Understanding torque converter/tranmission temperatures

[mountainbiker42]

I'd like to start a technical discussion about understanding transmission and torque converter temperature readings. I run a B&M cooler with an analog autometer temp gauge installed on the outlet of the torque converter, before the cooler. I'm not wanting to start a debate about where the best location is for a temp gauge, I want to be able to better understand what's going on in the torque converter based on temperature readings.

Let me give you an example...

Under normal city driving condition, the temp gauge will usually read around 150 (not operating temperature). As you can imagine, when I'm pulling a trailer or have a heavy load and I'm driving in a manner that prevents the torque converter from locking up, the temperature will reach 180-210 (this is operating temp when the cooler is fully open). Everything's cool so far and I completely understand what's going on. Here's where I get lost...

When the transmission is at operating temp and I turn off O/D, or drive in manner that allows the torque converter to lock up, the temp immedietly drops to around 125...until the torqure converter unlocks. Once it unlocks (even if I come to a stop and put the vehicle in neutral) the temp goes right back up to operating temperature. This is telling me that when the lockup clutch is engaged, the torque converter remains hot, but somehow fluid is being cooled or bypassed. I'm trying to understand why this happens.

I have two theories; 1. When the lock up clutch is engaged, the fluid flows around the torque converter and straight to the cooler. Therefor the fluid does not pick up any heat from the torque converter, hense a a lower reading. The torque converter is remaining hot and not being cooled off until the lock-up clutch is disengaged and fluid can flow through the T.C. and pick up heat (operating temperature.) If this is true, I wonder if your temp readings during T.C. lock up are internal transmission temps since the T.C. is being bypassed. If so, I've never seen these temps go higher that 125 during T.C. lock up.

2. When the lock-up clutch is engaged, the fluid stops flowing all together and fluid in the lines becomes stagnent and cools off rapidly. Once the lock up clutch is disengaged the fluids starts flowing out of the T.C. again, and the fluid in the lines is replaced with hot fluid in the tranny and your temp reading goes back up to operating temp. From my understanding the transmission uses the T.C. as a fluid pump rather than having a seperate internal pump (I'm only guessing). If this is true however, how does the tranmission create pressure to engage clutch packs if the T.C. does not create fluid flow during lock-up?

What are your thoughts? I'd love to be able to understand this, and maybe with some technical discussion we can figure that out.:shocked:

 

[jeremyc74]

The fluid is always flowing. If it stopped, the transmission wouldn't hold pressure on the clutches and it would slip. The pump is driven off the back of the torque converter, and is basically coupled straight to the crank. If the engine is running, the fluid is flowing.

The reason the temperature drops is because the torque converter is what generates the vast majority of the heat (and loss) in the transmission. It's a fluid coupling that is always in a state of slip until the lockup clutch is engaged. When the lockup clutch is engaged the crank and input shaft are coupled together and the torque converter is out of the loop and not generating heat. In that state the losses are much closer to those of a manual transmission, and the heat loss drops substantially. This is why a transmission cooler isn't needed on a manual transmission except in the most demanding situations.

 

[mountainbiker42]

I agree with you. Although that does not explain why the temp jumps up back to operating temp immedietly after the T.C. lock up clutch disengages. If I'm driving with the T.C. locked, temp at 125, then put the vehicle in neutral and cost to a stop, it jumps back up to 180-200 (where it was before the T.C. locked up).

 

[drodandsrod]

I've been wondering the same thing since I installed my trans temp gauge and noticed the same phenomenon. My trans is wounded so my temps run a little hot but on the highway my gauge indicates below it's min cool temp of 140 and will sweep back up to 180 + almost by the end of the off ramp. Converter lockup is clearly a major factor, I'm just surprised at how much heat the converter generates. I initially wondered if my converter wasn't failing. I worry that some of the rapid temp rise is due to moderate clutch slip in one of the clutch packs but I suppose if that were the case it would be evident whether or not the converter is locked up. It's a little puzzling though as I assume the converter can lockup at lower speeds and in gears other than OD.
My other thought is that we're seeing a significant increase in the thermal capacity of the cooling system at higher speed due to the significant airflow at 75 mph. What size cooler are your running, I've still got the stocker for now...

 

[mountainbiker42]

I'm also suprised at how quickly the T.C. generates heat, it's interesting. I know that the TC locks in at 48mph in 4th gear, and 42mph in 3rd. I've never seen the TC lock up in 2nd or 1st gear, or going downhill at any point.

I'm running a B&M 268 (I think, whatever the most commoon B&M cooler is on T4R is the one I'm running) as my one and only cooler. I agree though that the TC lock up and fluid routing is playing a major factor here, we just don't know what it is yet. I'm glad other people have noticed this as well.

 

[2014 Warthog]

Hope I don't make a fool of myself here, but I was always under the impression that an unlocked converter does run much hotter than locked. That the very act of the fluid doing it's job in the converter heats it up and is the main reason you want it to be locked up when towing as the more torque you put on it unlocked, the hotter it will get. The friction caused by an unlocked converter on the fluid itself causes heat. So if that's true, then the fluid coming out of the trans would cool down in a locked configuration as your cooler is cooling it faster than the tranny is heating it up.

From Wikipedia: "... impelling losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and virtually no power loss."

 

[mountainbiker42]

warthog-

That is correct yes. Couldn't have been said better. The question still remains though how it gets so hot so quickly (assuming the trans was at operating temp before the TC locked up) when the TC lockup clutch is disengaged, even with no load on the TC. The TC is not generating any heat when you take the exit ramp to slow down on the interstate like "drodandsrod" mentioned, but yet the temp quickly rises back to operating temp. This is the mystery

 

[jeremyc74]

warthog-

That is correct yes. Couldn't have been said better. The question still remains though how it gets so hot so quickly (assuming the trans was at operating temp before the TC locked up) when the TC lockup clutch is disengaged, even with no load on the TC. The TC is not generating any heat when you take the exit ramp to slow down on the interstate like "drodandsrod" mentioned, but yet the temp quickly rises back to operating temp. This is the mystery

The torque converter holds a lot of fluid, and when it locks up a lot of that just sits in there. I would imagine what you're seeing is the accumulated heat from it not being locked up, plus the heat from the engine.

It would be interesting to see what it gets up to after being locked up for a long period of time.

 

[jeremyc74]

It's a little puzzling though as I assume the converter can lockup at lower speeds and in gears other than OD.
...

Newer vehicles lock up a lot earlier, but I think in this case the lockup only happens when you're in OD and at steady speeds.

 

[19963.4lsr5]

When OverDrive is locked out lockup happens a little earlier than the 45mph when it shifts to 4th.

If I lockout overdrive in the jeep I get TC lockup in 3rd gear at 30mph. It be nice if the Toy did that.

The TC will not lockup until the trans shifts into the top gear. (3rd or 4th) Depending on your OD button.

Saturn S Series autos lock up the TC in every up shift. They sometimes referred to the TAAT trans in them as a "manual matic" I liked that feature when I had a Automatic in my 96 SL1.

 

[jgue467]

The torque converter holds a lot of fluid, and when it locks up a lot of that just sits in there. I would imagine what you're seeing is the accumulated heat from it not being locked up, plus the heat from the engine.

It would be interesting to see what it gets up to after being locked up for a long period of time.

That's the closest to understanding why it gets hotter with the tc locked. While driving in 80 degree weather the other day, mine slowly climbed to 205 and I didn't let it climb higher - switched OD off and within 60 seconds it drops 25degrees. I've learned just to switch OD while climbing hills in excess of 100feet. Once while driving it thru the davis mountains in west texas last summer it got to 237 (on uphill passes at 85mph; 100degrees), as soon as i turned OD off it dropped to 197, crazy. :wacko:

 

[drodandsrod]

I've been mulling this over on and off while sitting in my "thinking" room and I'm still not satisfied with the explanation. If the fluid just sits in the converter while it's locked up (and neither heats up or cools off which doesn't seem likely but I'll leave that alone for now) and the remainder of the fluid cools to say 150, which I routinely see in my rig at 80-ish, I'd expect to see the temp rise and fall as the captured fluid mixes with the cooler fluid in the pan and hydraulic circuits. Instead I see the temp rise to *at least* what the pre-lockup trans temp was and it takes a long time to come down.
Break for family dinner, remainder of thought to come...

 

[jeremyc74]

I've been mulling this over on and off while sitting in my "thinking" room and I'm still not satisfied with the explanation. If the fluid just sits in the converter while it's locked up (and neither heats up or cools off which doesn't seem likely but I'll leave that alone for now) and the remainder of the fluid cools to say 150, which I routinely see in my rig at 80-ish, I'd expect to see the temp rise and fall as the captured fluid mixes with the cooler fluid in the pan and hydraulic circuits. Instead I see the temp rise to *at least* what the pre-lockup trans temp was and it takes a long time to come down.
Break for family dinner, remainder of thought to come...

I figured this out. When the converter is locked up the flow to the cooling circuit in the radiator slows way down. The transmission temps drop because the transmission itself is capable of radiating the heat away and doesn't need the cooling circuit while it's locked up. Once the converter unlocks the flow to the radiator circuit kicks back in and the fluid temperature goes back up to the same temperature as the water in the radiator.

I came to this realization last week on a long road trip when I was watching the temperature on my ScanGage. When the converter would unlock the coolant temperature would actually drop by about 4 degrees, and then slowly rise back up the 186 degrees it's normally at in steady state cruising. This puzzled me because the engine is under more load when that happens, and it didn't make sense for the block temperature to drop. The only explanation is that the transmission fluid is cooler than the water and is pulling the water temp down as the two equalize. :yo:

 

[cashmoney]

The fluid is always flowing. If it stopped, the transmission wouldn't hold pressure on the clutches and it would slip. The pump is driven off the back of the torque converter, and is basically coupled straight to the crank. If the engine is running, the fluid is flowing.

The reason the temperature drops is because the torque converter is what generates the vast majority of the heat (and loss) in the transmission. It's a fluid coupling that is always in a state of slip until the lockup clutch is engaged. When the lockup clutch is engaged the crank and input shaft are coupled together and the torque converter is out of the loop and not generating heat. In that state the losses are much closer to those of a manual transmission, and the heat loss drops substantially. This is why a transmission cooler isn't needed on a manual transmission except in the most demanding situations.

This.

 

[SpidermanGeek]

I was just about to ask that to those who notice the temps jumping to 180 in neutral or whatever... I was wondering if they still had factory trans fluid cooling system.

I have mine bypassed and I never see the temps bounce. They'll slowly go up or down 10-20 degrees depending on the current situation/gear/whatever, but I've never witnessed a drastic sweep in temps on my Autometer Gauge.

Since installing the bypass and gauge, I have yet to witness temps go above 165F, with the average temp holding around 150F. No Armor, No Towing ,no engine mods. Just a 3 inch lift with 33's. '97 Limited with 4:30 gearing.