dumb elementary brake fluid question

[TechnoLoGs]

Shouldnt you take the sprayer bottle and get all the brake fluid out then put new in the master cylinder and pump the brakes to change it?

 

[TechnoLoGs]

Originally Posted By: berninicaco3
question about engine braking.

It's hard to find techs who really, really know about transmissions; and that definitely includes myself. I haven't pinned down the answer to this one yet.

For engine braking to work, the wheels have to want to be going faster than the engine, no?
So that the wheels are trying to drive engine compression (rather than engine combustion powering the wheels).

This works a LOT better if you actually have release valves to vent the compressed air, otherwise the compressed air returns a lot of its energy like a spring-- so there's some engine braking cam with engine braking valves on a truck's diesel. A feature that doesn't exist on a gasoline car engine.

Strike one.


But more importantly...

In my manual transmission'd car, when I'm in 5th gear and I let off the gas, the engine rpms stay in line with my vehicle speed. They are mated together with the clutch.

But in my automatic transmission'd car, let's say I need to get to 3000rpm to get to 70mph (and I've stayed in overdrive, and didn't downshift to 3rd). When I let off the gas, that rpm needle drops like a stone to 1200rpm. Try to accelerate again, and it needs to quickly rise back to 3000rpm before I start going faster.
It is NOT tied to the wheel speed.
What I've read/been told, and it make sense, is that the torque converter as designed is also a one-way fluid clutch. I've got a very basic torque converter from 1990 without locking overdrive, if that changes anything.
This is designed such that the pump must always be going faster than the turbine to drive it. The turbine cannot drive the pump: that is to say, if you let off the gas and the drive wheels are still spinning at whatever rpm is 70 miles per hour, they won't be able to drive the engine in the other direction. It's a one-way fluid coupler.

I mean, I'm pretty sure I'm seeing this every time I let off the gas on my automatic and rpms plummet right to idle, and every time I let off the gas on my manual and they don't drop any more than vehicle speed.

But that means that in my automatic, if the rear wheels cannot drive the engine in the first place (not to mention that the engine has no engine braking release valves),
I will get NO engine braking out of my automatic, no matter what gear I choose to force it into? The wheels cannot drive the engine, not in 1st, not in 2nd, not in any gear. Only the engine can drive the wheels.

I feel that this is right.
Going down pikes peak, my one and only experience with mountain driving to relate to, it seemed like engine braking wasn't doing anything and it was all due to the brakes. Let off the brakes, and I sped up as fast in forced-2nd as I did in overdrive, with nary a rise in engine rpms (seemingly untied to vehicle speed).

Maybe more modern, more sophisticated torque converters can do more. My sister feels that her 2010 nissan versa with CVT does do engine braking for her, although she may be wrong. I can't find whether it has a torque converter or not. But even if it does, maybe they lock the torque converter to use it for engine braking. Or maybe she's just imagining it. I'm curious about more modern cars/ CVT.

But it is it correct that my old 1990 ford with its 4 speed AOD and very basic non-locking torque converter, is by design incapable of any engine braking?


Apologies for the wall of text.
I wanted to lay out all of the information I believe I have, so you can follow my train of logic-- and if I can in fact engine brake, pinpoint where I've got some bad information.
I'm curious about this.


Dude, I think I have the same trans as you. 95 Taurus, so, AOD or AXOD. Trying to get it back to good.

To "engine brake," you downshift, the trans "grabs," RPMs race up, and you use the trans to brake. manual transmission.

I don't think it goes like that on an auto, you can put the column shifter in "1" or "2" but that does not affect the slowing down, only the gear shifting up, IIRC.

Correct me if wrong.

 

[Ken2]

Every automatic transmission car and truck I've driven has engine braking when it is shifted out of its highest gear. The Drive or Overdrive or whatever that model has on it's shifter often does have an overrunning clutch or something similar to save gas and let the vehicle roll when you take your foot off the gas. My Tundra, for example, begins engine braking when I switch off Overdrive. It engine brakes more when I downshift to 2nd or 1st. An exception are a few automatic transmission cars where the fuel is completely shut off when your foot is off the gas pedal, and the wheels continue to drive the engine even in the highest gear.

By the way, what is actually happening is suction braking. The major engine braking force is the pistons pulling against the closed throttle valve. The compression results in compressed air in the cylinders which pushes back against the descending piston returning much of that force. This is part of the reason why diesels are so poor at engine braking without an engine brake device--no throttle to suck against.

For anyone who says that brakes are cheaper than transmissions, so don't downshift...the answer is yes, but. But on a long, steep grade a transmission is cheaper than a crash. And, the transmission is made for engine braking, at least every owner's manual I've read has said to downshift for long steep downgrades.

The right way to descend a long steep grade, especially with a heavy load, is to downshift until you are running just a bit above the speed you determine is safe for your rig (and might be lower than the speed limit). Brake lightly to trim your speed. You aren't overheating your brakes and aren't in a risk of crashing due to overheated brakes.

 

[Sequoiasoon]

Don't forget the brake pads and rotors in the whole equation. New good fluid and all new parts and you can still get serious brake fade. Stoptech probably has it listed, I know raceshopper did. When the pads get too hot they off gas and create a cushion of gas between pad and rotor. Push all you want on the pedal but it won't slow down. Normal conditions no issue, extremes yes. That is why they have the slotted/drilled rotors not just for the cool look factor.

Also part of the reason for different materials and friction coefficients. All places say NO race pads for normal driving as race pads need to be hot to actually have the friction. You'll drive through every intersection trying to stop when they are cold.

 

[SHOZ]

I use EBC Yellow stuffs for daily driving and track use. Best brake pad I've used. And the drilled and slotted rotors are for race not the street. They will actually lower the mass of the rotor and shorten your pad life.