At 93k miles my truck shifts just as it did when I bought it 15 years and 63k miles ago, but 4th and overdrive have always had me scratching my head. Shifts from 1st-3rd are obvious, but 4th and overdrive without a load seam to be one-in-the-same. The truck simply goes from 3rd to 4th/overdrive. However, when I am towing I sometimes notice the converter will unlock going up a slight grade and the truck will run 4th gear and then the converter locks back up once the grade flattens back out. Why does the truck rarely use 4th unlocked? If I turn the overdrive off for heavy towing, 4th is locked out entirely and the truck uses 3rd. Why is the computer programmed to not take advantage of 4th unlocked so infrequently? Now that I have 4:10 gears, 4th unlocked would keep me at the torque peak of the engine, while 3rd has me turning more RPM's than I would prefer at highway speed.

Thanks

 

The 4R70W in your truck is a four speed. Fourth gear IS overdrive, they are not separate. You also have a lockup torque converter.

Fourth gear unlocked is infrequently used because it is very inefficient. You will get better fuel economy, and create less heat in the drivetrain, running in third gear with the converter locked than you will running fourth gear with the converter unlocked.

If you are running too many RPMS in third gear locked, turn the overdrive back on and run fourth gear locked. With 4.10:1 gears that shouldn't be a problem. The only time you need to turn off overdrive is if the transmission is frequently downshifting and upshifting. If it stays in fourth gear you'll be fine.

 

I made the mistake of thinking the converter only locks after 4th. I didn't know that the converter locked for the other gears too. I assumed overdrive off for towing was 3rd unlocked, but it makes sense now.

Thanks!

 

I found your reply very informative, but for my truck, a 99 5.4 4x4 Lariat it brings up a question. Column mount shift selector has 1 2 and D with a button on the end of the stick that says Overdrive. How would I run in 3rd gear?

 

When cruising, if you press the button on the end of the gear selector to turn off OD, the transmission locks out 4th/OD. 3rd becomes the highest gear you can use cruising.