Torque converter - Ford Powerstroke Diesel Forum
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Old 05-15-2013, 03:18 AM
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Torque converter

I have read about how the torque converter unlocks on the E40D trans. So, I tried what I had read about holding the throttle and tapping the brake to see if the rpm's went up a bit. They do. I also read somewhere that the TC will unlock while cruising down hill with no throttle use. So, how does one tow down hill, and while sitting at a traffic light, why does the truck still move when the brake is released? Does the TC "relock" on its own? Still learning and loving this truck more every day.

'97 F250 OBS
Turbo Diesel
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Old 05-15-2013, 09:14 AM
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This answer is very general and I'm trying to avoid more confusing things like the Stator:

There are two main parts of the TC. There is the locking clutch and there is the fins (Turbine) and veins (Pump). When, unlocked, fluid (pulled by vacuum from the centrifugal force described next) enters the TC (#1 to #2) and is immediately diverted back to the veins braised into the TC housing (#2 to #3 known as the "Pump"). These veins now cause the fluid to be re-diverted toward part #4 (known as the "Turbine") through it's fins and those fins catch the fluid against the face of those fins and deflect the fluid back in the reverse direction (up and out #5). This is what pushes the vehicle forward as the turbine is splined to the input shaft of the Transmission. The fluid is then directed to the Stator (#5 to #6) where magic happens and the fluid is diverted back out of the TC and to the tranny.

Since the TC is bolted to the flexplate (that is bolted to the crankshaft of the motor) the TC is constantly spinning with the motor (not the transmission). Thus the fluid is constantly spinning around the inside of the TC and creating a small amount of forward force (or backward if that gear is selected) as motor RPMs increase more and more driving force between the Pump (#3) and the Turbine (#4) is created as the fluid moves from one to the other. There is a point, called "Stall Speed", at which the fluid moving from the Pump to the Turbine is creating so much force that the fluid "backs up" on itself and starts to bog the motor down. You can find your stall speed by crushing the brakes and reving the motor as high as it will go in gear (truck stopped of course). The point that the motor will no longer increase in RPMs is the stall speed of the TC. For trucks it is normally in the 1800 to 2500 RPMs and depends on what the truck was built to do. The higher the stall speed of the TC the more torque is produced down low in the RPMs to do work (think Tow Truck) BUT the higher the stall speed the more the TC heats the fluid moving through it. Since our trucks are turbocharged the engineers at Ford settled on a mid stall speed so that our Turbos would start producing airflow quicker and we would notice less lag from a standstill. There is a give and take.

*If you try to find your stall speed do it quickly, this test creates a lot of heat in the tranny's fluid*



Last edited by BigFuel; 05-16-2013 at 07:48 AM.
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Old 05-16-2013, 03:21 AM
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So it never really "unlocks", it just releases some of the pressure? Ok. So, what is the proper way to come down a hill? Keep tapping the accelerator to keep the TC locked? Right now I only have a 7000lb trailer, but will be going heavier later. With my old gasser, I let the engine help keep things under control. Just trying to learn the best way to come off the big hills. Thanks.
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Old 05-16-2013, 04:11 AM
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With a stock trans you can take it out of OD which will help some... The best way to get the help from the trans like you are looking for is an upgraded VB and some tunes.
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Old 05-16-2013, 08:02 AM
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Agreed, custom tunes are written with deceleration in mind (unlike what the Engineers at Ford gave us). As long as the TC is locked you are getting no torque manipulation from the turbine BUT you are also getting no slipping as is the case when going down a hill. As stated above when you accelerate the fluid moving from the pump (attached to the motor) to the face of the turbine (attached to the Transmission) spins the turbine that spins the trans that spins the wheels. On decel everything is working backward, the engine is slowing down (so the pump is slowing down) BUT the wheels want to move faster (spinning the trans faster and then the turbine faster). The engine is no longer the dominating force in the TC and the truck speeds up (I know, long setup). With tunes the writers have programmed their chips so that as you decelerate the trans will stay locked allowing the engine to remain the dominating force to the wheels and thus uses it's compression to slow down the truck. My tunes Lock in 4th and in 3rd and will ride the engine, shifting itself, till the motor has slowed the wheels to about 20 MPH (depending on slope and trailer weight) before unlocking or me accelerating again. If you want more decelerating force then this you need to start looking toward a Exhaust Brake.
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Old 05-16-2013, 11:54 AM
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Ok, thanks. I know I will need an exhaust brake, there is an after market one on the exhaust now, but when I had some oil leaks taken care of, I was told the brand doesn't make the parts to rebuild it. I understand the system of the TC a bit better now. I was really at a loss as to how I was going to get down the hills in a normal fashion. This is my first diesel. And I think I did enough research to know to get an OBS 7.3. I'm on a budget, so I'll take things slow. But I do know that with this truck, I can at least haul the family camper around without wondering if we will get up the next hill or not. This forum is full of education, everyday I read something that educates an old dog. Thanks
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Old 05-16-2013, 12:30 PM
Can you hear the derail??

 

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There is a bnib pacbrake on Atlanta craigslist for like $400 Obo, and if I hit the od off button going down hill the tc locks in third and slows the truck a bit, unless it's steep, then it speeds up, but slower lol
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Old 05-16-2013, 01:00 PM
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Thanks for that bit of advice. So dropping out of overdrive, does that keep the TC locked in 3rd? Or, as I have read, after a certain amount time, will it still unlock?
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