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The 4HP24E in the Range, Will happily cruise in 4th locked, and with a gentle prod on the foot will unlock, and climb a gentle hill in 4th, then if need be will kick down to third. Most of the time I find it only needs to unlock in 4th, but the cruise usually makes it kick-down to 3rd.
This of course is Electronic controlled and i can imagine it would be quite hard to do this with a purely hydraulic box, and keep it in tip top condition.
So how does this work?
I've just increased the size of my tyres from 235/70R16 to 245/75R16.
Now this increase in tyre circumference changes my speedo reading, increasing it? over before. So the old 80km/h is now roughly 84km/h?
I know the standard speedo is quite often 5km/h below actual speed, so these larger tyres have probably corrected the speedo.
BUT, why does the 4th lockup still happen at exactly 80km/h on the speedo?
Shouldn't it have moved to 84km/h with the larger tyres?
The way I see it,
The overdrive is governed by the output shaft speed, and has nothing to do with the speedo. Therfore the gearbox doesn't care whether you've got 36" Simex's or 16" donut tyres, as long as the load conditions and toque converter speed to output shaft speed ratios are within tollerance.
B4 tyres:
Lockup at 76 Km/h Real speed = 80km/h Speedo Speed = Output Shaft 2000RPM guesstimate
After Tyres:
Lockup at 80km/h Real Speed = 80km/h Speedo Speed = Output Shaft 2000RPM guesstimate
Although your real speed has increased due to the tyres, the output of the gearbox is still turning at the same rate, just the tyres travel further with each turn.
Stu