A common concern passengers have while riding in my truck is, "Why does the transmission shift funny into overdrive." Then I have to explain to them that it's just the way DC has programmed the transmission to shift into overdrive and lock the TC at the same time thus causing a large drop in RPMs. So, today on the way to work today I did some experimenting with the overdrive ON/OFF button. My tests lead to the conclusion that the TC does NOT lock up with the overdrive off.

1. I turned the overdrive off
2. Took truck up to 60 mph and about 2500 rmps
3. Turned overdrive ON and it shifted once and then 2 seconds later sifted again.

The other thing I noticed is after the second shift the truck felt like the power was being put to the ground. It wanted to accelerate without having to apply additional foot pedal.

Then I thought to myself, this isn't right. If I turn the overdrive off it should only shift once. So then I decide to try it again, but this time on the highway.

1. Turned overdrive off
2. Accelerate to 70 mph and almost about 2900 rpms.
3. Turned overdrive ON and again it shifts twice.

Is the TC locking when the overdrive is OFF? Why does it shift twice? Also, I have had TSB 18-027-03 and TSB 18-015-03 applied.

 

I can tell you that my second gen locks up in third, when OD is locked out. Sorry don't know much about the third gens.

 

It didn't shift twice when you turned OD on. It first shifted up to 4th (overdrive) then the TC locked which felt like a shift.

When mine downshifts from OD into 3rd it feels like it's shifting twice. It is actually downshifting into 3rd (TC unlocked) followed quickly by TC locking.

Pete

 

Your tranny locks TC in 3rd, when it shifts to OD it unlocks TC, shifts into 4th, and then locks TC again.

Sometimes the lockup of the TC in 3rd is hard to notice, maybe just a drop in RPM's of about 100.

 

So I understand what you are saying.

1. With overdrive OFF, 60mph/3rd/TC locked
2. Turn overdrive ON, unlock TC and shift to 4th then approx 2 seconds later lock TC. (I imagine the 2 second delay is cause by the high rpms.)

Now under normal driving conditions with overdrive ON
1. 1st -> 2nd -> 3rd/TC lock -> unlock TC/4th/lock TC all at once to produce 400-500 rpm drop.

Is this correct? Do the 48RE's behave the same? Is unlocking necessary for the tranny to shift? I've also read that TC lockup can be hard on the tranny. Is unlocking/shift/lock doing the tranny and good?

 

from what i understand in the tow/haul or od off mode it locks in 2/3 with od it only locked in od.

Id like to be able to lock in all gears. Dunno if its possible but if you coudl run the lock up in first that would be alright.

 

Mr. Blonde,
You have it right.

The reason the TC unlocks on the shift to 4th is to go easy on the tranny. The TC unlocked 'absorbs' some of the abrupt application of torque. If it shifted locked under a load, it could do some real damage to the tranny, u-joints, rearend, etc.

There are some aftermarket goodies that can keep the TC locked, but most are used when drag racing, etc. But only a beefed up TC, tranny, etc. can take the punishment.