Downhill in lower gear with automatict ransmission

JHZR2

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Some seem to get it, others don't. Its all about necessity. Doing it all the time to save brakes is silly. ONLY braking on severe terrain to save the engine or trans is also silly. One has to make executive decsisions with a bit of smarts to avoid excess wear and tear when they can, yet ensue safety. My data points for camry and Accord ATs are from terrain where it is necessary due to severity of it.
 

tqh

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Thanks a lot for the responses. I have to regularly do a ~20 mins of steep descent, hence the question. If engine braking doesn't harm at all, I'll just do it smile If it harms only a little I'll use the brake.
 

jaj

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If you're doing a long, steep descent (we have a bunch within about four hours of where I live) you should disengage overdrive or drop the transmission by one gear - RPM should maybe rise to 2500RPM or so. You want additional braking; it's there to help, not to do the entire job. Then use the brakes to keep the speed under control: pick a max speed slightly faster than traffic and a slow speed slightly slower. When the car coasts to the max speed, slow it down to the minimum speed with a firm brake application, then release the brakes and allow it to coast back up the to max again. This will keep your brakes from overheating. What you're avoiding is brake fade or failure. If you just hold your foot on the brake to control the speed, heat just keeps building up and up and up until the pads fade or the fluid boils. This is because the pads stay pressed on the disks, and heat transfers from the disk through the pad to the caliper. The longer you hold the pedal, the hotter the whole assembly gets. However, if you brake and release, the rotor temps spike up more but they're cast iron, so they don't care. What matters is that between applications, the calipers aren't pressing the pads against the rotors. Heat transfer stops and they get a chance to cool down between applications; they're less prone to overheat.
 

JHZR2

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Originally Posted By: tqh
Thanks a lot for the responses. I have to regularly do a ~20 mins of steep descent, hence the question. If engine braking doesn't harm at all, I'll just do it smile If it harms only a little I'll use the brake.
20 minutes where it can stay in one gear and remain?
 

01rangerxl

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Originally Posted By: JHZR2
Some seem to get it, others don't. Its all about necessity. Doing it all the time to save brakes is silly. ONLY braking on severe terrain to save the engine or trans is also silly. One has to make executive decsisions with a bit of smarts to avoid excess wear and tear when they can, yet ensue safety. My data points for camry and Accord ATs are from terrain where it is necessary due to severity of it.
I wasn't trying to discredit your data points...both those brands have questionable FWD autos anyway. Someone else mentioned a blown Matrix trans...I've seen a Matrix fry its AT at 80K miles. I was just pointing out that in some situations for some transmissions, OD off is preferred for durability reasons because it may significantly reduce the number of shifts the trans has to make (which also raises temp). Ford seems to suggest turning it off whenever heavy loads are involved, and IIRC, in some towing manuals they say OD must be turned off (not just suggesting it). Bottom line, read the manual, it will tell you why they put the feature there if available, and when to use it. Not all "OD off" modes operate the same, so it's important to know what the manufacturer suggests for your particular vehicle. The way it functions in my truck is much different from my Colorado at work. With the Colorado, there is no "OD off" mode, but you can manually select the first three gears and it will hold those gears. Haven't played with it too much because I don't want to beat the work truck more than it already is, but on my truck you can manually select the first two gears and lock it out of anything above "4" with the button...the actual programming is more complicated than that, but that's the basic idea.
 
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Flatlanders just don't get it. But I guess everyone feels a need to contribute whether or not they have any experience whatsoever. Like all the people who are against additives that they've never tried.
 
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I don't get how the occasionaly downshift can wear clutches and heat fluid...they engaged/disengaged on the way up the gears, and will do so as the car slows...they are all perfectly normal activities in the trans. The clutches aren't doing (much) braking, the engine is. Not suggesting that you should "row" an auto tranny, by any means. I taught my partner to drive, partly in a 1978 Holden V-8 Auto (3 speed). Big (for Oz) car. Coming into town, there is a "hill", that can take a Prius from dead "empty" to "full" about half way down (and the Prius keeps charging). Brake smell at the bottom of the hill every time she drove, and I couldn't get her to pull "2" for the descents. Put on my best Robert Duvall voice one day, and said that we were going to do the hill my way, then do it her way. My way was in "2", for her way, I would zip it. When we hit the 60km/hr sign on the edge of town, she was to do a n emergency stop as 'though a kid had run on the road. Did it my way, and stopped, and put a brick on the kerb. Her way, and she was yelling as she tried to stop that the car wasn't stopping (it was, and it was stopping well, but there just wasn't as much available), and we pulled up maybe 20 feet further down the road. We don't "row" our autos (and manuals), but DO select a gear that provides appropriate engine braking ability, especially on long grades. As an aside, the vaccum modulator on that tranny went, and I discovered what I had previously read, that in the Holden Trimatics, they only apply the over-run band/clutch on 1st what "1" is selected. On the way up, power is engine to diff, so there is no need for engine braking and they don't apply that clutch. Vaccum modulator gone, it wouldn't change to second in drive, but had no engine braking.
 
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If I lived in or near mountainous terrain and was burning up brakes or auto trannies, I would consider buying a MT in my future vehicles. That's all I drove when I lived in PA and CO when I was a youngster and I never felt uncomfortable unless it was icy...it's all about praying then.
 
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Originally Posted By: SOHCman
If you have ever decended parts of the rockies, you HAVE to engine brake or you will melt your rotors into molten chunks. Even with engine braking in 3rd, I still had to pull off into a cool down chute to let my brakes cool down and stop stinking. When i finally got to the bottom an RV was off the side of the road with flaming front wheels. It had markings of a rental so i hope they got the insurance.
Originally Posted By: 01rangerxl
From my truck's owner's manual... Drive (O/D OFF) provides more engine braking than Overdrive and is useful whenever driving conditions (i.e., city traffic, hilly terrain, etc.) cause the transmission to excessively shift between Overdrive and other gears. Deactivate (Overdrive) when: • driving with a heavy load. • towing a trailer up or down steep hills. • additional engine braking is desired. It seems to imply that excessive shifting, especially with a load, is more of a concern than engine braking raising temps. I monitor temps with a Scangauge and with my truck they are about the same whether OD is on or off. I have turned OD off many times in the truck's life, sometimes for extended periods (hundreds of miles while towing), and it shifts like new at 150K miles.
Originally Posted By: JHZR2
Some seem to get it, others don't. Its all about necessity. Doing it all the time to save brakes is silly. ONLY braking on severe terrain to save the engine or trans is also silly. One has to make executive decsisions with a bit of smarts to avoid excess wear and tear when they can, yet ensue safety. My data points for camry and Accord ATs are from terrain where it is necessary due to severity of it.
Originally Posted By: jaj
If you're doing a long, steep descent (we have a bunch within about four hours of where I live) you should disengage overdrive or drop the transmission by one gear - RPM should maybe rise to 2500RPM or so. You want additional braking; it's there to help, not to do the entire job. Then use the brakes to keep the speed under control: pick a max speed slightly faster than traffic and a slow speed slightly slower. When the car coasts to the max speed, slow it down to the minimum speed with a firm brake application, then release the brakes and allow it to coast back up the to max again. This will keep your brakes from overheating. What you're avoiding is brake fade or failure. If you just hold your foot on the brake to control the speed, heat just keeps building up and up and up until the pads fade or the fluid boils. This is because the pads stay pressed on the disks, and heat transfers from the disk through the pad to the caliper. The longer you hold the pedal, the hotter the whole assembly gets. However, if you brake and release, the rotor temps spike up more but they're cast iron, so they don't care. What matters is that between applications, the calipers aren't pressing the pads against the rotors. Heat transfer stops and they get a chance to cool down between applications; they're less prone to overheat.
All of these receive +1.
 
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Originally Posted By: tqh
Thanks a lot for the responses. I have to regularly do a ~20 mins of steep descent, hence the question. If engine braking doesn't harm at all, I'll just do it smile If it harms only a little I'll use the brake.
A few years ago my wife and I were driving down from Pike's Peak in a rented car. The park rangers were stopping cars at a station half way down, using a heat probe to check out the front brakes, and waving folks whose brakes were hot over to a parking area for a cooling off before they drove down the rest of the way. The ranger got a green on my brakes, gave us a big smile, and on we went. It only takes one experience with brake fluid boiling or total brake fade before you "get it" about using engine braking instead of brakes on long downhill grades. To the saying "brakes are cheap, gears are dear" I respond "life is dear". If you think it's an issue for your transmission, have the fluid changed out for something better like Red Line or Amsoil and put an oil cooler on it.
 
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I manually downshift the auto in the Jeep regularly. Being a pretty dumb engine and transmission (electronics-wise), it takes well to a throttle blip for a rev-match on the downshift making it nice and smooth. I've dropped to 2nd at 4500 rpm a few times with no complaint from the trans and no other issues. I've done this since I've owned the Jeep (a little over 62k miles at this point) and at 162k total, the original trans is still quite healthy.
 
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Hello, Great, nuanced thread. May I add, with a nod to the subtleties offered by many, that in my experience with mountainous, downhill driving people are seldom given to selecting a "low enough gear". And, as always, they want to drive too fast all the time. jaj's post from p.2 addresses the method. Others have mentioned that making good judgements are required here. Inclines vary as do final drive ratios. You have to thoughtfully drive your vehicle. Whilst descending, I've always needed an additional gear between the two I can use. The thing to do there is to GO SLOWER. Remembering to take it real easy on the gear range selector lever is another big part of these scenarios. Kira
 
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People say the same thing about this in stickshift, yet I've yet to wear one out downshifting. Lets not forget the foolish auto Trans is shifting plenty going up the hill. If you let it, that is. I bet it makes more heat then. Also, if you have to stop at the bottom why not downshift?
 

MolaKule

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Originally Posted By: tqh
I have a Toyota Camry with an automatic transmission with a shift knob to force the maximum gear, i.e. if I move it to 3rd, it'll only choose among gears 1, 2, 3, but not 4 or 5. When going downhill I can shift to one of the lower gears to slow down the car without using the brakes. Is it bad for the transmission - as long as I keep the rpm within a reasonable range (< 3000 rpm)
Let's keep in mind the context and wording of what the OP said: He is driving a Toyota Camry in "hilly" terrain, not an 18 wheeler, not a Pickup pulling construction equipment or a horse trailer, not an RV, and he did not say anything about negotiating Pikes Peak.
 
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Originally Posted By: MolaKule
Quote:
tqh: Is it bad for the transmission - as long as I keep the rpm within a reasonable range (< 3000 rpm)
Consider this, high temperature in a transmission degrades the fluid through oxidation, and increases wear in the clutch packs, etc. Using a transmission in a lower gear to do your braking raises internal transmission temperatures. Brakes jobs cost much less than a tranny replacement. I think a better approach is to anticipate the hill and slow at the top so your descent speed will be slower as well.
Stirring the pot I see with bad info? smile
 

tqh

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I notice that the ATF temp goes up to ~200F when I get to the top (usually 140-150F on flat highways), it drops a little bit when I go downhill _regardless_ of whether I use the brakes or engine braking.
 

MolaKule

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Originally Posted By: tqh
I notice that the ATF temp goes up to ~200F when I get to the top (usually 140-150F on flat highways), it drops a little bit when I go downhill _regardless_ of whether I use the brakes or engine braking.
Are you observing the coolant temperature or actually measuring the ATF fluid temp? With my OBDII instrument I can measure ATF fluid temps but my gauges only show coolant temps.
 
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MolaKule

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Originally Posted By: GaleHawkins
Originally Posted By: MolaKule
Quote:
tqh: Is it bad for the transmission - as long as I keep the rpm within a reasonable range (< 3000 rpm)
Consider this, high temperature in a transmission degrades the fluid through oxidation, and increases wear in the clutch packs, etc. Using a transmission in a lower gear to do your braking raises internal transmission temperatures. Brakes jobs cost much less than a tranny replacement. I think a better approach is to anticipate the hill and slow at the top so your descent speed will be slower as well.
Stirring the pot I see with bad info? smile
You don't know me very well do ya? LOL
 
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Originally Posted By: MolaKule
Originally Posted By: tqh
I notice that the ATF temp goes up to ~200F when I get to the top (usually 140-150F on flat highways), it drops a little bit when I go downhill _regardless_ of whether I use the brakes or engine braking.
Are you observing the coolant temperature or actually measuring the ATF fluid temp? With my OBDII instrument I can measure ATF fluid temps but my gauges only show coolant temps.
I notice the exact same behavior measuring Transmission Fluid temperature through my SGII. The trans temp sensor is in the pan, bathed in fluid. Flat highway: 140-160 dependent on ambient Charging up a mountain: 180-200.(Seen 210 once) Going down the other side: Drops quick to 190 then slowly to 180. Once it passes 180 though it will never cool below that point unless shut off and allowed to cool.
 
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