2006 48re just lost 4th gear
 

Were a couple of states away from home and an aluminum cooler line sprung a leak.
No trans temp light came on and the only indication that this happened was that the truck kept going down to 3rd gear at 70mph.
Once we were able to find a safe spot to pull over (at night near Detroit), we found trans fluid all over the bottom and rear of the truck.
The fluid on the stick was showing just below safe so we assumed it was ok to limp to our destination if it didn't get worse.
The following day we cut the damaged line and hose clamped a length of hose in its place and refilled.
It actually took 6qts of atf4 to get to the full mark.
When we test drove it, we realized that there is now No 4th gear. I tried running the speed and rpms up at 70 to see if it would force a shift but nothing. I've also tried varied speeds and throttle for an additional 20 or so miles.
If I press the OD off button, it actually seems to drop the rpms slightly compared to being on.
I have tried removing the battery cables for a half hour, pulling fuses, checking for codes, and cleaning the oil soaked connector on the drivers side of the trans.
I'm at a loss. Hoping its an easy one.
We have no 4th and I'm worried that on the drive home, we could lose 3rd or worse.

One other thing that we know is that when it kept dropping itself into 3rd, the speedo also kept going in and out... reading 0 then bouncing up to actual road speed.

I found a bunch of codes using my reader but nothing that has anything to do with speed or transmission.

What codes were present?

Sometimes you can figure out a voltage issues, etc. if you have a companion circuit not working.

Does your TC LU still work?

I don't have the codes on me at work but they were like...intake temp circuit high etc.
It doesn't seem like lockup is working.
I assumed od off prevents 4th and lockup?
Rpms actually drop a couple hundred lower when they should actually rise up when driving at highway speed and disabling Id/lu

You need to check the wiring to the trans. The pinout for a 2001 trans connector is as follows. ( Yours should be the same, newest I have )

Pin 1 gets 12V from the Trans control relay, red wire.
Pin 2 is 5V for gov. pressure sensor circuit, orange wire
Pin 3 is Ground for the 5V above, Black.light blue
Pin 4 is Gov. pressure signal, Light green/White
Pin 5 is variable force solenoid, is grounded by PCM at C2/pin 29 violet/white
Pin 6 is Overdrive solenoid, grounded by PCM at C2/pin 21 brown wire
Pin 7 is TC LU, grounded by PCM at C2 pin 11, orange/black wire
Pin 8 is trans temp signal, violet wire.

I would start by insuring your Trans relay is providing the 12V necessary to drag in the solenoids.

Awesome advice and thank you! I will keep that info handy!
I ended up taking it to a local trans shop.
From my explanation, he's thinking governor went bad when we upset it.
He says he uses some "new,improved" Borg warner governor when I asked about the gm conversion.

Good, unless you are really set up to do it, it is hard to do much more than just check voltages in cases like this. I hate the newer stuff.

Well...
Turned out that it smoked the TCC and all of the 4th gear parts.
It has been fully overhauled to include a billet convertor and shift kit.
Shifts nicely now.

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