Transmission surging descending in 4Low

Hello Tw, I was wondering if any v6 AT owners have this problem: Transmission will "surge" or coast quickly then slowly when descending in 4lo, 1st or 2nd gear down a hill while offroading. it will feel like it is engine braking and the rpms will go up in sync with vheicle speed then it feels as if the torque converter is locking back up harder than it was then the rpms will die down. it will continue to do this unless i lightly tap the brakes (not even enough to slow or stop the truck on its own) so im convinced it has to do with the computer controlling the transmission lockup. anyone have this 'issue'? or is this normal

 

My truck does the same thing except I have a manual. I believe that what is happening is the slope isn't steep enough to keep the engine spinning on its own. Thus, when the RPM drop to a certain point the ECU turns on the fuel injectors which causes the increase in speed. Once the RPM get high enough the fuel shuts back off resulting in the decrease.

 

interesting, im a little confused though 1st gear at brisk walking pace in 4lo should be enough of a ratio to be well above engine idle. toyota should bring back cable throttle, would help with the chip shortage lol.

 

I've never experienced this myself, but I've heard others report that it is caused by the fact that the engine shuts off the fuel supply when engine braking but turns in back on if the RPM drops too low. I might not notice this because I tend to use the brake in tandem with engine braking,

 

I'm definitely not an expert maybe someone else will tune in with a better idea. My understanding of engine breaking is that the intake is closed and fuel is shut off creating a vacuum in the cylinders as the engine spins. Thus, the only thing spinning the engine is gravity as it pulls the vehicle down a hill.

If the breaking provided by the engine vacuum exceeds the force of gravity then the RPM will continue to drop in theory until the truck comes to a complete stop, essentially shutting off the truck. As we don't want the truck to stop the computer gives the truck some throttle creating the surge.

 

Everything you need to know about the Toyota Tacoma Transmissions.......

https://www.youtube.com/watch?v=qbCAEscrJ8E

 

Went off-roading yesterday again and experienced it again, anyone with kdmax or a tune feel this was solved? My buddy with a gladiator had superior control in 4lo and didn’t have to touch throttle or brakes during an entire descent with moguls, i was quite envious lol.

i know we have crawl control but to just have pure gearing and engine take care of a descent is cooler.

 

“Cooler”

All this for “cooler”

Next!!

 

Well if i were to use more correct vocabulary, “better” would be best to describe it.

 

Get 5.29s and/or crawlbox and you'll never have to worry about it again

 

@tonered might know if you can eliminate this feature with a tune?

 

I think you are feeling DFCO in action.

 

When you take you foot off of the pedal, the engine is literally using no fuel. So the combination of 4lo with the lower gear ratio, and little to no fuel causes the surge. Engine braking and zero fuel imputes is what you're experiencing.

 

Yeah but if i were to leave it on a longer constant hill (which i have done before) it will surge then come back down and repeat this cycle unless intervene

 

Use it cuz it fkn works.

 

If I'm in low range I manually use 1st gear only. I've never experienced that. Mostly because I've never downshifted from a higher gear to 1st when in low range. If I don't need low range 1st gear, I stay in high range. 1st gear, high range is about the same as 2nd gear low range.

But as long as I've owned my truck there is a delay when I downshift to 1st gear when in high range. My truck is an automatic, but in mountains I always manually downshift to the best gear for conditions for engine braking. Regardless of the gear when I drop down to the next lower gear there is a split second where the truck seems to go to neutral before downshifting. It is most apparent when going from 2nd to 1st.

It was a little disconcerting the first few times it happened, but I'm used to it and expect it. It's been doing it for over 15 years and has never caused a problem. I consider it a quirk of the Toyota transmission. Neither good nor bad, just different than what I was used to.

 

It’s DFCO.

 

Fact.

 

Happy new year!