Automatic Transmission

[Gino Caruso]

Does shifting an automatic transmission into neutral at a stop light (say sitting at idle for more than 3 minutes) do anything for prolonging the condition of ATF oil or A/T itself; i.e. Is it good preventive maintenance or just senseless. Do any experienced mechanics have a opinion on this? This is in relation to A/T temperature and shifting into neutral to cool fluid. Normally I sit at idle in Drive,

Thanks,
Gino

Topic

[Chris]

I think I saw a vid that Eric made that said it was best to shift to P when at traffic lights as this keeps the ATF in the box better than in N and doesn’t speed up damage to the rotors (over time) if left in D with foot on the brake…

[Andrew Button]

In most transes the fluid in P and N doesn’t change location. Keeping the car in drive at a high idle with the brake on, will in fact heat up fluid inside the converter, thus spreading heat throughout the box, that doesn’t mean, however that the box isn’t meant to remain cool when the idle is reasonable, which is the way it should work. Putting the car into N or P at a stoplight is actually WORSE under normal conditions as its very easy to put mechanic strain on the reciprocating mass when the light turns green when traffic is building from behind and with any significant idle speed other than slow, the trans gets a slam into gear which isn’t any better than hot fluild.

[Mike]

Technically yes it does. In a practical application kind of way, AndrewButton is correct that you are more likely to shift into gear under duress and have it slam than to ultimately save any wear on the trans or fluid. He is also correct that the where the fluid goes is the same in Park or Neutral. The difference is that Park engages the parking pawl to immobilize the car, which is why I prefer to use neutral when I do it. I do it for a different reason however, which is to prevent me from having to hold the brakes firm if they are hot, which can cause brake pulsation due to transfer of glues in the brake pad over to the rotor.

Getting back to your question, and the reason why being in neutral saves on ATF wear. The torque converter by design shears the ATF between the turbine, impeller, and stator wheels at all times that the torque converter clutch isn’t locked up (brake standing being the most damaging to the fluid). As Andrewbutton mentioned, this creates heat, and is the main point of heat generation in a automatic trans. As the fluid is sheared while passing between those internal components, the polymer chains that are the basic molecular building block of the fluid are cut into shorter pieces, changing properties of the ATF. This is how the ATF oil itself actually ‘wears out’, aside from being contaminated by clutch and metallic debris.

[Author]

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