Sorry to do it but I noticed something odd last night and was wondering if other 48RE's do the same thing. The following only seems noticeable in Tow/Haul mode.

When my TC locks-up in 2nd there is only a 100 RPM drop. From what I've read the lower the RPM drop the more efficient the TC is while unlocked. This seems fine and great to me.

However, when it locks-up in 3rd there is a very noticeable "thunk" and the RPM's drop 400-500. This causes the truck to gain about 10mph of speed in about 1-2 seconds. You pretty much have to lift the throttle or you'll be exceeding the speed limit before you know it. The thunk is still present when it locks in 4th but I don't notice the same sudden "boost" of speed or severe RPM drop that I do in 3rd. Maybe that's due to the higher speeds/speed limits making the increase less noticeable.

Does your 48RE do the same thing in Tow/Haul mode?

 

I have the truck at work today so maybe I will try this on the way home today. That is if I can get over 30mph in stop and go traffic. argghhh!

 

IIRC the last time I drove mine in TH mode with about a ton of weight in the truck it does the same as yours but I did not hear any "Thunk" sound from my transmission.

 

Same here, once the TC locks, it just takes off. On my truck, sometimes the lock is smooth, but under load it can be a little violent.

 

Thanks for the feedback guys. The "thunk" I described is really more of a feel than a sound. It seems pretty normal though I guess.

I still don't really understand why there's only a 100 RPM drop in 2nd and such a huge 400-500 RPM drop in 3rd though?

 

My truck is an '04 so the transmission function may be different, but mine does the same thing as yours in TH and out of TH (rpm drop is same).
With my 48RE, when in tow/haul, the converter locks up and there is a small drop in rpm as you stated.
When not in tow/haul however, the converter still locks up in third, but the large drop you notice is not the converter locking, but it is overdrive engaging. That's why there is the difference in rpm.
I've been told that the converter should lock at about 50 regardless of what gear you are in, so in many cases the shift to overdrive is with the converter already locked.
I have a lockup switch on my 48RE so I can lock it up in any gear, and it acts just the same when overdrive engages.