PROJECT - "Truck No More"

[Cochise]

Normal auto's don't really compression brake, or if they do, not much...

By compression braking I mean that when you go down a hill, in gear, you don't have to touch the brakes, the rig slows down using the motor, like a manual transmission.

In an auto, if you put it in 1rst gear, it'll compression brake, no? Or does the auto "slip" on purpose when doing this?

 

[Cochise]

It also depend on the TC and the compresion of the motor.



But don't let that one old detail scare you away from an Auto. unless your truck doesn't have brakes?

Err, let's sort out what I am missing here. Consider me a noob, one who may not be sensitive to insider gear-head jerk-offs.

TC gear ratios and motor compression are independent of transmission gear ratios, whether manual or auto. A given.

So what is different between an auto "locked" in first, versus a manual? When locked in first, is the auto "doing/managing" anything other than when you are in a gear in a manual?

If it isn't, I can see where gear ratios come in, but otherwise, what's the issue?

 

[Farm Boy]

Consider me a noob, one who may not be sensitive to insider gear-head jerk-offs.

HUH?


By "locked in first" are you refering to position of the shifter or function of the trans? They are not necessarily the same thing.

The issue is that most autos essentially freewheel down hill.

If you are driving your regular car/truck/whatever with an auto trans down a long steep hill, without using any breaks, the vehicle will speed up even without your foot on the gas and the shifter pulled all the way down into low gear (or any gear for that matter). There is nothing in the trans to stop the wheels from spinning faster than the engine. Think of a Detroit locker as an example. Neither wheel can spin slower than the driveshaft but they can spin faster (outside wheel in a turn)

If you are driving the same car/truck/whatever with a manual in the exact same manner (long down hill, no brakes, no foot on the gas), the engine will hold the vehicle back from speeding up (as long as you pick an appropriate gear) Think of a spool. Both wheels will always turn at the same speed as the driveshaft (tire scrub in a turn)

With compression braking in an auto trans, this freewheeling is eliminated. You get much more control on the downhills than relying on the brakes alone.

I hope this helps. :beer:

 

[Root Moose]

Dan, with the AW4 this is why people wire in switches to manually override the PCM logic.

The AW4 is shifted via solenoids, not hydraulically. Wire up a switch to keep 1st engaged, and another to keep the torque converter clutch (TCC) locked and you effectively have the same as a manual (2.8x:1 first gear locked behind the natural compression of the engine).

The thing with our OBDII Jeeps is that wiring in these switches will throw a code when they are used. Not a big deal, should have a scanner in your boonie box anyway.

Out of the box our transmissions will kinda/sorta hold first gear until revs get to a certain level and then switch up to second. I say sorta/kinda because when in 4 Low there appears to be some logic in the PCM to hold first for a while. But it isn't fully implemented like say the HDC on the newer LRs. Very annoying. Not so much because it is bad form to use brakes on downhill descents as much as that it happens suddenly when you are not expecting it if you aren't really familiar with the XJ/AW4 setup.

 

[aweber]

HUH?


By "locked in first" are you refering to position of the shifter or function of the trans? They are not necessarily the same thing.

The issue is that most autos essentially freewheel down hill.

If you are driving your regular car/truck/whatever with an auto trans down a long steep hill, without using any breaks, the vehicle will speed up even without your foot on the gas and the shifter pulled all the way down into low gear (or any gear for that matter). There is nothing in the trans to stop the wheels from spinning faster than the engine. Think of a Detroit locker as an example. Neither wheel can spin slower than the driveshaft but they can spin faster (outside wheel in a turn)

If you are driving the same car/truck/whatever with a manual in the exact same manner (long down hill, no brakes, no foot on the gas), the engine will hold the vehicle back from speeding up (as long as you pick an appropriate gear) Think of a spool. Both wheels will always turn at the same speed as the driveshaft (tire scrub in a turn)

With compression braking in an auto trans, this freewheeling is eliminated. You get much more control on the downhills than relying on the brakes alone.

I hope this helps. :beer:

What Jon said :beer:

Don't forget Torque Converter slippage as well as clutch slippage. When there is no pressure built up in the tranny, the clutches will slip, but when you have a Manual Valve Body, the pressure is MAX all the time, when the Torque Converter is spinning that is...... But it is all theory to me anyway,

 

[aweber]

It also depend on the TC and the compresion of the motor.



But don't let that one old detail scare you away from an Auto. unless your truck doesn't have brakes?

You, of all people, should know that brakes can fail :flipoff:

 

[aweber]

I don't have a problem going down hills with a properly set up Tranny and TC.

BTW - What is your setup? Just wondering what your Stall is and what mods you have done to your Tranny, if any.

 

[aweber]

I assume it is a TH350?

The low stall would help a lot for the compression braking thing I imagine.

Do you have the kick down hooked up? When you put it in second does it stay in second or will it kick down to first?

 

[Richard]

Bwahahaha! I was waiting for that one. I don't have a problem going down hills with a properly set up Tranny and TC. All you manual guys have to over compensate with dual t-cases, 10:1 gear reductions, a third leg and the best you can come up with is that old compression braking line.......LoL

There's also that they change gears when you don't want them to, won't run without oil, and tend to catch fire (or maybe that's just with you? :flipoff.

In the end I don't care about the pros of auto, I just like the technical driving part with a manual.

 

[kbrousseau]

Won't run without oil???????????

Most manuals I've seen won't run without oil either, at least not for long.

I really have no opinion either way, just:stir:

 

[Richard]

Won't run without oil???????????

Most manuals I've seen won't run without oil either, at least not for long.

It just needs to get you off the trail, then either one can be towed home at least.

 

[Mini-T]

In that case fill it with water. It will get me off the trail. It is a 700r4 as it states in my signature.

Is there a Coles notes for your sig?

 

[Root Moose]

I've been stuck with a trans with no fluid... anyone know the first boulder field at the beginning of the trail system in Ardbeg at the lake? Right in there things got bad.

I got it off the trail but so what?

If you don't treat your equipment with respect it won't treat you with respect. Very important lesson that day. Well, that and moving your Samurai to a location that is low under the transmission while creating clearance also tends to make changing the transmission a muddy job when the skies open up. LOL

So, which direction can we hijack Adrian's thread into now?

 

[aweber]

Is there a Coles notes for your sig?

It has been in-grained in my mind to ignore signatures :lmao: When reading any forum I don't even notice them anymore

 

[Twinkie]

and tend to catch fire (or maybe that's just with you? :flipoff.

Not just John, I remember a blue Range Rover being on fire on Quinn last year...

 

[Root Moose]

Yeah, but that was a Rover. They will always do something stupid like not start, catch fire, whatever.

 

[Cochise]

HUH?

Nevermind me, I'm on post-op meds, and a little fuzzy in the brain pan. That's my story and I'm sticking to it. :smokin:

With compression braking in an auto trans, this freewheeling is eliminated. You get much more control on the downhills than relying on the brakes alone.

I hope this helps. :beer:

Sure does. Thanks for the ssplaination! :beer::beer:

 

[Cochise]

Dan, with the AW4 this is why people wire in switches to manually override the PCM logic.

The AW4 is shifted via solenoids, not hydraulically. Wire up a switch to keep 1st engaged, and another to keep the torque converter clutch (TCC) locked and you effectively have the same as a manual (2.8x:1 first gear locked behind the natural compression of the engine).

Ahh, cool. Gonna do some searching on this.

The thing with our OBDII Jeeps is that wiring in these switches will throw a code when they are used. Not a big deal, should have a scanner in your boonie box anyway.

Yeap, got one of those.

Out of the box our transmissions will kinda/sorta hold first gear until revs get to a certain level and then switch up to second. I say sorta/kinda because when in 4 Low there appears to be some logic in the PCM to hold first for a while. But it isn't fully implemented like say the HDC on the newer LRs. Very annoying. Not so much because it is bad form to use brakes on downhill descents as much as that it happens suddenly when you are not expecting it if you aren't really familiar with the XJ/AW4 setup.

Hmm, yeah, unexpected surprises of that sort aren't fun.

Thanks for the info. I'm less noob than I was 10 minutes ago now. :clue:


:beer::beer:

 

[Cochise]

What Jon said :beer:

Don't forget Torque Converter slippage as well as clutch slippage. When there is no pressure built up in the tranny, the clutches will slip, but when you have a Manual Valve Body, the pressure is MAX all the time, when the Torque Converter is spinning that is...... But it is all theory to me anyway,

Ahh. Good to know, too. Thanks!

:beer::beer:

 

[BillaVista]

Ahh. Good to know, too. Thanks!

:beer::beer:

He means when the pump is spinning, not the torque converter. They are related, of course - can't spin the pump without the TC spinning, but you can spin the TC (at least the impeller) without spinning the tranny input shaft / pump.