Old transmission that must be drained better to redo valve body or nothing at all

So I am going to switch my transmission as part of my 4WD conversion project on a 2015 DC SR.

Draining the transmission is a must and I've heard old transmissions it's better to leave the fluid so is it better to replace the valve body with a remanufactured unit or just fill up and hope for the best?

The replacement unit is supposedly in good condition but I don't know if it was serviced regularly.

More background:

Since the transmission is old, I don't know if the transmission fluid was changed every 60K miles as supposed and since I'll have to drain it, it is possible and provable I will have problems with all the debris that will be dislodged with the new fluid and the most susceptible to clogging is the valve body so changing it would be an improvement if a whole transmission rebuild is not feasible, or is it?

 

Just curious, are you doing the work yourself or taking to a shop?

 

Is there a trans cooler? If so you’re gonna lose fluid disconnecting that right?

 

I'm doing it myself.

 

I know I'll have to drain the whole thing, so I want to know if I should change/service the valve body as preventative measure, since I'm draining all the fluid, or full refilling as is.

 

Are you taking out the whole engine/transmission together or taking it apart from underneath the truck? I'm contemplating if I want to switch out transmissions as well.

 

No one here knows the condition of the old transmission. If it was in good working order before, just add new fluid. The valve body will not magically fail because you changed the fluid.

 

Just taking the tranny... and transfer case, front ADD and rear diff.
I'm converting my 2WD SR into 4WD.

I'm taking it out from below.

 

I've read the problem with old transmissions and replacing the fluid is that the valve body gets clogged so I'm trying to prevent that as a cheaper measure than rebuilding the whole thing.

 

It’s either already clogged or it’s not. What’s gonna clog it in the new fluid? Worried you’ll dislodge some debris in the system that doesn’t come out with the old fluid? If you replace the valve body and then fill it with new fluid, whatever theoretical debris you were worried about would just clog the new one, right?

 

Personally Id replace all the old fluid with new. I would never put old fluid back in.

 

I think he’s putting new fluid in, and he’s concerned that it will cause the transmission to fail. The old stirring up debris theory

 

Not planning to put used fluid but the new fluid will have detergents that can dislodge particles that the old fluid left alone and it's all going to the valve body - that may already have a serious case of high "cholesterol".

 

I cant imagine pouring in fluid will blast debris into the system, the detergents is interesting.

 

Check this out: https://www.youtube.com/watch?v=jJr...hcmUgbnV0IHJlcGxhY2UgdHJhbnNtaXNzaW9uIGZsdWlk

 

Are you winging it or do you have a video/website that you’re gonna follow on how to remove the tranny?

 

I have an alldata.com membership and there are many videos on YouTube on how to remove the Tundra transmission. alldata.com is a provider of step by step instructions for professional mechanics shops.

It does not seem too hard to do, just time consuming because you have to remove one of the exhaust manifolds, then the starter motor, just to get access to the torque converter bolts...