odd overdrive behavior...

[e9999]

hmmm....
saw something odd/interesting the other day.
Was playing with the O/D.
Don't remember the exact conditions (downhill, flat, speed...), but several times, I was driving with the O/D switch on, then turned the O/D witch off and the engine RPM dropped a bit, say from 2,200 to 2000. Since the RPM did not go up, I imagine I was actually in 3rd at the time, but why would the RPM drop a bit?
Are the shift points different when the O/D switch is on?
thanks
Eric

 

[Beowulf]

Normal converter lockup.

-B-

 

[syrinxstar]

I believe it's the VC disengaging - that'll get you the ~200 RPM difference where a gear change will be a significant jump. Landtoy has nice explanation of this a few months back.

 

[firetruck41]

[quote author=Beowulf link=board=2;threadid=16862;start=msg162667#msg162667 date=1085373745]

Normal converter lockup.

-B-
[/quote]I wondered, do we have a lock up torque convertor then? Just in 3rd and 4th?

 

[Rich]

Like B stated the drop in rpm is due to the torque convertor locking up.

The transmission control logic has a little bit of odd behavior. You can be running in third gear, at a certain rpm and throttle setting (most often encountered when going uphill) with the torque convertor unlocked; if you then disable overdrive (4th gear), the torque convertor locks, which reduces the engine rpm.

 

[e9999]

well, I can certainly believe that this was the TC locking up as that would explain a drop in rpms (the eliminated slip), but aren't these things generally pegged to a speed for locking and unlocking? Sure seems like a quirk of the control system.
E

 

[RavenTai]

depends on how it is programed, my truck locks up in 5th in normal mode and in 2nd through 5th on tow/haul mode

I would be interested to know how the 80 is programed

 

[krich97]

Mine did exactly the same thing when I experimented with it while going up a hill - I assumed there was a shift, but it was only 2-300 rpm drop. I have gotten a different response if I slow the vehicle down first or once when going down hill - makes sense above - knowledgeable folks!

 

[Extreme_Trans]

Hi Guys it's Rodney here from Wholesale Automatics. It's a completely standard feature as the 80series transmissions can only get Lock-Up in forth what the ECU does is when you turn 4th off ( Overdrive ) the ECU comands the Lock-up to start working in 3rd gear instead. A great idea when you start towing and hit the hills. As soon as the Landcruiser slows enough for 4th to come off quickly turn the overdrive off and the lock-up will come back on and stop the convertor from generating so much heat. In second gear the engine load is so small that the convertor doesn't generate no where as much heat as it could in 3rd and 4th.
Regards
Rodney