Down shifting at highway speeds on it's own.

[Musicars]

I sitting on the side of the highway waiting for AAA which could take all based on "high volume". Thought I would start looking into the cause. Just had the transmission rebuilt and was heading up north to the cabin. About an hour out the truck suddenly revved up to about 5000 rpm and the speedometer needle bounced back and forth and dropped to zero. I had been doing about 65 mph and dropped to about 50 mph when a realized it felt like a down shift. I hesitated for second throwing it into neutral because I wasn't sure if it was a runaway throttle or a downshift. I took a chance on it, hit neutral and the rpm dropped.
Now, I had been getting an occasional P0521 for low oil pressure even though I was running between 40 psi and 18 psi hot to cold and was search this forum the last couple of days for more info. I was keeping my code reader in the truck for that reason. I checked the codes and that was the only one.
I cleared the code and head out to try again. It happened again after a few miles. I sent text to transmission guy and he said he has never seen before and I would need to get it back to him. We decided to turn around and head home. I had been checking the transmission temp when I first realized there was an issue before the rebuild. It was 163 degrees. I checked after the rebuild still 163 degrees. After this shift into a lower gear (if that's what happened) I thought I should check it again even though my engine temp was still 210 degrees. The trans was now at 264 degrees and I now also had a P0011 code for cam sensor. That's when I called AAA.
Since I have already rambled on too long I will only give a brief history that may or may not be relevant. It threw the P0011, P0700 (transmission), P0523 (high oil pressure for a bad sensor I since replaced), P0796 (stuck solenoid), and a P2771 (transfer case which seemed to kick out of gear) code the day the transmission basically went out and I had it towed to the transmission shop. The transmission tech thought all but the oil pressure code was the result of the transmission going crazy.
Sorry for the long post. Been waiting over an hour for a tow. Trip up north not looking good.

 

[kbuskill]

Some more info would be helpful, such as Year, Make, and Model.

This will help to determine which transmission you have.

 

[mikez71]

Sounds like he should have a 6l80 for 5+ gears.
260 deg trans temp probably? enabled the HOT shift maps, which maybe explains the downshift at a speed you weren't used to.

But it sounds like you have more serious issues than that. Hopefully the shop will take care of you..

I wonder what @NickTransmissions can tell us!

 

[NickTransmissions]

About an hour out the truck suddenly revved up to about 5000 rpm and the speedometer needle bounced back and forth and dropped to zero.

If this vehicle is the same as the GMT 800 Tahoe in the pics in your profile page, you have a 4L60E. The above indicates your VSS signal to the PCM was lost and as a result you downshifted to first gear. Given all the other codes you have, Id suspect some sort of powertrain C3 problem.

C3 = communication, command and control".

The trans should come out to inspect for and replace any damaged clutches, steels, band, hard parts. All of the other problems listed must be fixed. A complete review/repair of vehicle grounds and electrical system should be done...

 

[Musicars]

Some more info would be helpful, such as Year, Make, and Model.

This will help to determine which transmission you have.

Sorry, it is a 2011 Suburban LTZ with the 6L80 that was just rebuilt last week. I was a little distracted. I had previously posted the tranny going out the day after I bought it. So, this pretty much overloaded my brain. I think in my head I was continuing my transmission trouble post. It took 7 hours and multiple calls to get AAA to finally get someone there. After that post we let it cool down waited an hour and drove 10 miles to a the next bigger town after being told by AAA that it might help speed up service if we got closer. On that short drive it went from 109 degrees to 175 degrees and that was as hot as I was willing to go. My '06 Yukon stayed under 200 even when towing a loaded car trailer. So, I was freaking out at 260. Didn't want to take a chance.

 

[NickTransmissions]

Sorry, it is a 2011 Suburban LTZ with the 6L80 that was just rebuilt last week. I was a little distracted. I had previously posted the tranny going out the day after I bought it. So, this pretty much overloaded my brain. I think in my head I was continuing my transmission trouble post. It took 7 hours and multiple calls to get AAA to finally get someone there. After that post we let it cool down waited an hour and drove 10 miles to a the next bigger town after being told by AAA that it might help speed up service if we got closer. On that short drive it went from 109 degrees to 175 degrees and that was as hot as I was willing to go. My '06 Yukon stayed under 200 even when towing a loaded car trailer. So, I was freaking out at 260. Didn't want to take a chance.

Simply apply everything I said above in post #4 re your VSS to a 6L80 - same sh_t, diff toilet bowl. Bring it back to the builder for teardown and inspection and have the shop diag the overheating issue; something's not right. My 6L80 in my 09 Sierra never climbs above 200 in stop/go when it's 110 out. I just have the factory ext trans cooler and pan.

 

[Musicars]

Here is latest on the transmission. Got the truck back today. According to the guy that built it, the brand new updated TechM was defective. He says that is the first for him. He tore the thing down, inspected it, didn't find any damage. He reassembled and put all new fluid in it. Temperature only hit 160 on the way home and today was a lot hotter than this weekend was.
He also said he noticed the brake light switch was bad and replaced it at no charge. I had noticed that the cruise control was not working. And thought that might be the culprit.
Then on the way home the Traction control light went on. This truck is cursed. That went out later. I had gone through some big puddles leaving the shop and the previous owner removed the auto ride shocks and left the wires hanging. Hoping that is it and they dried out.
Then I discovered my brake lights are stuck on. The left side and center come on and stay on even after I unplug the brake switch. I think the right side is burned out. I'll dog into that tomorrow. I see that it could be a broken wire at the switch. So, it never ends.