99 GMC Z71 - Changed my transmission fluid and filter bc the old fluid smelled burnt and looked old. Trans was working fine before I did the changed, just seemed a little weak when taking off at times. I've only had the truck about a month, has 240k miles on it and I have no idea if the trans has ever been worked on or the fluid changed by the previous owner. Anyway, I changed the fluid and went to test drive it. Put it in reverse and it went in easily and seemed more powerful than before. Put it in drive and pressed the gas...it barely moved. Put it back in park and checked the fluid level again. Fluid level is good. Tried it again...same thing. I put it down in first and it moved a little better but still not good. Tried it in drive once more but it will barely move. Reverse seems to be the only position that it works well in. Someone mentioned that when I was changing the fluid I could have damaged one of the solenoids in the trans? I've also heard that the fluid change itself can mess up a trans? I'm just trying to narrow it down as to what exactly I've done to it and what I need to fix.

 

I doubt the actual new fluid did the damage, so I would get in there again and see if any of the solenoids got bumped. They can get real brittle. Also did you make sure the old filter seal came out and that the filter is fully seated if not the pump can suck air.

 

Thats a lot of miles on that tranny. If the fluid smelled burnt then you could have burned up clutch bands that were on their last leg....

 

Unfortunately trans, diffs, and transfer cases seem to only get attention with there is an issue felt and usually by that time it is too late. I like how the original poster says - "Trans was working fine before I did the changed, just seemed a little weak when taking off at times." Most people do a fluid change hoping it will fix all the years of neglect and it just doesn't work that way. Still I don't think just a pan drop and refill/filter change would make it go from "weak taking off at times" to barely moving. It is probably a solenoid causing the trans to start out in a higher gear. I bought a 98 Z-71 a little while back that barely wanted to get going and it was one of the solenoids that was busted. I got a trouble code for it though.

 

I'm not getting a check engine light but I do have a service 4wd light that stays on most of the time while I was driving it. I thought it may have been bc the 4wd selector switch is broken.

 

The sun shield in the tranny might have broken. I have a '99 Silverado and the sun shield in mine broke at around 140,000 miles. I was told that they break pretty often after 135,000 miles. If it was replaced around that time on your truck, it's getting close that 135,000 mile mark again on. Hopefully, for your sake, that isn't the problem, because it's a costly repair.

The reason I say that it may be a broken sun shield, is because when mine broke, I lost reverse, and my drive gears would hardly pull at all. For you, it's the opposite, so it could be a different problem.

 

What is a sun shield? And today I cranked it up just to let it run for the first time since it broke and put it in reverse...it didn't go. I put it in park and then reverse again...it backed up. Idk why it works, then stops, then works again. Drive still acts the same though.

 

I'm not exactly sure what a sun shield is. That may just be a slang term. I do know that it is an important part of the transmission, that works all the gears. And I know that GM doesn't exactly make them all that strong, which is why they break more times than not after 130,000+ miles. Depending on how hard or easy you were on the tranny, that mileage number could vary a lot.

 

Yeah, that's something else that I don't know. I've only put about 2k miles on the truck, I haven't had it long and I have no idea what the previous owner did to it. I'm guessing nothing bc he didn't even fix minor stuff on it to make it work correctly (which I have fixed) so I seriously doubt he kept up with servicing it.

 

I'm with Shamrock pull the pan and check everything over real close.