In and out of O/D a lot - Hello all, New guy on the block here! I have a 05 silverado 1500LS 4X4 with a 7 suspension lift and 37 ...

Hello all,
New guy on the block here! I have a 05 silverado 1500LS 4X4 with a 7" suspension lift and 37" tires. Recently i pulled an enclosed trailer a good way which may have been too much for this truck. Now I notice the truck has started shifting out of O/D into 3rd for like a split second then back into overdrive when around 1500 to 2000 rpm. Fluid does look a lil darker than new color so I was thinking about servicing the trans. Before I do that I wanted to see if I could get any recomendations. Could my tire size be the cause of this?? I have never changed any gears in the rear and speedometer is still off, so I was wondering if that would have any effect on the shiftpoint. Trans shifts smooth all the way to O/D, but once there about every 5 seconds it jerks in and out. Think I should replace the solenoids while im servicing it?? Could this be a computer problem?? Any advice would help, and be greatly appreciated. Thanks in advance!

We have some great trans experts here. I am sure they will chime in.
Are you sure it's going from 3rd to 4th? Or, could it be the torque converter not staying locked?

Most likely its the torque converter going in and out of lock up. When it does it put foot on the brakes just enough to turn the brake lights on if quits going in and out it is lock up.

Change your gears to 4.56 if you plan on running that size of tire. That will put you a little lower then stock. It will feel like you added 150 hp.

Not sure its actually going from 3rd to 4th because it happens so fast. But i will def. try driving it tomorrow and touch the brakes to see what happens. And will let u know what happens here. If its going in and out of lock up, im guessing thats due to my tire size? And is the fix just as easy as changing the gear? or were u just recommending i do that too? Also i haul my racecar alot during summer months and was considering an ex. tranny cooler. From reading other posts looks like transman installs a lot of true cool stacked fin style? Lil off topic, but I installed one of those "s" shape cheapos on my astro van (4L60E) and didnt see a change. Wish i would have found this website first, cuz i agree they are scrap metal. Thanks alot for the info.

It is very hard to fix in and out problems with big tires and not stock ratio. Thats why i sugested gear change, plus it will help the truck all around preformance. You can never go wrong with a trans cooler.