Here's a new one - transmission

[dbphillips]

Started out from Bodega Bay with the travel trailer this morning and looped around to I-80 and north toward Oregon and stopped early in Orland for fuel after about 180 miles. Fuel mileage was terrible, like 8 MPG. Continued heading north and realized I had never seen the gear indicator on my iDash above 7 today. Called the Chevy dealer in Redding, whose service department was supposedly open and left a message asking if they could run a quick diagnostic on our way through, then answered their confirmation text for good measure. Never got a response.

About 10-20 miles later (after 180 miles, a power cycle, and some messing around to convince it to shift after I called the dealer), the truck started shifting into 8th and 9th, under comparable driving conditions and stayed working all the way to Medford, OR. Has to be pure coincidence, or could they have sent something OTA? Anyone else had this issue?

 

[Silverado4x4]

Started out from Bodega Bay with the travel trailer this morning and looped around to I-80 and north toward Oregon and stopped early in Orland for fuel after about 180 miles. Fuel mileage was terrible, like 8 MPG. Continued heading north and realized I had never seen the gear indicator on my iDash above 7 today. Called the Chevy dealer in Redding, whose service department was supposedly open and left a message asking if they could run a quick diagnostic on our way through, then answered their confirmation text for good measure. Never got a response.

About 10-20 miles later (after 180 miles, a power cycle, and some messing around to convince it to shift after I called the dealer), the truck started shifting into 8th and 9th, under comparable driving conditions and stayed working all the way to Medford, OR. Has to be pure coincidence, or could they have sent something OTA? Anyone else had this issue?

Alot of reasons this could happen here. How heavy is your trailer, how fast were you towing,, were you driving into heavy headwinds , slight inclines , I know towing a travel trailer is like towing a sailboat sideways with the sails up and far as them sending a update Ota not for this.

 

[Doubeleive]

Alot of reasons this could happen here. How heavy is your trailer, how fast were you towing,, were you driving into heavy headwinds , slight inclines , I know towing a travel trailer is like towing a sailboat sideways with the sails up and far as them sending a update Ota not for this.

not lilkely from the road, that trip is thru the cascades (mountain range) so a lot more than just "slight" inclines, more like 6% grades.
Towing thru there is a workout for sure. elevation peaks at about 6500ft, it's like a roller coaster ride.

 

[dbphillips]

Hadn't even gotten to the mountains. Was wondering about wind, but like i say 10-20 miles later, comparable conditions, and it started allowing higher gears. Same through mountains on the easy bits, but it did hold lower gears in going downhill. Was in tow mode the whole time. Also, been towing the same rig for over 12k miles, from Seattle to Maine and back and Seattle to San Diego and back. Seen it all.

I went to check the transmission fluid, even though temps were fine and highly unlikely to be the cause and discovered the dipsticks removed the dipstick.

 

[Jay P Wy]

Were you in Tow/Haul mode? Shift points are determined by throttle/pedal position and speed. If you look at the shift tables for a modern transmission in EFILive or HP Tuners the speed at which the shift happens goes up the more the pedal is pushed down (both tow/haul and normal mode). Tow haul just raises the shift points (and line pressure on some transmissions).
As for the dipsticks, they have been gone for a long time in Tahoes/Yukons (and 1500 trucks). I'm not sure if the 10 speed in the 2500/3500 has a transmission dipstick but the 2019 6 speed Allison did still have a dipstick.

 

[dbphillips]

Were you in Tow/Haul mode? Shift points are determined by throttle/pedal position and speed. If you look at the shift tables for a modern transmission in EFILive or HP Tuners the speed at which the shift happens goes up the more the pedal is pushed down (both tow/haul and normal mode). Tow haul just raises the shift points (and line pressure on some transmissions).
As for the dipsticks, they have been gone for a long time in Tahoes/Yukons (and 1500 trucks). I'm not sure if the 10 speed in the 2500/3500 has a transmission dipstick but the 2019 6 speed Allison did still have a dipstick.

Always use tow/haul, mostly use cruise, watching EGT.

Today running about 13 MPG, well above average. ‍

 

[JKeller]

This may be antiquated knowledge, but I was always taught to keep a transmission around 2k rpms when towing anything heavy and/or with wind resistance. This is to keep the engine where it is making power and to keep the transmission from hunting for gears and burning up. I tend to think this is still good information as I notice my transmission will hold in higher gears if, for example, I am towing a trailer with a lot of wind resistance into the wind. It sounds to me like your truck was in tow mode and doing what it's supposed to do. For reference, I get 9-10mpg towing a 4,000lbs trailer with lots of wind resistance at 70mph.

 

[dbphillips]

This may be antiquated knowledge, but I was always taught to keep a transmission around 2k rpms when towing anything heavy and/or with wind resistance. This is to keep the engine where it is making power and to keep the transmission from hunting for gears and burning up. I tend to think this is still good information as I notice my transmission will hold in higher gears if, for example, I am towing a trailer with a lot of wind resistance into the wind. It sounds to me like your truck was in tow mode and doing what it's supposed to do. For reference, I get 9-10mpg towing a 4,000lbs trailer with lots of wind resistance at 70mph.

Sure, but I had downhill sections and it would not shift up. There was definitely something off in the transmission control.