It's pretty simple but hard for many to understand.
In normal operation, the transmission is designed for the input shaft to be "connected" to the output shaft. It's part of the reason to use an automatic, there is never a moment where power isn't transferred through the trans.
In each gear it is mechanically tied to the output shaft by the clutches.
In 1st gear and 2nd gear you have gear reduction/torque multiplication.
In 3rd it's direct.
The trans isn't intended to be shifted to neutral while moving.
With any VB except the "clean or safe neutral" you shift to neutral and the clutches all release. With a full manual VB with no intermediate band, 2nd gear is similar to neutral as far as the direct drum is concerned.
When you shift to neutral, the clutches disengage, specifically the direct clutches. Then when you decelerate, the input shaft is no longer tied to the rest of the transmission.
The output shaft becomes the driving member.
You have reversed the power flow in the trans, so the gear reduction planetaries have just become an overdrive unit driving the 14 lb cast iron, 40 yr old direct drum at a 248% overdrive ratio.
If you crossed the traps in 3rd at 120 mph, 6800 rpm engine rpm and driveline speed, you kick it to neutral and let out, the direct drum will soon be spinning over 16,500 rpm.
The result is usually ugly.
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Jake, I posted your reply to my question on another site.
This is another members opinion. >>>
The problem is this is not correct. The way the TH400 works is that it contains 2 planetary gear assemblies hooked together as a Simpson set. The sun gears are mechanically linked. The forward planets and rear ring gear are also linked. For forward gears the forward ring gear is the input. The rear ring gear is the output. For each planetary gear set you must have an input and a hold piece. The third piece is the driven piece. When power is applied to the forward ring gear either the planets on the sun gear must rotate. If the sun is not held the weight of the car will hold the forward planets that are attached to the output shaft.(Newtons Law) This forces the sun gear to rotate backwards at about twice the speed of the input shaft. If you hold the sun gear to the case then the planets rotate forward slower than the input speed (this is 2nd gear). Back to the sun gears rotating backward, when this happens it forces the rear sun to rotate backwards. Since the rear ring is being held by the weight of the car, the rear planets begin to rotate backwards. If the planets are held to the case then the sun gear will rotate roughly twice the speed of the output shaft except in the opposite direction. This combination of reverse rotating sun and forward rotating forward planets (output shaft) causes to output shaft to rotate at 40% of the input shaft speed. This is first gear. It happens because the rear planets are prevented from rotating backwards by a sprag. During first the drum rotates backwards at 80% of input shaft speed. When the drum is prevented from rotating backwards the rear sun is held still, the rear ring rotates forward and the rear planets are forced to rotate forward over-runing the sprag. In the TH400 when the direct clutch engages the drum is forced to rotate at the input speed. This over-runs the 2nd gear sprag that was engaged to prevent reverse rotation in the drum. Here the first conclusion we can draw is that if the engine was a 6000 RPM in direct then a down shift to first would cause the direct drum to reverse rotation to about 12000 RPM the other direction. It would also spin the engine 15000 RPM. Bad Idea.
Now lets look at this from the other side. The output shaft is rotating at 6000 RPM and the clutches all are dis-engaged. HEre all elements are also rotating at 6000 rpm forward. The only holding element still engaged is the low sprag. It prevents reverse rotation in the rear planets. Remember the rear planets are still spinning 6000 RPM forward. The sprag does nothing. In fact the whole assembly continues spinning near output shaft speed because that is the lowest energy state. If the gears are energized and start rotating at different speeds then friction will try to stop them. Without the low/reverse clutch being engaged to force the rear planets to stop, the whole assembly keeps doing exactly what it was doing before the clutches were disengaged. It is just like pedaling a bike wheel and then letting go. The wheel keeps spinning at the same speed (inertia) until friction gradually slows it down.
John
) but if you dont want to listen to Jake, Coan, or any other performance trans builder then nothing I say will matter. They put that warning in big bold print for a reason!