Good morning, I own a 2018 1500 Silverado 4 x 4 6.2 L Max trailer Package I am not sure which rear is in her. The truck incurred the chattering issue at 75k. Brought it to the dealership , they flushed and supposedly updated the computer. It did correct the problem. I am now at 112k. I have been experiencing some bang issues at low acceleration. So when I start to accelerate at slow speed, have to let off accelerator for a moment and then hit the accelerator again, the transmission (?) bangs/clunks. I have also noticed, when I come to a stop, I can feel the transmission downshift into its lowest gear, which i never felt before. My personal mechanic and my local chevy dealership both diagnosed it and neither came up with a code. Chevy seemed to think i should be okay towing.

Now I have towed this trailer several times before with is truck without absolutely no issues. Dry weight of trailer is approx. 6600 lbs. I just towed the trailer down to Myrtle Beach from Long Island New York.
After about 150/200 miles, the truck, on inclines, the truck/transmission? would chug/bang for lack of a better term. I was in cruise control on the first occurrence, afterwards stayed out of cruise control.
Conditions of occurrence. Approx. 1800/2000 rpms, between 55/65 mph. always on an incline. In order to maintain speed, I would have to accelerate hard to get the transmission to downshift. Once she downshifted, she ran fine. just ate up tons of fuel to get up these minor inclines. once the terrain leveled off, she ran fine. but always happened at this same condition. approx. 60 mph, and approx. 1900 rpms. The truck was always in tow mode. I tried once taking it out of tow mode, just made the incident occur more often. I made it to my destination. Unhooked the camper, took the truck for a test drive. She ran fine, other than the previous issues I mentioned earlier. Driving the truck home, absolutely no issues. Gas milage on the way down (Towing) 8 mpg. On the way home 21 mpg cruising @70 mph. The only thing I changed on the way down was upgraded to using Premium (93 octane) vs 87. It seemed to help minimally. I have had several thoughts. Torgue converter, clutches, Syncros, modules, Universal joints. Someone even suggested coils possibly. They had an older blazer with the same exact problem. changed the coils and bingo! Problem solved. Wondering if anyone has incurred the same or has any ideas where to start looking.

 

if you had a coil problem you would have misfires and a check engine light. torque convertor shudder is one possibility but i can't confirm without a test drive. have a good trans tech test drive with you when the fault is occurring.

 

Just a thought in my head, Sounds like you have an 8 speed trans, see if you have p0711 stored in the transmission control module (trans codes don't always set a p0700 in the ecm to turn on the check engine light). Those transmissions had issues with the fluid temp sensor (part of the internal trans harness) that could be possibly causing an issue. Again just a quick thought .

 

interesting. Because the reading on the instrument board was higher then I am used to seeing, but could that be a false reading creating the problem?

 

How much higher? If it is only a few degrees higher then that is not going to make the ECM/TCM control things any differently (based on transmission temperature alone.) However, what is causing the higher temperature could be a source of the problem. For example, maybe you have a slipping clutch. Were the engine speeds while in cruise control at highway speed the same as what you saw in the past? Or were they higher?

 

Well My Transmission fluid temp normally never tops out higher than 170. This trip I saw as I as 193.

Yes to the Highway speed.

 

I found the TSB about trans temp wire harness. The one I found is for MY15. Here is the link. https://static.nhtsa.gov/odi/tsbs/20...89896-2280.pdf
The problem was "This could cause a harsh shift when starting the vehicle at temperatures below 32ºF (0ºC); however, shift quality improves once the transmission oil warms up." My feeling is that what you are experiencing is not caused by incorrect transmission fluid temperature. But here is how you can check it - the DIC can display transmission fluid temperature (TFT). It may be turned off by default but you can add it under the options for what is displayed. Let the truck sit overnight so you know for sure your transmission temperature is at ambient. Then key on the truck and check TFT on the DIC. If it is reading correctly then you know for sure that isn't the issue.

A 23F rise in steady state TFT while highway driving on level ground is indicative of a substantial increase in spin losses inside the transmission. The most likely cause is a slipping clutch (range clutch OR torque converter clutch) but there could be other reasons as well. When you say "yes to the highway speed" I'm assuming you mean you did see an increase in engine speed at steady state highway cruising speed. If so, that's another indicator that you have slipping going on inside the transmission somewhere.

Another potential cause of 23F temperature rise is a problem with the transmission cooling circuit. My guess is you have 8L90 transmission (standard with 6.2L); it has a transmission cooler bypass valve. The bypass valve doesn't allow flow to the transmission cooler until the TFT reaches a certain setpoint. I know for 6L80/90 GM did a TSB for the bypass valve to change it from 90C (194F) setpoint to 70C (158F) setpoint. If the TBV is acting up this is a much easier fix, it is on the outside of the bell housing on the driver's side so it is a 15 minute driveway job to change it.

Here is an article discussing various 8L90 failures.
https://nextgendiesel.com/blogs/tran...lems-solutions

Here is an article about class action lawsuit for 8L90.
https://gmauthority.com/blog/2023/03...action-status/