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Thanks for the suggestion writing to the manufacturers. I might do that some day.
>>>

It's probably the only way you'll actually get anything authoritative to resolve your quandry. Otherwise, as it stands, you'll merely get bickering around the points.

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Que: Among the five rpm drops in D postion, which one is the clutch lock?
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If I'm understanding you correctly - assuming you accelerated up to a cruising speed, in D, then backed off the throttle, so that you were cruising at a fairly constant speed, and at the threshold at which 4th and lock-up would be used, you'd probably get 1st -> 2nd -> 3rd -> 4th -> lock-up.

So from what I'm getting from your question, probably the last one, but I don't understand why you'd get 5 rpm drops, I would have expected 4, unless your acceleration was reasonably gentle, or paused in stages, in which in may engage lock-up in the intermediary gears (2nd or 3rd). I'm puzzled as to why you are getting 5.

I understand why you may use the term "clutch lock", but the lock-up is within the torque convertor. The other clutches that are present within an autobox are actually within the gearbox (ie not the torque convertor) and are merely used momentarily to engage distinct gears.

However, if this was - say - slower roads, with varying times at varying speeds (ie not constant acceleration) you may get lock-up engaged in any of the intermediate gears (ie 2nd and 3rd, too).

But in normal, constant, acceleration up to a reasonable cruising speed - the 'box will probably not engage lock-up in any of the intermediary gears, as it probably detects you still want to accelerate.

At some point, when in 4th / top, the ECU will decide to engage lock-up - probably likely rationale being throttle position, road-speed, gearbox mode (ie sport or economy) - that sort of thing.

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Lo Pan's Ghost