Odd occurrence while driving this morning

As Supra TT said... his TAC drops into 5th as he start going down hill.
Which I would assume is normal operation.
I understand if i tap the brakes it could downshift... but at some point if I just start rolling after that it should drop into 5th on its own.

is there docs from toyota that anyone knows of that describe this function?

 

As stated above the auto tranny does downshift when going downhill. It's more noticeable in cruise but I've felt it in manual throttle operation as well. It's normal for this transmission. My theory is it's a downshift to keep the truck from accelerating too much down big hills, giving you some engine braking to help maintain a steady speed. Also, the injectors shut off in this scenario saving a few pennies in gas.

 

This is actually getting interesting.
SO if I understand correctly - There is a system in my ride that tries to decide my RPMs for purposes of fuel economy and/or emissions control.

So before I can research this system I need to know whats it is officially called and what devices/sensors are being considered. Also is there a notion of acceptable variance from car to car? is it user "tuned" ?

I'm off to goolge. Wish me luck.

 

From:

http://tacodoc.com/?page_id=732

"The 1GR-FE engine is a 4.0-liter, 24-valve DOHC V6 engine. This engine uses the VVT-i (Variable Valve Timing-intelligent) system, DIS (Direct Ignition System), ACIS (Acoustic Control Induction System), and ETCS-i (Electronic Throttle Control System-intelligent). These control functions achieve improved engine performance, fuel economy, and clean emissions."

So it looks like this ETCS-i is our guy...

 

So is the consensus here that a toyota dealer would say there is nothing wrong?

 

Unless the torque converter clutch is engaged, and all the over-run clutches are secured, the wheels cannot "drive" the engine. Just a little FYI.

 

There is a difference though between engine braking and engine driving from the rear. With the TC lockup NOT engaged, and the drive wheels "driving" the transmission, the torque converter will load up, but is not capable of actually "driving" the engine. It will load up because of the way fluid moves inside the torque converter, and this causes the traditional "engine braking" that is talked about. But it will not actually DRIVE the engine itself. It's kinda hard to explain without trying to get into detail about how automatic transmissions work in detail.

As for the DFCO operation, there is a graph that indicates the RPM range for fuel cut off. Normally the lowest RPM is around 1200. Other factors though have relevance like which transmission (Auto and manual transmissions have a different logorythm) as well as speed, cruise engaged or not, brake pedal engaged, AC clutch engaged, etc... All of these are input factors on the DFCO. I am not privy to the specifications, but it was discussed in another thread.

 

Mine did this once before when the truck was still in warranty. Mine however was combined with no overdrive, and no reverse.

I would be driving and then it would feel like the driveshaft was disconnected... like the engine was just free. Then all of a sudden SLAM and it would start pulling again.


In my case the valve body in the transmission had to be replaced. Hope yours is something simple and covered by warranty!

 

Yeah. These days there is still a limited amount of information given to a tech who is trying to diagnose problems. That is part of the reason there are 500+ diagnostic codes these days. We can't figure everything out with just a book and a multimeter anymore. It is beginning to get to the point where to be a good tech you need to dang near have an engineering degree! I have been doing this for over 20 years now, and I STILL take about 50 to 60 online classes and training webinars each year to try and keep up!

There are two vehicle owners that I find hilarious over everyone else:
1. Those who think that we simply "plug in that computer there and it tells you what is wrong with it, right?"
2. Those who bitch because we charge $80-$90 per hour to diagnose and repair their vehicle, but will pay a doctor $500 per hour without complaint, even when the doc is wrong and charges you ANOTHER $500 per hour to go back over your symptoms and find a problem.

Years back I remember reading an article that actually compared a doctor to an automotive tech. Two things were key that I remembered the most. First is that the human body has changed less than .5% over the past 100 years, yet auto technology goes through thousands of changes every other year. The other is that the article stated something like a doctor has about 500,000 pages of printed text that they have to reference during their careers, but an automotive tech has over 5 million pages that they will reference of the course of THEIR career.

 

My take on it: the truck will down shift when it senses you are going downhill and need to brake for an extended amount out time, it will also hold gear after ascending a hill. Either of these times it will continue to hold these low gears longer than it really needs to, seemingly causing the engine to surge unnecessarily. IMO it's doing this to prevent the trans from hunting gears if that low gear is suddenly needed again.

 

I'm sorry... this cant be a feature.
Specially when when we have cases of TACs operating " normally".
If I'm going up a mountain towards the crest and I'm in 3rd gear @3K rpms the truck is NOT supposed to stay @3K rpms once I go over the crest.
I'm sorry but every auto trans car i ever drove would just drop into OD and let me coast downhill.
Whatever this ETCS-i system is it's crap.