

 

Trans ECU probably done for.

 

Yup

Transmission goes into fail safe to prevent further damage .
As mentioned above , scan for codes
Do you still have warranty at 100K
If no warranty , stay as far away from the Stealership as you can .

Not the same transmission , but another example of how Toyota,s computer controlled transmissions operate . Yours is wired very similar
https://gearsmagazine.com/magazine/dropping-out-of-5th-gear-yes-theres-an-app-for-that/

 

Sounds like a faulty trans control module to me. But I dont know how Toyota does that on these trucks, whether they have a separate CM or if its built into another.

 

A little more background on the truck...

Warranty was up at 100k miles, so naturally problems occurred just after it rolled over.

It started when the fan clutch blew apart as I turned on my street. Looked and sounded like a bomb went off in the engine compartment. Took out radiator and overflow, water pump, power steering reservoir, any and all plastic covers and shrouds it could find, as well a few electrical connectors and the trans cooler lines. All components replaced, trans flushed and filled, oil change, diff and transfer case service, spark plugs, 100k service, etc. Can NOT confirm services were actually performed even though I was charged for them. Have proof/pictures oil was not changed, maybe just topped off at best, rotate was not completed (LF tire covered in oil before and after), coil overs still covered in dirt/dust, etc. Immediately took back after getting it home and smelling oil burning and fluid drips in driveway (didn’t even hose it off) to have services redone (Different dealerships issue for another day). I did unscrew the trans inspection plug, and had what I consider an excessive amount of fluid come out (no, it wasn’t the drain plug), prior to having it towed back, after it acted up on the highway the first time.
I do not know what the code/codes are. Dealer has not provided any paperwork for repairs, nor have they charged me for any work performed or parts thrown at it since ( good faith or admission of responsibility?) This issue started after all the work was completed. It is currently still at second dealership.
My background is a degree in automotive technology, and was a Ford tech years ago. Tons of side work, performance builds, quad and motorcycle work, etc, so I have a pretty solid knowledge/ understanding of vehicles and do most of my own work, however, this is my work truck which is why it goes to dealer.
That being said, there are only so many things that could cause this I would think. My first guess would be inline with most of you, trans control unit/ module, or whatever Toyota calls it, putting it in safe mode. Causes could be fluid, valve, sensor related, but code(s) should indicate that. Torque converter, clutches, low fluid, would be more like slipping, but that’s not the case. I have not replicated without cruise on, my drive is 50 miles each way, all highway and after each incident, I parked it to avoid damage.
Sorry for the book, I’m seriously confused and frustrated that 2 different dealerships and multiple techs including shop foremen, can’t figure this out. It seems pretty upfront, should be able to be replicated with scanner hooked up, and repaired.
Thanks for all the responses so far!

 

@gearcruncher would too full cause and fail safe?

 

When you checked the trans fluid , was the engine running ?

 

Very possible

 

Maybe a transmission shop could help

 

With it being such an intermittent problem, this smells like a control problem to me. The 4L60E in my chevy did a very similar act after a transmission flush and the valve body caught a bunch of junk, it would just drop out of gear in the same way. New valve body fixed it and never had an issue with it, sold it over 100k miles later.

I think the weird thing is that it started after the fan clutch went. Makes me wonder if maybe in that time or the cleanup afterward some gunk got swept into the transmission somewhere.

 

I fuckin hate the 4L60E haha. I feel like they haphazardly just through oil in the trans. But who knows

 

Yes, truck running, at temp- D on, not blinking.
Loosened plug, pulled it to the side briefly, full stream of fluid like you would expect from drain, stuck plug back in quickly and tightened. Expected a trickle

 

Lol. I never really had a problem with it. It had a corvette servo in it so it shifted more solidly, but other than that it was a stocker.



That said, my next vehicle was an MT I was driving 6.0l chevys at work with NV4500's and having a blast, so driving the auto felt sad.

 

I went through 3 or 4 in my Silverado