Purpose of lockup switch

[AH64ID]

23 minutes ago, JAG1 said:

No John, not at all. The only thing I am saying is it engine brakes better with it locked and do not understand why some say otherwise.

 

Because there is OEM documentation from at least one OEM that engine braking is better with an unlocked torque converter. I have driven multiple vehicles that also unlock the tq converter for enhanced engine braking, thou they were rentals and I don’t recall the make/model. I owned a Toyota that did it so I recall that info more specifically. 

 

I brought it up because it’s the exact opposite of what you guys are saying here, and I found that interesting. I trust what you guys are saying it’s just odd to me. 

 

It makes me wonder why. It honestly makes no sense to me that a locked torque converter can provide more resistance than an unlocked one. There should be more drag, and higher rpms, both of which should give better engine braking. That being said I’ve never driven a 47/48 trans and cannot provide any experience with those, I just my experience with other auto’s. 

Edited January 6, 2018 by AH64ID

[Tractorman]

1 hour ago, AH64ID said:

It honestly makes no sense to me that a locked torque converter can provide more resistance than an unlocked one. There should be more drag, and higher rpms, both of which should give better engine braking.

 

Here is a different way to look at it.  Let's say the truck is going down a grade at 1700 rpm's with the torque converter locked up, no throttle input from the operator, and the truck's speed is being held constant with just the friction of the engine being forced  to run at that rpm (the drag).  If the torque converter is switched to unlock, the engine rpm's will fall immediately, not increase.  The falling engine rpm will reduce engine braking.  The truck will begin to accelerate.

 

I have no experience with Toyota transmission controls, but I am guessing that there is more going on than just unlocking the torque converter.

 

- John

Edited January 6, 2018 by Tractorman

[AH64ID]

2 minutes ago, Tractorman said:

  If the torque converter is switched to unlock, the engine rpm's will fall immediately, not increase. 

 

 

And there is the difference. All the auto's I've driven that unlock while braking increase engine rpms, and thus increase load and increase engine braking. 

 

I know lots of new cars also drop the rpms while coasting, no braking, to reduce drag. There seems to be a lot going on within the modern auto trans.  

 

AFIK Toyota just unlocks the converter, at least that's what the bulletin I read said... it has been a few days thou. 

[Killer223]

Here's a thought for you. Why do they sell the kit that requires the converter to stay locked when you install the EB on a 47/48 truck. 

you want direct coupling to the engine, with it unlocked it can freewheel. no braking effect. 

my 04 for instance, i can go down a 17 mile 6% grade hill unlocked and coast the whole way if i've tapped the brakes at the top, however if i just let off and never touch the brakes and the converter stayed locked, it'll slow my down to around 35 before it unlocks. 

Edited January 6, 2018 by Killer223

[AH64ID]

2 minutes ago, Killer223 said:

Here's a thought for you. Why do they sell the kit that requires the converter to stay locked when you install the EB on a 47/48 truck. 

you want direct coupling to the engine, with it unlocked it can freewheel. no braking effect. 

my 04 for instance, i can go down a 17 mile 6% grade hill unlocked and coast the whole way if i've tapped the brakes at the top, however if i just let off and never touch the brakes and the converter stayed locked, it'll slow my down to around 35 before it unlocks. 

 

The exhaust braking is different to me than coasting... but as Tractorman pointed out an unlocked 47/48 will decrease in rpms and the transmissions the intentionally unlock for braking will increase in rpms. 

[hex0rz]

Maybe I'll be able to add a little to this. I think the reason why this works as stated as compared to the Toyota's you are referring to is that it works principally the same as a manual transmission. When you lock the TC you have direct coupling with the engine and transmission.

 

When your in a specific gear, you're restricting your engine speed to that gear ratio. Now there's drag involved and that momentum is soaked up into the drive train and engine. I'm sure terminal velocity of the drive train has some play in limiting the rpms also.

 

When you coast down a grade with the TC unlocked, you no longer have that coupling going on between the engine and transmission. I notice a significant difference between the transmission manually placed in neutral and when in drive with the OD on. When in neutral it will coast much faster than coasting downhill in drive. But i also notice that I'm also not being restricted by gear ratio and engine drag as much either when in drive even.

 

When i used to drive a manual truck, gearing down and coasting downhill, the gearing and drag made a big difference in slowing. But if i pushed the clutch in or had it in a higher gear, away i would go and gain in speed down the hill. 

 

Maybe i muddied things up more instead of clarified...

Edited January 7, 2018 by hex0rz

[AH64ID]

Based on what Tractormanm posted earlier the Toyota transmission acts completely different than the dodge transmission, and that’s why it seemed odd to me. 

 

The Dodge trans will decrease rpms and drag when unlocked, whereas the Toyota transmission will increase drag and rpms when unlocked. 

 

A Diesel engine has so little drag itself, which is why exhaust/engine brakes are needed, that it seems that the Toyota style trans would actually work better. 

[notlimah]

My guess is he technology just wasn’t there when these trucks were rolling off the line. Any time you left off the throttle when the trucks in D rpm will drop tomidle and the truck will coast with the only resistance being the friction between the tires and the road. Obviously it’ll hold back a bit if you drop it down a gear and are headed down a hill but it does it better when dropped in the appropriate gear and can keep the TC locked. 

 

I understand you know the differences but trying to give you an idea of how the trans runs since you haven’t driven one *inserts slight jealously here.*

 

I will say, the automatic trans in the 4th gens with automatic exhaust brake, primo! 

[JP907ATB]

On 1/1/2018 at 10:50 PM, notlimah said:

One big variable is how much work has been done on the trans. This will dictate what gears it’ll lock in, and what gears the trans can shift to and from without causing damage. 

 

Most people use it to have a locked 3rd-4th shift which is nice. I have my trans setup so I can also downshift from 4th-3rd locked no problem. 

 

The other main reason/scenario people use the switch is for slower speed pulls up hills either in a steep grade that the truck just can’t pull in 3rd so you’d naturally drop into 2nd. Being able to lock in 2nd to keep your trans temp down when hauling can save you some headaches.

 

Last scenario is like @JAG1 mentioned and using the lockup switch as a sort of engine brake, which is does a surprisingly good job at.

 Yes, however i think you need at least custom valve body to lock up in 2nd, when i lock up going down hill and hit the engine brake at the same time i wouldn’t even have to use the trucks brakes.

[Tractorman]

My 6 spd manual transmission locks up in every gear, even reverse!  However, it is very annoying the way you have to unlock it - you can't just flip a switch.  If you don't unlock it during shifts, it makes grinding noises.

 

- John

Edited January 7, 2018 by Tractorman

[JAG1]

1 hour ago, Tractorman said:

My 6 spd manual transmission locks up in every gear, even reverse!  However, it is very annoying the way you have to unlock it - you can't just flip a switch.  If you don't unlock it during shifts, it makes grinding noises.

 

- John

One custard filled Donut brought to my shop and I will put in an unlock switch that will work when you press the left pedal. Me have good coffee too.

[notlimah]

22 hours ago, JP907ATB said:

 Yes, however i think you need at least custom valve body to lock up in 2nd, when i lock up going down hill and hit the engine brake at the same time i wouldn’t even have to use the trucks brakes.

 

If I'm remembering this correctly, the truck won’t lock in 2nd on a stock valve body when the gear selector is in 2nd. It mechanically locks a stock valve body from locking up. If you try to lock in 2nd with the truck in D, it WILL lock on a stock valve body. I’m sure you can see why that’ll be a problem.

[jlbayes]

On 1/5/2018 at 8:04 PM, notlimah said:

 

Arent you on a built trans? I personally don’t worry about it but I’ve had it built so I wouldn’t need to worry. I just think it’s good for people to know the potential risks, not saying it’s for certain.

 

No. this is in my stock 12v.

[notlimah]

Ahh, well either way, I don’t think these guys will blow their stock trans using a lockup switch, you just need to know how/when to use it, and just know that you’re at least running a risk of damage with a stock trans doing things these trans don’t do stock. I know you’ve been around these trucks long enough to know more then the average guy so I’m sure all this goes without saying.

[trreed]

There is a reason I refer to my car's transmission as an automagic...