New clunk, prob not axle wrap; time for trans fluid change?

My '99 auto 2.7l Prerunner has picked up a clunk only when shifting into drive from park. I had the typical axle wrap clunk for a while until I beefed up the leafs; this sounds and feels different. I don't know if that is related to fluid service or not, but it got me considering. While I occasionally check the level of the trans fluid, I never really paid attention to the color. So I got a pic of that fluid next to some new fluid. The truck has 170k miles and never had a trans service AFAIK, at least not since 2011.



So... change? Half drain and fill, wait, another drain and fill? Leave alone? Full service?

 

After all the helpful suggestions, I decided to just drain as much as I could via the drain plug (exactly 2 quarts, 10 oz, the color of maybe Hershey's syrup, but no burnt smell) and replace it with the same amount of Valvoline Maxlife ATF. Walmart had it for $25/gal so I loaded up. I drove it around the neighborhood a bit then came back to check the level. It was right on at the "HOT" marker on the dipstick, so it seems there are no leaks. But already (or perhaps coincidentally) the trans did not clunk going from Park to Drive in the 10 or so times I tried it running errands today. Also, this may just be in my head, but I swear it shifts better. It used to hang in a lower gear longer than it should before shifting. And a few times randomly it would not shift out of third at all. I would switch the O/D button on and off a few times, or slow down to downshift then get moving again and eventually it would shift into the right gear. No such issues today. Again, maybe it's just coincidence.

Anyway, with no indication that the transmission has ever been serviced, I'll order a gasket and drop the pan soon and clean out all the goo. I'll button it up and refill with the Valvoline, then drain in another few hundred miles and refill. It won't be perfect, but it will be cleaner for sure.

 

I have been told if you have high mileage and you have very dark automatic transmission fluid and put new stuff in it will do more harm than good.

 

Also been told if you do change it you should drive it for awhile then do another transmission fluid change. That’s the safest way but yea can cause the clutch to slip putting new stuff in a vehicle that has never been serviced and is that black.

 

I dont think that is true.

For what its worth, folks usually say color is not a good indicator of fluid life. But either way, good to get some fresh fluid in there IMO OP.

 

It actually is true I have also done research from it and trust worthy mechanic told me that. Fortunately Toyota makes some of the most solid transmissions so it should be fine changing it but on a lot of make of vehicles you can cause serious damage to changing it with high mileage. But like I said drive it for a bit and change it a second time then you shouldn’t have to worry about anything.

 

I make sure I change my transmission fluid every 60 000 km 400 000km on my truck and transmission shifts like it’s new

 

It seems this concern is the kind of handed down experience that have become part of the shade tree mechanic's lore. As far as I have learned, neglected transmissions build up debris here and there like blood clots. It's not really doing any harm as long as a big chunk of it isn't floating around. Draining out all of the dirty fluid (assuming the clutches are still ok) and replacing with clean fluid acts like a detergent and loosens these chunks of residue that may end up clogging up the passages in the valve body. Doing a drain and fill (in the case of my '99 AT) only replaces 1/3 of the fluid. So it should improve the overall performance of the fluid without the extreme effects of 100% new fluid, and certainly not of a power flush.

The other issue is when the fluid is super dirty and full of metallic particles (thankfully mine didn't seem to be), the clutches need the suspended particles for grip. In this case, draining and replacing the fluid will make the worn clutches slip since they now have no suspended material to help engage.

Great info here:

https://www.youtube.com/watch?v=o690DovjDAc