Electronic Torque Converter locking and unlocking in OD frequently while towing

[mike_cox]

With a serious case of cabin fever, I have been pondering this situation. Last year I towed in 3rd gear at 60mph to allow the TC to stay locked up almost all the time while using cruise control (mostly MI flatland). When I towed in 4th gear (OD) at 60 or 65, the engine would bog on light hills and unlock the TC to maintain speed in cruise control and then lock it back up once the target speed was reached. This would happen very often. I have heard that frequent locking and unlocking of the TC can lead to excessive heat build up. Is this true?

THE BACKGROUND:

I have a 1996 S-10 blazer with 135k and ~35k on the rebuilt transmission, has 3.42 gear and heavy tow package (heavy for S-10's ) Truck is rated for 5,300 to 5,500lbs depending where you look. I do not have a trans temp gauge or an external trans cooler. I would like both, but the budget won't allow right now. I tow a pop up with a GVWR of ~2,400lbs. I can say we usually have it loaded near that (2,200+lbs most of the time).

The idea:

I tow at 60mph in 3rd gear, but would like to tow at 65 in 4th. Even with the TC unlocked in 4th vs locked in 3rd, I wouild still be turning less rpms at 65 in 4th. Move with traffic better and use less gas right?

The solution:

I have contemplated installing a switch or relay (depending on if the TC needs 12v to unlock or just a broken circuit) on the TC lockup wire to force the TC to stay unlocked while in 4th gear.

Question:

Does "excessive" TC locking and unlocking heat up the trans fluid? If so, is this remedy a good idea in theory? Will this allow my transmission to run cooler? Will there be any other forseeable side affects?

Thanks for all your input!

Mike

 

[Carl L]

I have heard that frequent locking and unlocking of the TC can lead to excessive heat build up. Is this true?

Yes.  No only that but the thing wastes fuel sloshing all that fluid around to heat it up.

A slush box can run into trouble  even on the flats if your road is windy and requires frequent shifting between direct- and over-drive.  In fact, we had our unit overheat and slip into safety mode hard shifting on a level but twisty road leading into Sedona AZ.  The truck was NOT towing at the time.

The cure is to lock OD out and stay in direct drive (3rd).   I would not tinker with the tranny -- that is the road to misery.

 

[John From Detroit]

In another forum people often ask about using Overdrive under assorted conditions.. This is what I tell them:

If the transmission is constantly shifting in and out of O/D then TURN it off.. If it picks a gear and for the most part stays there Leave it on    (Example of the latter,, If I come home via US 23 as I get ready to ramp over to I-96 it will downshift two times, and once I'm over the hill it will upshift 2 times  it's a 70 mile trip.. I run it with O/D on.

Now if it shifted say every mile or even 5 miles, I'd consider turning it off

But one set of shifts per 70 mile trip (not counting slowing for stop lights and such) I don't turn it on

So if the Torque conveter is switching between locked and unlocked all the time when you tow in overdrive

TURN OFF OVER DRIVE

 

[mike_cox]

So if the Torque conveter is switching between locked and unlocked all the time when you tow in overdrive

TURN OFF OVER DRIVE

No need to shout, sheesh...   

To clarify, the trans very rarely downshifts to 3rd when at cruising speed while towing in OD- it just locks and unlocks the TC a lot to assist in maintaining the cruise control speed.

By the logic that you're using -----> If I disabled the locking of the TC when in OD (mind you the trans very rarely downshifts to 3rd), then the TC would cease locking and unlocking all the time and it would then be safe to tow in OD?

Carl - I appreciate and respect your input - unfortunately I'm one of those who can never leave well enough alone...point taken though.

 

[Carl L]

unfortunately I'm one of those who can never leave well enough alone...

Well, I suppose even transmission shops gotta make a living and the economy does need stimulating.  ;D

Just a thought.  Wasn't one of the reason for locking torque conventers to eliminate the penalty paid in fuel mileage and tranny maintenance by old fashioned slush boxes?

 

[John From Detroit]

No need to shout, sheesh...    

To clarify, the trans very rarely downshifts to 3rd when at cruising speed while towing in OD- it just locks and unlocks the TC a lot to assist in maintaining the cruise control speed.

By the logic that you're using -----> If I disabled the locking of the TC when in OD (mind you the trans very rarely downshifts to 3rd), then the TC would cease locking and unlocking all the time and it would then be safe to tow in OD?

Carl - I appreciate and respect your input - unfortunately I'm one of those who can never leave well enough alone...point taken though.

Well, just so you know.. It's like the old joke Patient: "Doctor it hurts when I do tihs"  Doctor: "well then don't do that"

Actually I'd rather turn off the O/D and let the torque converter remain locked, several reasons for this, #1 being that it's better for the transmission,  Less heat, Less wear

 

[herd]

By turning off the Overdrive, is that the same as engaing "Tow/Haul" or is it necessary to downshift from "D" to "3"?"

 

[Gary RV_Wizard]

A switch to force lock-up is asking for trouble, in my opinion. As far as I know the tranny can't downshift when locked up and it is possible to stall the engine if you leave it locked up when it needs to use the torque converter to multiple torque or needs to shift to a lower gear. Too easy to forget you have the lock-up switch engaged when you come to a small grade or some traffic forces you too slow down and then accelerate. A lock-up switch is handy for going downhill, though! It increases engine drag.

 

[Gary RV_Wizard]

By turning off the Overdrive, is that the same as engaing "Tow/Haul" or is it necessary to downshift from "D" to "3"?"

Dropping out of OD is NOT the same as Tow/haul, though Tow/Haul mode will have an effect on when the OD engages. Tow/haul still allows OD to engage, but alters the RPM range for each of the shift points.

It is generally not necessary to shift to a gear other than "D" unless the tranny is shifting frequently ("seeking"). The tranny will usually find its own most efficient gear, but sometimes gets fooled when road/traffic conditions change constantly. That's when you override it by selecting a gear.

 

[mike_cox]

A switch to force lock-up is asking for trouble, in my opinion.

I was actually thinking of adding a switch (or relay) to defeat the lockup and force the TC to stay unlocked, thereby keeping the RPMs up while in OD enough so that the engine would be operating at an RPM with more torque available ---- but mostly just to stop the TC from locking and unlocking all the time while in OD.

In light of the comments by John and Carl, I'm thinking that the transsmission would be much happier to operate in 3rd gear with the TC locked up (it seems to never need to unlock the TC to maintain the CC speed while in 3rd) and the engine at a higher rpm to be nearly on its torque peak than for it to be in OD unlocked the whole time.

I think then the smartest decision is to burn a little more fuel (still gets 15mpg while towing), but have a situation where I know the transmission is happy instead of a situation where I could have a potentially unhappy one. What is the saying? "penny wise but a dollar short"? <----- sometimes that's me alright! Now that I'm the ripe old age of 35, I think now I'm starting to at least be able to know when I'm doing it!

Thanks all for your insight, I welcome any other comments too!

Mike

 

[Gary RV_Wizard]

Sounds to me like you need a numerically higher ration rear axle, maybe a 3.73. I guess a lock-up defeat switch is a lot cheaper, though.

 

[John From Detroit]

The problem with defeating the lock is heat..  When the torque converter is "working" it makes heat (When it's locked it is resting) and this can become critical over time when towing

If simply turning off O/D makes it stay locked.. Then think about this

In Highest gear with O/D off the transmission is a straight through device, no gear pushes against any other gear under load. ONE revolution of the crankshaft = one revolution of the drive shaft, 1-1

With O/D on well, the very defination of over drive is one revolution of the crank means more than one revolution of the drive

So a gear is pushing a gear under load and thus generating heat

So if just turning off overdrive is all that is needed to keep it locked..you will run lot cooler that way,, And longer too

 

[dave95.1]

The problem with defeating the lock is heat..  When the torque converter is "working" it makes heat (When it's locked it is resting) and this can become critical over time when towing

If simply turning off O/D makes it stay locked.. Then think about this

In Highest gear with O/D off the transmission is a straight through device, no gear pushes against any other gear under load. ONE revolution of the crankshaft = one revolution of the drive shaft, 1-1

With O/D on well, the very defination of over drive is one revolution of the crank means more than one revolution of the drive

So a gear is pushing a gear under load and thus generating heat

So if just turning off overdrive is all that is needed to keep it locked..you will run lot cooler that way,, And longer too

I agree with everything here and would like to add that adding a switch WILL give you a check engine light and possibly cause the ECM to adjust how your transmission operates.  The ECM compares input speed and output speed to calculate slip within the transmission.  If it thinks the torque converter is locked up when it's not, it will think there is slip happening that should not be.  This will cause harsh shifts to start with and could lead to defaulting to 3rd gear only - limp home mode.  You also create alot more heat when the converter is unlocked.

A '96 S-10 blazer has a 4L60-E transmission, the replacement for the 700R4.  It's biggest enemy is heat, and towing in OD creates unecessary heat.  The clutch pack for OD in that trans is also....we'll just say not very stout.  Not made to handle much load.

 

[John From Detroit]

Dave,  I never even thought about error codes messing putting it into "Limp home"mode

Yup. yet another reason to just kill the OverDrive

now, how you kill overdrive depends a lot on the tranny model and SUB MODEL  For example.. I flip a toggle