lazy fluid changes

[youngbuckwithapickuptruck]

I always wondered why you cant change fluids on a vehicle by just replacing small amounts at a time. actually seems like it would be ideal for replacing the ATF on a high mile automatic transmission with a questionable service history...

so I have a 2nd gen prius and it says I need an expensive computer tool to completely drain and replace my brake fluid. why couldnt i just drain a couple oz out of the reseviour(sp?), replace with fresh fluid and then drive around, and repeat the procedure a few more times over the next couple thousand miles? i know it wouldnt be the ideal way to completely clean the system and remove trapped sediments that sunk to the bottom, but at least it would improve the quality of the fluids within the brake system, ya?

 

[Curious Hound]

With the transmission, you are often stuck with doing it that way because some fluid remains stuck in the torque converter, cooler lines and other nooks and crannies. But you want as much as possible drained and replaced. Shops often have a machine with a pump to actually flush the transmission clean.

The problem with not draining completely is that if the remaining oil is contaminated or burnt, then it immediately contaminates any new fluid you put in. That means particles of metal or dirt are still in the system to continue wearing moving parts. Water is still in the system to cause corrosion. Burnt fluid which no longer has the proper lubricating properties is still in the system diluting your new fluid so that it is not as effective as it should be.

With engines, differentials, manual transmissions, the amount of old oil that remains is negiligible - maybe a few tablespoonfuls. That has little to no appreciable effect in the new oil. With an automatic transmission, it can be close to half the system volume, not just a few ounces, but quarts, that does not gravity drain. That has a much bigger effect on the new fluid. If the fluid is in decent condition when you inspect it, not burnt, not a lot of metal particles on the magnet, etc. And you replace the filter, that should be fine. If the fluid coming out is extremely dark and dirty, do what you mentioned. Drain and refill. Drive for a few miles and do it again. If the fluid is burnt or there is a lot of particle buildup on the magnet, that transmission needs more work before you put new fluid in.