Torque converters and lockup clutches?
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Staging Lane
Joined: Sep 2024
Posts: 58
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Torque converters and lockup clutches? Just want to make sure I have this figured out correctly. Cause I think some on the youtube got it wrong.
Most lock up clutches get apply pressure from the torque converter internal pressure, after the pressure in front of the apply plate is released? Meaning that pressure is bled off through the input shaft. Some say the apply pressure is going through the input shaft? I thought I studied this some time ago, but you know it can be an age thing, and a bit of laziness too. Is it different on the different makes of transmissions?
Most lock up clutches get apply pressure from the torque converter internal pressure, after the pressure in front of the apply plate is released? Meaning that pressure is bled off through the input shaft. Some say the apply pressure is going through the input shaft? I thought I studied this some time ago, but you know it can be an age thing, and a bit of laziness too. Is it different on the different makes of transmissions?
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From: BERWICK PA
Early lockup was pretty simple . On /Off with a release and apply side fluid was dumped on one side and directed to the other, Todays still work similar but not with just dumps but by varying the pressure on each side to allow limited slip of the clutch.
It can seem difficult to understand as it not a simple apply like a trans clutch pack . I'm sure a converter company could explain it better than myself however. But it is not a super simple operation. This is why diagnosis can be trifling when problems happen. Leaks either side, pwm issues, pump leaks, vb leaks . bushing leaks, stator leaks the list goes on .
It can seem difficult to understand as it not a simple apply like a trans clutch pack . I'm sure a converter company could explain it better than myself however. But it is not a super simple operation. This is why diagnosis can be trifling when problems happen. Leaks either side, pwm issues, pump leaks, vb leaks . bushing leaks, stator leaks the list goes on .
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Lockup in the converter and how it applies is normally dictated by the type of transmission it is made for since the transmission is ultimately what controls lockup. Your more common applications like 700s, 4L60Es, 4L80Es, 6L80s, and 8L90s all use pressure on the cover side of the piston to keep the piston unapplied, there is always pressure on the opposite side of the piston. To apply the piston the transmission simply removes that oil on the cover side of the piston which allows the piston to apply. A lot of the more common Chrysler and ZF converters are actually using the addition of pressure apply lockup, they even have an apply piston.
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You are correct. Most systems apply the lock-up clutch using torque converter pressure itself, not a separate feed through the input shaft.
The pressure in front of the apply plate is released (bled off), allowing the main converter pressure to push the plate and engage the clutch. The input shaft passage
The pressure in front of the apply plate is released (bled off), allowing the main converter pressure to push the plate and engage the clutch. The input shaft passage
Thread Starter
Staging Lane
Joined: Sep 2024
Posts: 58
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So after watching some fairly good video's on the torque converters, what it seems everyone doing said videos completely ignores, is what seals the largest outside diameter to the cover or?, I've looked for any hint of a seal and don't see any. Yes Seals on shafts etc. Does anyone have a good picture of a seal for the major diameter of the TCC piston?
So if no seal I really don't see how there can even be a pressure differential to keep the clutch disengaged? Seems like there would need to be a huge volume of fluid coming from some place to accomplish that. Why no seal?
And all the worry about TCC seals leaking and valving leaking and then have one huge internal leak? I guess I'm just wearing my stupid hat tonight.
Edit here.
Yeah no outer seal. After a huge bit of research, all I can say is what where they thinking, a bit of a stupid design, at least after a huge (yeah I know) discussion with chatgpt, that said I was 100% correct.
I'd like to know how tight the clearance is between that piston and the bore it rides in? Isn't this almost similar to leaving out some seals on a clutch pack piston? Yeah I know it works and then sometimes things do wear out. Is there some kind of upgrade for this design issue on converters?
So if no seal I really don't see how there can even be a pressure differential to keep the clutch disengaged? Seems like there would need to be a huge volume of fluid coming from some place to accomplish that. Why no seal?
And all the worry about TCC seals leaking and valving leaking and then have one huge internal leak? I guess I'm just wearing my stupid hat tonight.
Edit here.
Yeah no outer seal. After a huge bit of research, all I can say is what where they thinking, a bit of a stupid design, at least after a huge (yeah I know) discussion with chatgpt, that said I was 100% correct.
I'd like to know how tight the clearance is between that piston and the bore it rides in? Isn't this almost similar to leaving out some seals on a clutch pack piston? Yeah I know it works and then sometimes things do wear out. Is there some kind of upgrade for this design issue on converters?
Last edited by bob4360; Sep 9, 2025 at 01:00 AM. Reason: Did some more study on the topic
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Joined: Mar 2023
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From: BERWICK PA
So after watching some fairly good video's on the torque converters, what it seems everyone doing said videos completely ignores, is what seals the largest outside diameter to the cover or?, I've looked for any hint of a seal and don't see any. Yes Seals on shafts etc. Does anyone have a good picture of a seal for the major diameter of the TCC piston?
So if no seal I really don't see how there can even be a pressure differential to keep the clutch disengaged? Seems like there would need to be a huge volume of fluid coming from some place to accomplish that. Why no seal?
And all the worry about TCC seals leaking and valving leaking and then have one huge internal leak? I guess I'm just wearing my stupid hat tonight.
Edit here.
Yeah no outer seal. After a huge bit of research, all I can say is what where they thinking, a bit of a stupid design, at least after a huge (yeah I know) discussion with chatgpt, that said I was 100% correct.
I'd like to know how tight the clearance is between that piston and the bore it rides in? Isn't this almost similar to leaving out some seals on a clutch pack piston? Yeah I know it works and then sometimes things do wear out. Is there some kind of upgrade for this design issue on converters?
So if no seal I really don't see how there can even be a pressure differential to keep the clutch disengaged? Seems like there would need to be a huge volume of fluid coming from some place to accomplish that. Why no seal?
And all the worry about TCC seals leaking and valving leaking and then have one huge internal leak? I guess I'm just wearing my stupid hat tonight.
Edit here.
Yeah no outer seal. After a huge bit of research, all I can say is what where they thinking, a bit of a stupid design, at least after a huge (yeah I know) discussion with chatgpt, that said I was 100% correct.
I'd like to know how tight the clearance is between that piston and the bore it rides in? Isn't this almost similar to leaving out some seals on a clutch pack piston? Yeah I know it works and then sometimes things do wear out. Is there some kind of upgrade for this design issue on converters?
I am not a converter guy, I do understand the basics , But also know alot is happening in there, You have stall, ratios, sprag, power transfers, A type of fluidic gear ratio, Torque multiplication and of course lockup which is also a bit more complex than a piston with a seal applying a clutch set.
So again I yield to Converter builders, designers and engineers.
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Thread Starter
Staging Lane
Joined: Sep 2024
Posts: 58
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I know the old converters some what since in the old days they all bolted together, if memory is correct even some stators back then had variable vanes, then there was the turboglide I actually started to make one a cutaway in highschool, I think the Dynaflow was goofy too.
I'm discovering that the ? phase one TC like a 4L60E there is a sloppy fitting like in videos photos I've seen about 1/8 clearance piston to cover or shell on the OD. Yeah on lockup the friction is the seal of sorts, but on release its a huge iffy deal of I guess enough fluid to float the friction off the cover. It sure doesn't look like a positive control deal to me, Any fluid diagram photos make one think there is actual fluid pushing that piston back, no OD seal. Yes I really need to see the insides of one. Sure they work but, looks like a lot of hope to me?
I didn't watch all the way but SIU had a Sonnax engineer explaining the different TC's but everyone no matter who never mentions this seal issue.
I'm discovering that the ? phase one TC like a 4L60E there is a sloppy fitting like in videos photos I've seen about 1/8 clearance piston to cover or shell on the OD. Yeah on lockup the friction is the seal of sorts, but on release its a huge iffy deal of I guess enough fluid to float the friction off the cover. It sure doesn't look like a positive control deal to me, Any fluid diagram photos make one think there is actual fluid pushing that piston back, no OD seal. Yes I really need to see the insides of one. Sure they work but, looks like a lot of hope to me?
I didn't watch all the way but SIU had a Sonnax engineer explaining the different TC's but everyone no matter who never mentions this seal issue.
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Staging Lane
Joined: Sep 2024
Posts: 58
Likes: 5
I finally got a hold of a cut open and cleaned Torque converter, I think its a 4L60 ish unit, Here is what I think happens. The clutch friction material is the major seal for the piston. The clearance between the piston OD and the cover is quite large, have not put calipers on it yet.
To release of course fluid through the center of the input shaft that has to be at a higher pressure to keep the clutch off the cover and the fluid will make it act like a rock skipping on water to prevent engagement. Take away that pressure, and leave open a drain, (center of input) then of course pressure for the inside around the pump/ turbine area etc will help move the piston into contact once in contact friction material seals enough for a good build up of pressure to hold it locked. PWM makes it a just wow deal, I guess it never seals good and it is aways in wear mode or semi skip condition on the fluid.
Is there a way to post pictures here? Most sites have an insert icon?
To release of course fluid through the center of the input shaft that has to be at a higher pressure to keep the clutch off the cover and the fluid will make it act like a rock skipping on water to prevent engagement. Take away that pressure, and leave open a drain, (center of input) then of course pressure for the inside around the pump/ turbine area etc will help move the piston into contact once in contact friction material seals enough for a good build up of pressure to hold it locked. PWM makes it a just wow deal, I guess it never seals good and it is aways in wear mode or semi skip condition on the fluid.
Is there a way to post pictures here? Most sites have an insert icon?