Why do autos get hot towing?

[SurferUK]

I've only ever driven manuals until now, where you can put whatever you like on the back and go. I get how a torque converter would heat the ATF up, but I thought anything modern was supposed to lock up. So I don't understand where the heat comes from. I don't think manual gearboxes even have cooling fins let alone a link to the radiator.

Help me out auto experts :bounce:

 

[2021nightshade4x4]

I've only ever driven manuals until now, where you can put whatever you like on the back and go. I get how a torque converter would heat the ATF up, but I thought anything modern was supposed to lock up. So I don't understand where the heat comes from. I don't think manual gearboxes even have cooling fins let alone a link to the radiator.

Help me out auto experts :bounce:

Every time it shifts, it goes out of lock-up, then back in. With the reported gear hunting of these transmissions, that’s a LOT of time spent out of lockup, with a load, to have no transmission cooler.

Need SOMETHING. Whether external cooler, thermostat delete… something in that equation has to change or heat will just build up.


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[SurferUK]

Every time it shifts, it goes out of lock-up, then back in. With the reported gear hunting of these transmissions, that’s a LOT of time spent out of lockup, with a load, to have no transmission cooler.

Need SOMETHING. Whether external cooler, thermostat delete… something in that equation has to change or heat will just build up.


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Thanks, that makes sense. Now you mention it. I wonder if the partial lock-up is responsible for ATF smelling burned sooner than expected, and why there's a variance on ATF sample analysis (Clutch material etc) A lot of urban driving would do that. When I first heard about it, my first thought was "Slipping the clutch to save fuel???" although don't get me wrong I'm sure Toyota have the system well worked out given the mileages these can do. But frankly I would rather the slip comes from the TC, and spend the extra couple of percent on fuel.

 

[fourwd1]

Not to mention the added stress on trans and engine working harder under a heavy load towing. Engine and trans are going to run hotter while towing.

 

[2021nightshade4x4]

Thanks, that makes sense. Now you mention it. I wonder if the partial lock-up is responsible for ATF smelling burned sooner than expected, and why there's a variance on ATF sample analysis (Clutch material etc) A lot of urban driving would do that. When I first heard about it, my first thought was "Slipping the clutch to save fuel???" although don't get me wrong I'm sure Toyota have the system well worked out given the mileages these can do. But frankly I would rather the slip comes from the TC, and spend the extra couple of percent on fuel.

Not just unlocked and locked, but the A750 has a “controlled slip” (in between) mode designed to make the transmission feel and act as if it has more gears than it does. The clutch material in the fluid is almost entirely because of this in-between state.

Flex Lock-up Clutch Control (See Page CH-31)

Controls the solenoid valve SLU, provides an intermediate mode for when the lock-up clutch is between ON and OFF, increasing the operating range of the lock-up clutch to improve fuel economy.

https://shop.ukrtrans.biz/wp-content/uploads/catalogs/A750E.pdf


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[SurferUK]

Not just unlocked and locked, but the A750 has a “controlled slip” (in between) mode designed to make the transmission feel and act as if it has more gears than it does. The clutch material in the fluid is almost entirely because of this in-between state.

Flex Lock-up Clutch Control (See Page CH-31)

Controls the solenoid valve SLU, provides an intermediate mode for when the lock-up clutch is between ON and OFF, increasing the operating range of the lock-up clutch to improve fuel economy.

https://shop.ukrtrans.biz/wp-content/uploads/catalogs/A750E.pdf


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From a purely mechanic point of view that's a really horrible way of doing things...but they seem pretty robust so I guess Toyota made the idea work as well as possible.