Understanding the overdrive on/off switch and why

[pennyanguy]

Stupid question ... or maybe I just need an explanation... I have a 2007 Expedition EL 5.7 Limited, 117K mi...I have not really towed a lot of heavier loads with it long distances... but did today... and some things about the overdrive on/off don't really make sense to me.
So I was towing a 24ft boat, about 5500-6000# on the trailer, fairly flat highway towing from Buffalo to Rochester NY. In drive with the Overdrive OFF (button on gearshif and light on dash says O/D off) I am doing about 2700RPM at 60 MPH... seemed high but I did that for about half the trip. Then I turned the Overdrive ON (button on gearshift).. at same speeds 60 mph I am doing 16-1700RPM...truck feels fine, I have plenty of ability to accelerate and the engine does not seem to be struggling... what I really do not understand is if I do accelerate a little heavily (not shove to floor just try to gain a few MPH or go uphill) it kicks down (out of OD I believe) and engine RPMs only go to 2000-2100... and plenty of power there... so if that's "out" of OD (2100)via the tranny auto kickdown... why with the OD shut (button on the shift knob) does she scream to 2700 RPM?

 

[JamaicaJoe]

Unless I am missing something, you are describing exactly what O/D is supposed to do. O/D is the highest gear ratio. It will always settle to the highest ratio/lowest RPM possible at a given speed. When O/D is off, the gear selected is the next lowest and the RPM will be higher. The only reason to turn O/D off is if you are in hilly terrain and the transmission is hunting. I am new to the 3rd gen (2013) but I think works the same as my 1st Gen 2001.

 

[GaryB]

Just wanted to say I know that stretch of road well. Grew up near Buffalo, and lived in the Finger Lakes for a decade or so while working in Rochester. It doesn't seem like it should have needed OD off for that. I think I see you're point about the difference in RPM from 2700 to 2100? If so, the only thing I can come up with is maybe the torque converter doesn't lock out when O/D is shut off. still seems weird.

 

[bman440440]

what I really do not understand is if I do accelerate a little heavily (not shove to floor just try to gain a few MPH or go uphill) it kicks down (out of OD I believe) and engine RPMs only go to 2000-2100... and plenty of power there... so if that's "out" of OD (2100)via the tranny auto kickdown... why with the OD shut (button on the shift knob) does she scream to 2700 RPM?

what is going on is that under mild acceleration (with O/D on) the computer will unlock the torque converter to give you (that 3-400 RPM's) what you need to accelerate, it's not actually kicking out of O/D... but if you do need more and press even harder on the accelerator pedal it will not only unlock the converter it will kick out of O/D

 

[chuck s]

I had an '07 5.4V8 6-speed and towed our 6,000 pound travel trailer many times across NY and PA and left the OD set to ON. Unless the transmission is "hunting" for the right gear all is fine. I had an auxiliary temperature gauge on the dash to read the transmission temp and lockup as I was initially concerned but after a couple of trips just ignored it. My '17 has the same basic transmission behind the 3.5EB engine and tows much better over the same routes.

-- Chuck

 

[Pilgrim]

I think OD off means it is in 4th gear, as 5th and 6th are OD. That will explain the RPMs.