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I saw a statement that a transmission will be damaged by flat towing in neutral, How? Doesn't putting a vehicle in neutral disengage the transmission?

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  • No, the transmission is always connected to the driven wheels. Putting it in neutral "breaks" the connection between the engine and the transmission so that the transmission is no longer driven by (or driving in this case) the engine.
    – dlu
    Commented Aug 27, 2016 at 17:56
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    @dlu - fill that out a little and make it an answer. Add to it why this is very important to automatic transmissions (no lubrication without engine running). Manual trannys are dependent upon the design. Commented Aug 27, 2016 at 18:10
  • For RV owners that insist on four wheel flat towing (not desiring a tow dolly or trailer to deal with) a pump kit is sold by RV supply houses and towing supply companies. I prefer the tow dolly. Commented Aug 27, 2016 at 21:45

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Unlike most manual transmissions that rely on "splash" lubrication, automatic transmissions use an engine-driven pump to circulate the fluid, which serves as both a lubricant and a hydraulic fluid for various servo actions within the case.

Flat towing in neutral spins various components in the output section of the transmission, many that require lubrication. Since the internal pump isn't running, and length or distance of towing in this condition can cause permanent damage.

The driveshaft should probably be disconnected in these conditions if a flat tow is not avoidable.

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    Many manual transmissions that rely on splash lubrication depend on movement of the input shaft and the gears attached directly to it to ensure proper lubrication of the entire gear train. If only the output shaft is moving then excess heat is being created in the bearings between the gears that are turning and the input shaft that is not. Even heavy truck manual transmissions have to be disconnected by either pulling the driveshaft off the front of the differential or pulling axles out before towing any kind of distance to prevent burning out the bearings on the transmission's input shaft.
    – Michael C
    Commented Aug 28, 2016 at 1:31
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    @MichaelClark Agreed. Especially in the conventional arrangement where the layshaft is the only one low enough in the "sump" to stir the stuff up. Bottom line is to consult the owner's manual or relavant OEM information before a flat tow ... which, in my opinion, is a last resort idea in any case.
    – SteveRacer
    Commented Aug 28, 2016 at 3:18
  • Yeah, but when it's a truck tractor hooked up to a loaded trailer you don't have the choice of loading it on a roll-back.
    – Michael C
    Commented Aug 28, 2016 at 6:47
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The transmission is always connected to the driven wheels, so when the car moves the transmission will turn. If the transmission is in gear then the engine will also turn, putting the transmission in neutral will break the connection to the engine. This means that the input shaft of the transmission is no longer turning, but everything else will be.

This is a big problem with most, maybe all, automatic transmissions as their lubrication pumps are driven off of the "engine side" of neutral – in normal circumstances this ensures that there is lubrication in neutral, but when the engine is stopped and the transmission is being driven by the wheels it is a *Bad Thing®."

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Towing an auto transmission is bad because the driveshaft turns the rear pump. The pump will increase pressure to the control system according to speed .At some point the pressure will fill the torque converter/fluid coupling and shift into some gear for the speed. This will try to turn the engine. If the torque converter is empty there is no problem .Auto trans have different designs so you don't know what "gear" it is in when trying to turn the engine unless you know the specific transmission. The old hydromatics would engage fourth gear at 30 MPH with pressure from the rear pump. That is why they had to reach that speed for a push start to turn the engine ( I never did it). Lube has very little to do with it.

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