torque converter lockup question...

[vdubnut]

1991 burb 350TBI - 700R4 - 4:10 gears

I'm trying to understand the logic of the way my TC lockup works.

when im cruising on the freeway going steady the lockup is engaged.
If i then push the gas pedal down a little further the TC disengages.
If push the pedal down even further(pretty much WOT), then it downshifts.

is this how other peoples trucks are working?

here's my question...why would the TC disengage rather than the trans downshifting? the reason i ask is because when i am towing, my TC almost never is locked due to the extra power i am demanding from the engine. It seems like it should downshift rather than disconnect the TC lockup. The problem with all this is a concern about trans temps...

One explanation someone mentioned was that it was trying to reduce the load on the 4th gear clutch packs. But it seems to me that it would actually increase the load on them because its staying in 4th/OD and increasing the RPMS of the motor thereby increasing the load on 4th gear. Am i missing something?

 

[4X4HIGH]

Yes that is how is should work.

Think of the lock up as a gear between a gear, you hit the skinny pedal and you don't need a full gear drop so the TC just unlocks and raises RPM's about 300 or so, if more is needed then the trans will drop a gear.

NEVER TOW IN OD, that is a very quick way to kill a 700R4.

 

[vdubnut]

ok...that makes sense but not for towing...my 3rd and 4th stay unlocked when towing....im worried about burning it up, and im only towing about 3k lbs at 60mph

i had a previous suburban which was a diesel that something went wrong with the TC lockup solenoid or the circuit...the shop i took it to did something that eliminated the electronic control so the TC would lockup. that trans would downshift into 3rd and keep the TC locked up and would only unlock it if i really pushed it or when i was slowing down to stop. it seemed to be a better way to control it for towing purposes. i took this burb back to them but they didnt know what that tech had done years ago....they said they could make it lock up in 4th only without electronic control but not 3rd. thoughts?

 

[4X4HIGH]

Simple solution is DO NOT TOW IN OD. It is ok if you're towing in 3rd gear and the convertor isn't locked as it will lock when needed. Most 700R4's will lock-up the convertor in 2nd, 3rd, and 4th. If you have a tach it is really easy to see if the convertor is loced or not by lightly tapping the brake pedal to see if the RPM's go up by about 300 or so. If they do go up then the convertor was locked even though you didn't notice that it was.

 

[vdubnut]

i can definetly tell when the the converter is locked or not....by engine sound and how slight variations in the throttle change the RPM's or not...but when you say "It is ok if you're towing in 3rd gear and the convertor isn't locked as it will lock when needed." what do you mean by when needed? will it lock up if its getting too hot or something?

not trying to be a pain...just trying to understand and be thorough...

 

[Thunder]

Heat has nothing to do with TC lock up. There is no temp sensor in 700r4.
Lock up is controlled by engine load as the ECM sees it. The ECM uses data from the TPS, MAP sensor, and the pick up coil (RPM) and VSS. To control the TC lockup.
It is programed to give you the best fuel economy and save wear and tear on parts.
It may not be ideal program but it does work if you dont tow in OD with heavy loads or in any kind of a headwind.
Pretty common for the TC to stay unlocked when towing. The lockup clutch for the TC is small and does not have a lot of surface area.. It can be easily over heated and worn out with too much load. The ECM is pretty smart and knows a thing or two about a thing or two.
That is why the TC unlocks when towing. TC lock is mainly a gas mileage thing. Not for towing.
Just get a good trans fluid cooler and you wil be fine. Dont worry about the TC being unlocked.

 

[vdubnut]

Ok....thanks for the info!

 

[Greg Ducato]

A torque converter multiplies engine torque whenever it is not locked up. This allows you to pull slight grades and remain in 4th while you are cruising without the jolt of a 900rpm downshift. It is a terrific system that GM uses to help the driveability and one that owners of earlier GM products would like to have. Towing in overdrive is pretty hard on the trans but especially so when the converter is unlocked as it severly reduces lube flow to the trans and can result in some pretty major trans overheating and planetary damage, whenever you are in overdrive with the converter unlocked you are building serious heat. As I have told some of my customers in the past, you can't save enough fuel towing in overdrive to pay for another transmission....