Overdrive Conditions

SBUBandit

Original poster
Member
Dec 5, 2011
597
After a recent fuel fillup that calculated out to 12 mpg (whoops, too much playing in the snow), I decided to try an experiment with the TB, and have been driving it the last few days based on the tach readings, and trying to keep it cruising along at 1500 rpm, trying to never allow it to go above 2K when accelerating. My question is what parameters must be met before the computer tells the trans to go to overdrive? Seems like when it will shift to overdrive might depend on whether the engine/trans is warmed up? Just curious if anyone knows exactly what the computer is looking for. Thanks.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Overdrive is 4th gear. It should always be capable of being in 4th gear. The final RPM reduction if you're gentle with the torque is the lockup clutch engaging, otherwise known as the TCC. I didn't locate the exact conditions in the shop manual in a short search just now. Sorry.
 

Boog2006

Member
Dec 3, 2011
151
Interested in your results. I did the same test on mine before the lift and improved from 18mpg to 20 mpg driving below 60.

Now 16 is a good day.
 

Sparky

Member
Dec 4, 2011
12,927
I still can't figure out how you can accelerate at any reasonable pace while staying below 2000 RPM. Especially when trying to merge on the highway. I easily hit 2500 normally, and 3000 when doing a highway merge on an onramp (especially uphill). I'll open it wide on occasion because never doing so can be bad long-term.
 

SBUBandit

Original poster
Member
Dec 5, 2011
597
Sparky said:
I still can't figure out how you can accelerate at any reasonable pace while staying below 2000 RPM. Especially when trying to merge on the highway. I easily hit 2500 normally, and 3000 when doing a highway merge on an onramp (especially uphill). I'll open it wide on occasion because never doing so can be bad long-term.

On the highway you probably can't. I live fairly rural, and I've been planning my route to and from work accordingly to be sure I have room to accelerate easy. Also helps if you can plan it to use a downhill to help get up to speed.

Thanks for the info Roadie, I guess the TCC is what I was refering to. Once that locks up I can cruise at 1500 rpm at 50+mph. Never realized that's what the RPM drop was, although I have found in the past merging on the highway I sometimes I have to put it in 3rd to avoid it, just because it kills power unless you punch it to downshift anyways.

FWIW I just googled Trailblazer and TCC. What are the chances that Tarrant County College's team name is the Trailblazers?
 

eutechnyx

Member
Mar 31, 2012
375
Sparky said:
I still can't figure out how you can accelerate at any reasonable pace while staying below 2000 RPM. Especially when trying to merge on the highway. I easily hit 2500 normally, and 3000 when doing a highway merge on an onramp (especially uphill). I'll open it wide on occasion because never doing so can be bad long-term.


Sorry to thread jack but I NEVER WOT my truck I drive it like a grandpa,Ive always heard getting WOT helps clear carbon but is this true that never WOT is bad long term?
 

SBUBandit

Original poster
Member
Dec 5, 2011
597
Well, doing a little reading and getting my learn on, and found an interesting site. This is intended for older Vettes, but explains how the 4l60e TCC works and determines when to lock up. About halfway down this page there is a nice graph that shows it is a combination of throttle position %, gear, and speed.

How To: 4L60/65E trans tuning. Shifting/TCC/TM (w/ Pics) - LS1GTO.com Forums
 

Sparky

Member
Dec 4, 2011
12,927
eutechnyx said:
Sorry to thread jack but I NEVER WOT my truck I drive it like a grandpa,Ive always heard getting WOT helps clear carbon but is this true that never WOT is bad long term?

Doesn't even really have to be WOT (and frankly the stock PCM probably won't let us anyway). But yeah, opening it up can help. Maybe not as much as the old carb engines however. We got my great grandma's car years ago and it would barely do 55 at first lol.
 

RayVoy

Member
Nov 20, 2011
939
SBUBandit said:
Well, doing a little reading and getting my learn on, and found an interesting site. This is intended for older Vettes, but explains how the 4l60e TCC works and determines when to lock up. About halfway down this page there is a nice graph that shows it is a combination of throttle position %, gear, and speed.

How To: 4L60/65E trans tuning. Shifting/TCC/TM (w/ Pics) - LS1GTO.com Forums

Keep in mind, the above link is to help folks understand the questions that are asked when requesting a "tune" from a 3rd party supplier. The link, may, or may not, correctly describe the factory operation of the 4L60 trans.

The overdrive/converter lockup is pretty straight forward for the factory 4L60e.

The PCM will step the transmission up the gears (1st to 4th) as the road speed and engine speed increase. The only way the driver can stop this process is to move the shift lever to a lower gear number, such as 3. The trans will not shift into 4th if the shift lever is at gear 3.

The torque converter clutch locks in 4th gear (when D is selected) when the engine reaches a certain temperature. and power is not being called for (as in passing, or climbing a steep hill). The above is also true when 3rd gear is selected on the shift console.

In other words, if the engine is warm, and you are not asking for passing, or climbing, power, the converter clutch is locked.
 

SBUBandit

Original poster
Member
Dec 5, 2011
597
RayVoy said:
Keep in mind, the above link is to help folks understand the questions that are asked when requesting a "tune" from a 3rd party supplier. The link, may, or may not, correctly describe the factory operation of the 4L60 trans.

You are correct, however one of the charts does show the factory setting, albeit, as i mentioned, that forum is for vette owners

Your description of how the TCC works does seem to correspond nicely with what I'm seeing. On cold starts, it takes a while for it to lockup, even at 50+ mph.
 

Sparky

Member
Dec 4, 2011
12,927
I think the TCC lock takes transmission temperature into account also.
 

Wex

Member
Dec 4, 2011
124
RayVoy said:
In other words, if the engine is warm, and you are not asking for passing, or climbing, power, the converter clutch is locked.

I think the TCC also unlocks if the brake pedal is tapped. My memory is not 100% on that but I think it's true. I have not had much sleep this week. :smile:
 

Mark20

Member
Dec 6, 2011
1,630
I also remember tapping the brake and watching for the RPMs to increase (?) was a quick way to test the TCC.
 

willn513

Member
Dec 4, 2011
918
'Blowing out the carbon' isn't really something that is necessary on these modern vehicles. I can remember on my dads 1984 Buick Riviera having to open it up every couple months or so to do this- TONS of smoke from that thing. Newer cars just really don't have a need for it anymore.
 

Ed H

Member
Oct 18, 2012
167
The TCC is definitely temperature related. Ever notice how long it takes to shift when the outside temp is brutally cold?

Sorry to thread jack but I NEVER WOT my truck I drive it like a grandpa,Ive always heard getting WOT helps clear carbon but is this true that never WOT is bad long term?

I believe there are numerous benefits of high rpm blasts - clearing carbon (minimal) plus ring lands, oil pump, PCM mapping / learning, and on ours TBs the CPAS, Phaser, etc, etc.

I remember a thread on here (or the OS) where a mechanic explained he would take cars (i.e. Buicks / Caddys) from blue hairs out for 10-15 minutes of punishment if they were brought in for "running rough". He would do that before diagnosing anything. He advised the blasts cured many issues by themselves. I don't think he advised what he charged for the service though, LOL.

I am a firm believer in a couple high rpm blasts (5000+) at least once per week. I have done this on ALL of my cars, weather permitting.
 

SBUBandit

Original poster
Member
Dec 5, 2011
597
After going super easy all week, I managed to only go over 2000 rpm once or twice, and even then only to 2100-2200. I also did go out on the expressway once and was able to keep rpm down then too, but there was no one right behind me when merging and I was able to use a pretty long downhill on-ramp to get a good start anyways. After all that, she got 16.3 mpg. Compared to its lifetime avg of 17.4 mpg, that still sucks, but I'll try it again in the summer after we get rid of the nonsense winter gas. Really wish there was a ethanol free station near us.
 

SBUBandit

Original poster
Member
Dec 5, 2011
597
Put 1500 miles on the trailblazer this week, from Michigan to Tennessee, around the mountains, took it up to 6600 feet, then back to Michigan. 21 mpg there, 16 while there, 22 mpg back
 

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