Coasting in Neutral - Page 2 - Mercedes-Benz Forum

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post #11 of 12 (permalink) Old 11-28-2006, 12:14 PM
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It really sounds like there's a fair amount of confusion as to how ATF is circulated though an automatic transmission. First, I'm not a transmission guy, and have never rebuilt one. I've done clutches in a manual transmission and replaced front and rear main seals, but that's about it. The very little I know about slush boxes is what I'm about to post.

Unlike in an engine, where oil always follows the same path, ATF doesn't always follow the same route. ATF typically flows from the pan through the filter, to the pump. Oil pumps are driven by the pump drive hub of the torque converter. This means when the torque converter cover is rotating, the oil pump is pumping. After the fluid leaves the pump, the flow of ATF is dictated by a part called the valve body. The first stop for the ATF in a valve body is the pressure regulator valve. A pressure regulator valve is one of the few parts I know of that exist in all automatic transmissions. However, sometimes that valve is physically separate from the valve body. At any rate, after leaving the regulator valve the fluid goes to one or more of the other valves in the valve body. The job of valve body is to regulate (hence the regulator valve) and direct fluid (the purpose of the other valves in a valve body).

The valve body, which is in charge of directing fluid, knows whether or not the transmission is in drive, or whether it's in neutral. <Caution WAG alert> It is entirely possible that parts that ordinarily receive "x" amount of ATF at 60MPH while in drive, only see a fraction of that amount of ATF while in neutral. If so, those parts which are supposed to be bathed in ATF at speed may severly overheat if they don't receive the correct amount of ATF. </caution> If anyone here KNOWS the ATF flow schematic of their automatic transmission, then they KNOW whether shifting into neutral while moving is safe. Those that don’t know, like Peter Guenther said, are playing "russian roulette" with several thousand dollars worth of transmission parts.

Lastly, although ATF shares the radiator with the engine coolant, the ATF is cooled by air.
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post #12 of 12 (permalink) Old 11-30-2006, 08:06 AM Thread Starter
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Nice post - full of thoughtful information, which is just what this forum is all about.

Hopefully someone who is a specialist on MB auto transmissions will finally put all our minds at rest....
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