I just got a new-to-me 03 QC Dually with the auto trans in it. it is bone stock, 28K miles and only towed a 20-something foot pleasure boat through a Class IV receiver. It's so well kept, everyone, including dealers, are saying that it looks brand new., so I am doubting previous mistreatment.
I've read the threads here and only one came close, but isn't close enough to apply.
My truck seems to hold 3rd until 40mph, no matter what. And then TC lock up won't go until around 50mph.
Otherwise it shifts fine. My only reasons for posting this is that it just annoys me for the most part and is killing my fuel mileage now that my fuel station changed over to ULSD (went from near 20mpg to about 15.5mpg over course of two tanks... no different driving and only a little cooler in temps).
Additionally, I use the truck every day to take my daughter to daycare on city streets that are 35-40mph speed limits. Running around in 3rd or 2nd is eating me up inside as I can watch my fuel mileage gauge just drop through the floor.

Any ideas are welcome.

Thanks in advance!

 

Sounds like you have a 48re trans.....same as mine. If you do the TC will not lock until 80 kph. or approx 50 mph.......no matter what. It used to drive me crazy too but I learned to ease off the throttle when in 3rd. gear and the truck will maintain about 30 mph. at about 1100 rpm. If you keep your foot in it, it's easy to run around 1500 rpm and still only be doing around 35 mph. The trick is get it up to speed in 3rd. gear and then back off the accelerator.

I had the 47re trans. in the old truck.....it was real "slushy". If you step on the accelerator hard with your truck you find the 48re will shift firm and fast with little hesitation.

How do you know if you have the 48re......If you have the HO motor you have the 48. Some early build 03's with the SO motor have the 47re. There is also a code in the glove box......can't remember the letters and numbers but if you do a search you'll find info.

 

So, this is normal? Annoying, but I'll live with it, I guess.
The truck was built in 10-03 and it shifts fast on the change from cruise to WOT.
It is the 305/555 engine. I think the window sticker the PO left in the glovebox says HO motor.

 

I have the same motor/tranny, hit the overdrive lockout button when city driving. Don't lug it around at 1100-1300rpm's in fourth gear below 40 mph. Yes the TC doesn't lock till 50 mph in fourth or with the OD off it locks on mine at 46 mph. Since you don't have gauges you can't see that the ecm increases fuel (boost) to maintain or meet speed when in fourth gear. Going on the third year of ownership and I can confirm, you will get better mileage in the city and create less heat ( trans temp). Now don't baby it, make sure to romp on it... enjoy!

 

Right now I am getting used to the quirks of the truck, so romping isn't done much at all.
I'm also trying to find my fuel mileage so I know what to budget for fuel LOL

I've romped it a couple of times. Seems to open up better when at speed versus from a stoplight.
I think this may be due to the turbo already being in a bit versus off-idle and having to spool completely.

 

When I say romp on it mikey I mean after all your city driving make sure to work the motor once in a while. Otherwise these new cummins tend to carbon up the injector tips when run real easy. You know what a cold smokey fire is? As for mileage it is critical not to go by the overhead. Fill your tank till you see fuel in the neck and it stays there. Takes an extra 10-15 min. The best I got no load highway was 23 mpg at 65 mph. The worst was 13 pulling 8500 lbs racing semi's thru Oregon. The 305/555 motor only has two fuel injections since we have no cat!

 

I put it on the highway at least twice a week and keep it at or as close to 65-70 as I can in Phx traffic. So I'm not just lugging it around town
I got right about 20mpg with that sequence first tank (which was non ULSD) and now I am looking at about 15.5 with ULSD... calculated, not computer. Even though the computer was within .2 at both calcs.

 

Okay at that speed your exhaust gas temperatures are around 600-700 F. Your boost is anywhere from 6-8 psi. That, sorry to say is not enough to work the motor. Remember this engine is a true medium duty motor, by design made for serious work. The cummins 5.9 isb is found in many industrial, marine and semi's. The way you and I use these motors with proper maintenance they easily go 1/2-1 million miles. What I have found by watching my gauges, that only towing or hauling get exhaust temps up high enough and long enough to clean carbon from the injector tips. Don't let me worry you thinking you'll have a problem. I'm just trying to pass on what I've learned. Hey, check out my gauges in my photo gallery.

 

Well, I do have a 40' race trailer/toy hauler that I can put behind the vehicle. Weighs in at about 8 tons loaded That and the 2-vehicle triple axle trailer are the primary reasons I bought this truck.
I am getting the gooseneck installed soon.
Then can I consider myself working the engine
I'll definitely take a look at the gauge installs, as I'm getting ideas for my own (preferrably the above mirror mount or I'll form one myself to mold into the dash somewhere. Fiberglass is easy to work with, carbon fiber's a little tricky, though)

 

Many or most of us use fuel additives especially when driving without a lot to keep the injectors clean. Search on additives for tons of info.

Pete