Downhill in lower gear with automatict ransmission

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If you take your trans out of OD and into 3rd gear, I don't see how there is so much heat generated the tranny anyway. It's just in 3rd gear, transferring energy thru it's gears to the engine. The engine is doing all the energy transfer, not the tranny. The transfer thru 3rd gear is probably 97% efficient. I highly doubt any significant damage is happening in the tranny in this case.
 
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Originally Posted By: Olas
If your brakes overheat you need better components. If your transmission overheats you need a thermostatic cooler. Or, if you want better control of what gear you're in, you need a manual.
Precisely, and my sentiments exactly. I personally have never heard of, or experienced a transmission failure from manually downshifting auto's when the situation required. However there is limited choice available in terms of available transmission with some vehicles makes/models that are otherwise fit for purpose in accordance with the owners core requirements. My Darling Sweet Sweet has a Mercedes sedan with an auto(the only option available in oz), which is very frustrating in terms of it's ability to adapt to variations in operating parameters unless it's driven normally and gently in D. The same vehicle had a manual transmission option in other markets. In sport mode it's sort of an improvement under limited circumstances. IMO it's the worst auto I have ever encountered, but she prefers an auto, even though she grew up on a farm and is quite capable of manually shifting gears herself. I would rather have a manual any day with my preference being for the versatility of having a 6 speed. It can effectively be driven as a 3 speed under normal/gentle conditions, with the flexibility of having more ratio options available when required. To get back on topic. With reference to the OP's question there may be an inherent design flaw with the auto which contributes to the failures. Either that, or the units are being operated outside of the design parameters. Only the manufacturer can make the determination. All I know is that "our" next car will most likely be an auto. Experience has shown that anything with a well programed paddle shift ZF 6 speed auto will be the only alternative as far as I can see.
 
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Originally Posted By: supton
My wife has not figured out shifting on an auto. I'm about to forbid her from driving my truck. Not that I blame her, I find driving an automatic to be more work than stick.
In some makes and models of vehicles, I would agree this is certainly the case. However, the Honda vehicles mentioned above would appear to the pinnacle of auto transmission development. It's taken a very long time( over half a century) since the first mass produced auto's were developed to get to this point where the auto transmissions are finally working with us, instead of against us.
 
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Originally Posted By: Hyde244
^^ This is a good strategy most of all because you will have power available quickly, not have to wait for transmission shifting lag. For my 4-speed Automatic, leaving it in overdrive meant the transmission would shift to 4th when I was not accelerating, and then required shifting to 3rd when I pressed the accelerator. Removing overdrive means the transmission stays in 3rd through, and is able to have better throttle response.
Agreed. Or in the case of slower cornering speeds, a timely downshift to 2nd may be advantageous. Modern Transmissions tend to be set up to hold in a higher gear or have the converter in a locked condition for longer than is best for some conditions. It's a product of the relentless pursuit of better fuel economy. Electric cars will negate the issue in the fullness of time. For the time being, a pre-emptive downshift helps to provides a balance to the vehicles dynamics if one can select the correct gear ratio in a timely manner. This can only provide increased safety and control with no downside as far as i'n concerned. This is opposed to the instability created by the vehicle reacting to circumstances after the fact when left up to it's own devices.
 
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Unless you are going down the mountain for miles, all of the discussion is pretty moot. Tell me if your wife is going to take your truck and go for a week out and end up having to drive down a mountain on her own.
 
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Originally Posted By: philipp10
If you take your trans out of OD and into 3rd gear, I don't see how there is so much heat generated the tranny anyway. It's just in 3rd gear, transferring energy thru it's gears to the engine. The engine is doing all the energy transfer, not the tranny. The transfer thru 3rd gear is probably 97% efficient. I highly doubt any significant damage is happening in the tranny in this case.
I don't see it as a problem either since it is a final drive gear. In other words if you were towing with the vehicle locking a 4 speed AT out of OD is acceptable, and recommended. I would use 3 going down a grade w/o hesitation. OTOH second gear can generate a lot of heat in the transmission and might cause problems over time. I've been using third gear in a 4 speed OD transmission and the brakes going down mountains for a lot of years. No problems to report.
 
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Originally Posted By: Vikas
Unless you are going down the mountain for miles, all of the discussion is pretty moot. Tell me if your wife is going to take your truck and go for a week out and end up having to drive down a mountain on her own.
Why does it have to be miles? Can pick up speed pretty quickly going down short 6-8% grades.
 
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These ocd guys who don't want to get their transmissions hot will use up all the thermal reserve in the brakes and go over the side of the cliff with a nice cool tranny.
 
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Quote:
Why does it have to be miles? Can pick up speed pretty quickly going down short 6-8% grades.
Because you can use the brakes sparingly to control the speed without worrying about boiling the brake fluid.
 
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Even in second gear, or first, below redline, the fluid won't get hot. It is much easier to raise engine rpms by engine braking, than to move a car up hill. As I saaaid, that's too little energy to dangerously heat the fluid.
 
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Originally Posted By: demarpaint
Originally Posted By: philipp10
If you take your trans out of OD and into 3rd gear, I don't see how there is so much heat generated the tranny anyway. It's just in 3rd gear, transferring energy thru it's gears to the engine. The engine is doing all the energy transfer, not the tranny. The transfer thru 3rd gear is probably 97% efficient. I highly doubt any significant damage is happening in the tranny in this case.
I don't see it as a problem either since it is a final drive gear. In other words if you were towing with the vehicle locking a 4 speed AT out of OD is acceptable, and recommended. I would use 3 going down a grade w/o hesitation. OTOH second gear can generate a lot of heat in the transmission and might cause problems over time. I've been using third gear in a 4 speed OD transmission and the brakes going down mountains for a lot of years. No problems to report.
Why would 2nd be any different than 3rd? I am not questioning your judgement, just want an explanation.
 
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Originally Posted By: Vikas
Quote:
Why does it have to be miles? Can pick up speed pretty quickly going down short 6-8% grades.
Because you can use the brakes sparingly to control the speed without worrying about boiling the brake fluid.
Sure. But why not drop a couple of gears at the top of the hill and use no brake? That is what i try to do, I know my commute quite well. Although it seems I wind up having to use the brake anyhow--our cars would only need one gear down and they would control speed very well, this truck likes to barrel down hills. I can see why the dealer had to put brake on after only 73k.
 

MolaKule

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The amount of energy dissipated in downshifting is approximately 50% of that used in upshifting. This means upshifting would create about twice the internal heat as downshifting. SO if you can downshift at the crest of the hill, you create less heat. However, downshifting usually involves disengaging the TCC so you're going to have a temp increase at the torque converter's fluid coupler.
 
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I think the answer to the OP's question is really dependent on both the vehicle and road in question. And like somebody said, this is a situation where safety ought to trump transmission fluid. I keep thinking about the three summers I spent working in Wyoming, where a trip to town meant going down a 13-mile, 10% grade. With a 3.42-geared, 4-speed Silverado, I went down that hill in 2nd and still braked quite a bit going into the switchbacks. In 3rd, it would pick up more speed than I felt safe with, and I'm sure I would have overheated the brakes keeping it under control. All summer, I followed minivans with flatland plates and constant brake lights, breathing in the smell of their brakes.
 
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Descending the west side of Rabbit Ears Pass, I guarantee you you will have whatever transmission you have in a lower gear. In two decades plus, I saw a lot of cargo and wreckage spread out along that stretch of US-40. The runaway truck ramp there sees heavy use.
 
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