Automatic trans and braking


#1

When driving a manual trans car, some people – in addition to applying brake pedal – down-shift or “gear-down” slow down the vehicle. Does an auto trans automatically (and quickly) gear down as one is braking. In my 2002 Toyotoa Echo (auto), when switching between Neutral and Drive WHILE braking, I’ve felt very little diff. in applied brake pressure. In other words: 40MPH->0MPH in Drive seems to require the same pedal brake-force as 40MPH->0MPH in Neutral. I’ve tried this while slowing down from a hill.


#2

You can override the automatic transmissions shift program by manually selecting one of the lower gears but I really see no benefit in doing that. Today’s brakes are so good that they don’t need help from the engine except in extreme situations like a mountain pass. It just puts additional wear on your transmission and engine and makes the engine use more gas to boot. Brakes are a lot cheaper to overhaul than transmissions.
Even when I drive in the mountains of New Mexico, I don’t have the need to do that, because most mountain roads follow rivers, and rivers snake side to side a lot but they don’t go up and down much.


#3

I only downshift when going up or down steep moutain roads. It certainly saves the brakes, going down and helps the engine going up. Under other circumstances you are just speeding up the wearout of the transmission shift mechnism; it fulfills no useful purpose.


#4

An automatic transmission will automatically shift the the proper gear, but it doesn’t gear down like you can with a manual transmission. I don’t think downshifting or gearing down is really a good idea unless you are going down a long steep hill and do it to keep your brakes from overheating, in which case you can manually shift an automatic tranny to a lower gear. Keep in mind whether it is a manual or an automatic, downshifting in everyday traffic to prevent wear in the brakes only increases wear on the transmission and with a manual tranny the clutch also. Brake jobs are cheaper than transmission and clutch repairs.


#5

You are correct. An automatic transmission is simply not programmed to provide reasonable engine braking. It shouldn’t. Sometimes the driver merely wishes to coast at speed rather than decelerate. You can always downshift, if you must, but this action really defeats the purpose of the auto tranny.


#6

Frankly except for race cars and long steep hills, no one needs to use engine braking. The cost usually is more than any savings. Brakes are cheaper than transmissions.


#7

The previous posters are wrong about automatic trannys being able to downshift on their own. Honda Accords automatic transmissions do this when braking. There must be other conditions met in order for this to happen. It’s not predictable, but it happens every now and again. The computer decides when to downshift.

About 10 years ago I rented a Plymouth Breeze. While in cruise control and without touching the brake pedal the transmission would downshift in order to maintain the set speed.


#8

I always turn off the overdrive when in mountain roads. It slows down the shuttling between gears and I suspect downshifting will eleminate a lot of shifts, also.


#9

lars46 wrote: “The previous posters are wrong about automatic trannys being able to downshift on their own. Honda Accords automatic transmissions do this when braking. There must be other conditions met in order for this to happen. It’s not predictable, but it happens every now and again. The computer decides when to downshift.”

I agree that certain cars/models and/or certain conditions can cause an auto trans to downshift, tho’ that doesn’t happen the way it does in a man. trans. vehicle. Personally, when driving a manual on level terrain, I split the difference for braking or slowing: randomly 50% downshift, randomly 50% put-in-Neutral-and-brake.

My orig. query about auto trans and braking had more to do with a type of potentially-fuel-saving driving technique I employ: shifting the vehicle into Neutral well in advance of the next anticipated stop, such as a stop sign or stoplight. Why this may save fuel: Referring to the article ?Plug-in Hybrid Vehicles for a Sustainable Future? (pp 158-165, March/April 2007 edition of American Scientist magazine) it was noted: ??fuel is even consumed in slowing the car, because this is normally done by throttling back the engine after running at speed.? For safety reasons I only use my ?technique? in my daily, familiar driving route – one in which I can predict traffic and stopping/slowing-down distances.

How much fuel/$ have I saved thus far? I haven?t conducted this experiment long enough to gather any quantifiable metrics. I?ll report back if/when I do.

American Scientist article:
http://www.americanscientist.org/template/AssetDetail/assetid/54788


#10

Some auto transmissions have positions marked l,2,3,4,5…on those you can down shift as long as you do so within the speed range of the appropriate gear…I haven’t found it as effective as down shifting a manual.


#11

Typically touching the brake pedal and activating the associated electrical switch causes the TC to unlock. Are you sure you did not mistake this for a downshift?


#12

Note that you should not be in Neutral for any significant period while the car is moving. With a lot of auto transmissions this will damage the trans.


#13

Beadsandbeads wrote:

“Note that you should not be in Neutral for any significant period while the car is moving. With a lot of auto transmissions this will damage the trans.”

How precisely is this possible? What’s the mechanical explanation for this?


#14

The Echo is like a normal car and won’t downshift while braking. I don’t know the cars that will. You won’t do any harm by shifting out of OD but it couldn’t hurt at all just to leave it alone. When you shift to neutral, you could miss and get into reverse. One time and you could lose the transmission. It happens enough for you to want to be careful. If your shifter doesn’t have a guard to prevent that, you should never shift to neutral while going forward. Don’t downshift a FWD on slippery surfaces or you will end up in the ditch. RWD straightens itself out, FWD doesn’t.


#15

According to Tom and Ray’s books, shifting back and forth between drive and neutral will cause unnecessary stress to the drive train components. However, when Beadsandbeads says that leaving a car in neutral will damage a lot of automatic transmissions, he might be referring to the damage that can be done to a car that is towed with all four wheels on the ground without being modified to make it safe. This is basically the same thing as coasting in neutral for an extended period. The wheels are still connected to the automatic transmission and the gears turn, but the transmission fluid doesn’t circulate.


#16

If the engine is running, the transmission has oil pressure and oil is circulating. Howeve, I recommend just leaving the transmission in drive for all normal driving. The guys who designed the car’s transmission probably know what they are doing. I pretty much shift gears on a manual transmission about the same that automatic transmissions are programmed to shift and I never downshift through the gears for engine braking. I just coast to a stop in neutral or in fifth with the clutch disengaged and go straight to first gear when it’s time to go, unless I’m still rolling when a light turns green, then it goes into whatever gear is appropriate for that speed.


#17

Jeremy_R_Hoyt wrote:

?This is basically the same thing as coasting in neutral for an extended period. The wheels are still connected to the automatic transmission and the gears turn, but the transmission fluid doesn’t circulate.?

I had assumed ? right or wrong ? that Neutral was like an ?unlocked? Parking ?gear?. If Jeremy?s statement is generally true for all modern cars and/or for specific brands/models, then this is important. Or is B.L.E?s statement more correct: ?If the engine is running, the transmission has oil pressure and oil is circulating.?
I encourage anyone with more info on this topic to respond.

Shifting between N and D can be done with JUST the shifter lever. Going into R or P requires one to depress the lock-release button on the shifter, minimizing accidentally shifting into R while driving.

Just to note: when I switch from N to D and D to N, the transitions are very smooth (one?s foot must be off the accelerator, of course). Maybe it’s just the Echo ? other makes?/models? auto transmissions may not handle N to D and D to N shifting as easily? However, one thing I have noticed in all auto-trans vehicles is less noise/shuddering when car is stopped and in N vs. stopped and in D.


#18

I know that this was true, back in the '50s and '60s with some automatic transmissions–the ones that had a fluid pump in the front half of the transmission but did not have a fluid pump in the rear of the trans. However, transmission design has changed a great deal since those days and I question whether this is still true today. Is Transman out there to answer our question?

A check of the Owner’s Manual would likely tell someone if their transmission would be damaged by this type of behavior.


#19

In the '50s and early '60s, many automatic transmissions had two oil pumps. One on the input shaft and also one on the output shaft. The rear pump made it possible to push start these cars. In the mid sixtys or so, the rear oil pump was omitted and cars with automatics could no longer be push started.
Without hydraulic pressure, an automatic is in neutral regardless of the gear selector position. Hydraulic pressure pushes on pistons that put pressure on clutch plates that engage various gears. On planetary gear sets, brake bands that hold the ring gears stationary act as clutches. A lot of front wheel drive automatics are built a lot like manual transmissions except the synchronizers and dog clutches are replaced by hydraulic clutch packs that resemble wet clutches used in motorcycles.


#20

That’s true. If the engine is running, it isn’t the same as towing it on four wheels.