Downhill in lower gear with automatict ransmission

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Originally Posted By: Colt45ws
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.
I notice that one of the vehicles in your sig is listed as out of service due to a blown tranny. Does that have anything to do with downshifting during hill descents? wink
 
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Originally Posted By: NMBurb02
Originally Posted By: Colt45ws
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.
I notice that one of the vehicles in your sig is listed as out of service due to a blown tranny. Does that have anything to do with downshifting during hill descents? wink
No.
 

01rangerxl

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Originally Posted By: Colt45ws
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.
Mine is similar on flat highway, except it will not go above about 193*F max. That is the absolute highest I have seen it, usually it tops out at 191. The data is from the factory temp sensor, not sure where it's installed on the trans. It drops quick to 180 and will continue cooling to about 170. This scenario would be towing or hammering it up grades in 90-100 ambient temps. It has a factory installed transmission cooler in front of the radiator, and it also runs the fluid through the radiator cooler. Seems like overkill for a truck that did not have a factory towing package, but I'm happy to have all that cooling.
 
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Originally Posted By: 01rangerxl
Originally Posted By: Colt45ws
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.
Mine is similar on flat highway, except it will not go above about 193*F max. That is the absolute highest I have seen it, usually it tops out at 191. The data is from the factory temp sensor, not sure where it's installed on the trans. It drops quick to 180 and will continue cooling to about 170. This scenario would be towing or hammering it up grades in 90-100 ambient temps. It has a factory installed transmission cooler in front of the radiator, and it also runs the fluid through the radiator cooler. Seems like overkill for a truck that did not have a factory towing package, but I'm happy to have all that cooling.
I was running it hard to get that 210. I dont slow for grades, just bomb up them at 75-80. On a summer day with A/C on, carrying near GVWR in people + luggage and it happens. The factory cooler is pretty large and gets the job done. Its thermostatically bypassed and does not route through the radiator.
 

tqh

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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.
Yeah it's ATF, took me a while to get the correct code to fetch that temp with the ScanGauge. Coolant temp is a lot more stable.
 

tqh

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Originally Posted By: Colt45ws
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.
Yes! I only seen 240 once when I charged up Mt. Washington Auto Road. I stopped right away when I saw that temp.
 
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I have an SGII in my Tundra, and watch ATF very often. 160-180F on the flats; in the summer, on long highway drives it basically sits at coolant temp (hardly a surprise, as routes coolant to the trans to heat it) which is 190F in summer (183 in winter for the record). Climbing hills in summer can spike it to 200+F on the ATF; I've seen 230 with very little effort on my part. But not higher than 230... Best I can tell the sensor I'm seeing is the convertor outlet, but there are two and no one seems to know which is which. I should figure out how to install some thermocouples at some point. The temps come down with seconds of lockup though, so I strongly suspect it is convertor outlet. I do see temps rise slightly while using engine braking. I do not think the convertor locks back up under braking, ergo the bump. But it is not the 50F bump I can see while going up the hill. Which is not surprising: how much hp does it take to spin a motor with the throttle plate closed? I presume much less than it takes to climb the hill. The convertor probably has lousy coupling going the wrong way, but I have not seen much more than 10F bumps. Yet.
 
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Originally Posted By: supton
I have an SGII in my Tundra, and watch ATF very often. 160-180F on the flats; in the summer, on long highway drives it basically sits at coolant temp (hardly a surprise, as routes coolant to the trans to heat it) which is 190F in summer (183 in winter for the record). Climbing hills in summer can spike it to 200+F on the ATF; I've seen 230 with very little effort on my part. But not higher than 230... Best I can tell the sensor I'm seeing is the convertor outlet, but there are two and no one seems to know which is which. I should figure out how to install some thermocouples at some point. The temps come down with seconds of lockup though, so I strongly suspect it is convertor outlet. I do see temps rise slightly while using engine braking. I do not think the convertor locks back up under braking, ergo the bump. But it is not the 50F bump I can see while going up the hill. Which is not surprising: how much hp does it take to spin a motor with the throttle plate closed? I presume much less than it takes to climb the hill. The convertor probably has lousy coupling going the wrong way, but I have not seen much more than 10F bumps. Yet.
That does sound like the converter outlet. 50F rise while unlocked and powering up a hill sounds about what I thought it would be. No they typically unlock the converter when you lift off. But, like you said the HP to keep a 4000lb brick going 70mph up a hill is far more than the amount to spin a engine while going down the other side.
 
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If you ride your brakes down a mountain, they might not be there is a few miles. It is not about wear... it is about safety. As someone who lives in the mountains and commutes over a range regularly: downshift (just not into 1 or 2nd at highway speed). Folks need to remember to keep their tranny fluid in good condition and change it every so often. If you do that, you should be golden. First, if you are not normally driving through the mountains, the once in a blue moon downshifting is not going to kill a tranny unless you neglected your fluid change. If you drive up and down constantly, keep your fluid fresh. Downshift for your brakes are cool and ready in the case you need to use them. People (especially those from flat areas) get stupid in the mountains and think the speed limit dropping from 70 to 50 is a joke. It is easy not to have high RPM and be in a lower gear without a substantial load. Plus, not only is there fading issues, if the hill is bad enough, you can seize your calipers too...
 
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Some transmissions will do this on their own. The grade logic in Honda transmissions will downshift to 4th or 3rd if brakes are used and the car isn't rapidly slowing down (so the driver is "riding the brakes"). The PCM senses that and downshifts the transmission to help maintain speed. I've never owned a transmission with excellent shifting logic until we bought our MDX (and then later, our CR-V). The first summer we owned it, we drove to Pittsburgh to see my wife's brother and it was a pure pleasure to drive in the ups-and-downs of the West Virginia hills.
Originally Posted By: FutureDoc
People (especially those from flat areas) get stupid in the mountains and think the speed limit dropping from 70 to 50 is a joke. It is easy not to have high RPM and be in a lower gear without a substantial load.
Sounds like you're talking about I-40 near Asheville. I've driven that numerous times. I'm more familiar with the climb up to Fancy Gap, VA, on I-77 coming out of the Mount Airy area. Beautiful country, but with roads that demand respect.
 
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Originally Posted By: Hokiefyd
Some transmissions will do this on their own. The grade logic in Honda transmissions will downshift to 4th or 3rd if brakes are used and the car isn't rapidly slowing down (so the driver is "riding the brakes"). The PCM senses that and downshifts the transmission to help maintain speed. I've never owned a transmission with excellent shifting logic until we bought our MDX (and then later, our CR-V). The first summer we owned it, we drove to Pittsburgh to see my wife's brother and it was a pure pleasure to drive in the ups-and-downs of the West Virginia hills.
Originally Posted By: FutureDoc
People (especially those from flat areas) get stupid in the mountains and think the speed limit dropping from 70 to 50 is a joke. It is easy not to have high RPM and be in a lower gear without a substantial load.
Sounds like you're talking about I-40 near Asheville. I've driven that numerous times. I'm more familiar with the climb up to Fancy Gap, VA, on I-77 coming out of the Mount Airy area. Beautiful country, but with roads that demand respect.
Yup, I26 too. My wife's family is all over the Apps so we do the 40, 26, 77, 81 a lot. My Subaru downshifts if you are going down a certain grade if you just tap the brakes. The downside, it that it likes to hold onto the lower gear long than I want. I quick flip from D to 3 to D upshifts.
 
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Our MDX will downshift on the downhill with steady brake application of about 2-3 seconds. It'll hold that gear most of the way down the hill, and 3-5 seconds if it flattens out. Then it'll upshift. If you start up another hill at the end of the last, the MDX holds the lower gear up the next hill. It's just sweet. We've made this trip twice, and the first time was with our Chrysler T&C. The hunting of that transmission was annoying, bordering on the ridiculous. At the slightest let-up of the accelerator going up a hill, it'd upshift and lock the TC in 4th, then it'd have to go right back into 3rd when the engine speed dropped. And it wouldn't downshift going down a hill unless you pulled it into 3...so I was doing a lot of that. You had to "manually" drive that transmission to make it livable.
 
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Sometimes you're just plain smarter than the transmission. I have a section of road near me that my truck is just better off in 5th than sixth. Rolling hills at 50-55, with an overall elevation gain--going one way I can get it to stay in top gear but going the other way I'm almost better off staying in 5th for at least a mile. That 9 mile stretch often knocks a half-mpg off my "per tank" mpg readout (depends upon how many miles on the tank). For whatever reason my transmission would rather unlock the convertor than downshift. I often will force a downshift, and yes I can hear and see the rpm climb. Like 100rpm. Or less, if I were to use a bit more throttle with it unlocked it'd probably match rpm--but certainly not heat generation.
 
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Originally Posted By: satinsilver
The Acura transmission shifting sounds like the Grade Logic feature at work.
Right, and that was my initial point. The OP shouldn't be concerned about manually shifting the transmission into a lower gear, because many transmissions are programmed to do this already. In other words, it won't (or shouldn't) shorten the life of the transmission.
 
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With all of the years that I have traveled from NY down into PA, through the VA's, NC, SC and all the way to FL, I have never touched my auto trannys. I have left them in "D" and...have a nice day! I'd let the tranny just do what it does. Often stopping in various states including GA to visit friends along the way. Never burned up a tranny! smile
 
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Going downhill in a lower gear is *engine* braking, not transmission braking. It's the pumping losses in the engine that use up the potential energy. Thought experiment -- would it make a difference if you were doing this with a manual transmission? I just can't see how engine braking causes abnormal stress on the transmission.
 
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Energy is still being transmitted between the wheel and the engine... thus the tranny gets some. Basically some of the energy is being scrubbed off into heat in the tranny through mechanical inefficiency. Just like some of the energy is lost going from the engine to the wheels, the same thing is happening with engine braking. If your tranny or fluid is not efficient at dealing with that bit of heat, then there might be a problem.
 

MolaKule

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I think I know why I am seeing a momentary ATF temperature rise when doing a “coasting downshift” in both vehicles. Coasting downshift is defined as a transmission downshift (gear change to a lower gear) and a subsequent decrease in fuel flow. In a situation in which the Torque Converter Lockup Clutch is disengaged during a coasting only with no downshift, the temperature spike is very short. System Energy Considerations: When moving on a flat surface or up a hill, the engine is transferring mechanical energy to the vehicle, via the Torque Converter (TC), and either maintaining or increasing its kinetic energy, respectively. Engine coolant temperature increases on the hills, and reaches equilibrium when on level roads. When the vehicle is coasting or going downhill during deceleration, the potential energy of the vehicle is being transformed as reverse Torque, via the TC, back to the engine in the form of mechanical energy to increase engine RPM for drag. Engine coolant temperature decreases because little fuel is being burned with the majority of the heat coming from the internal friction of the engine. Detailed Mechanical Considerations: Review: Let’s consider a four-element torque converter (TC). The engine is on the left, TC and AT is toward the right. The flywheel is on the crankshaft of course, and the flexplate is connected to both the flywheel AND TC Impeller Pump. The Impeller pump forces fluid forward (to the left) through the stator and into the Turbine. The Turbine, splined to the transmission input shaft, reacts and drives the input shaft to the transmission gearing. Thus mechanical energy in the form of Torque is transmitted via the fluid coupling of the TC assembly. In addition, the stator has a “one-way” clutch free wheeling mechanism. The free wheeling one-way clutch prevents stator backward rotation and stator blades redirect the oil with little energy loss so the fluid enters the impeller in the same direction of the rotation as that of the impeller. The extra (fourth) element is the Torque Converter Lockup Clutch which is also splined to the transmission input shaft and controlled by actuator valves. In most transmissions, the lockup is only programmed to activate in 3rd or 4th gear. In certain transmissions such as my PathFinder, the Torque Converter Lockup Clutch can be manually forced into a “lockout” condition at any time by a switch on the console shifter. I find this feature to be especially useful in ice and snow conditions in four-wheel drive, since extra drag is induced when decelerating on slippery surfaces. During a coasting downshift transition, the turbine accelerates the oil flow towards its outside and into the impeller where the oil’s energy is absorbed in trying to increase engine speed, due to which the stator is also forced to overrun. This type of operation, although not efficient, helps to slow the vehicle by transferring some of the vehicle’s potential energy to the engine. The fluid in an ATF provides these primary functions: 1. Lubrication, 2. Cooling, 3. Friction control at the friction surfaces 4. Wear control in the planetary gearing system and bearings. In addition to the oil flow description given in the previous paragraph, gear translation is further impacted by a rise in the interface temperature of the on-coming band and drum and/or the interface temperature rise of the on-coming clutch and plate, which increases the likelihood of a torque reversal during the gear shift. If interface temperatures rise, so does the ATF temperature. When a transmission’s Torque Converter Clutch is locked up to the corresponding Impeller friction surface, power flows directly from the engine to the transmission gearing. TC fluid temperature drops. When a transmission’s Torque Converter Clutch is unlocked, the energy coupling is provided by the TC’s fluid coupling. Hence TC fluid temperature rises momentarily until equilibrium is reached. When a “coasting downshift” is initiated, both the TC fluid temperature rises and the internal fluid temperature from the clutch packs rise. Because the engine coolant temperature is decreasing, the ATF fluid temperature (lagging behind the coolant temperature) will be decreasing as well.
 
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Why would the clutches run hotter? I can see heating from a shift, but once engaged they should not slip. If trans pressure was low, I guess they could be slipping, but that would be a trans design problem, no? I've reviewed some Toyota document about their six speed, and it appears to have power flowing through the trans changing during engine braking. Only in reverse and gears 1-4, 5&6 do not appear to do this. I sorta understand how the planetaries work, but w/o spinning a stack of three of them I don't know exactly what is going on when they engage something during braking--I'd think the ratio would change, but that really doesn't make much sense to me...
 
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