I've spent some time researching our transmission and it's fluid maintenance procedures. When it comes to changing the ATF there are 3 main options.
1.) Drain and Fill - Replaces ~ 3 Quarts
2.) Fluid Exchange via Cooling Line - Enables essentially a full flush w/o machine:
ATF Exchange (using the cooling line)
3.) Flush @ Service Provider
To understand the mix condition of the ATF after drain and fill has been performed several times I made some basic assumptions and iterated.
Assumptions:
- Transmission Capacity - ~ 12 Quarts ATF
- Drain and Fill = 3 Quarts ATF Drained, 3 Quarts New ATF Added
- The drained ATF is homogeneously mixed
- ATF Expiration - 60,000 miles
- Drain and Fill Interval - 30,000 miles
My findings suggest that just using Drain and Fill every 30,000 miles as your maintenance plan for ATF will result in your fluid always containing 6.75 Qts of fluid that is expired. In other words, more than half of your fluid has 60,000 miles or more use on it.
See attachment for graph. There is a bit going on here, so allow me to try to decipher the plot.
Y-axis = Qts of ATF after Drain and Fill procedure is completed. Each column adds up to the total 12 Qt capacity. The column sections represent how many Qts have how many miles.
X-axis = Vehicle Mileage and Mileage on Fluid.
Key: You can calculate the miles on the fluid for each section color. The calculation is performed separately for each column.
So, after performing your second Drain & Fill @ 60,000 miles your fluid consist of:
6.75 Qts w/ 60,000 miles of use
2.25 Qts with 30,000 miles of use
3 Qts with 0 miles of use
After performing your fourth Drain & Fill @ 120,000 miles your fluid consist of:
3.8 Qts w/ 120,000 miles of use
1.26 Qts w/ 90,000 miles of use
1.69 Qts w/ 60,000 miles of use
2.25 Qts w/ 30,000 miles of use
3 Qts w/ 0 miles of use
That's 6.75 out of 12 Qts that will always have 60,000 miles or more use.
Let's shorten the Drain & Fill interval to every 15,000 miles. You can just half all of the mileage values on the plot
So, after performing your fourth Drain & Fill @ 60,000 miles your fluid consist of:
3.8 Qts w/ 60,000 miles of use
1.26 Qts w/ 45,000 miles of use
1.69 Qts w/ 30,000 miles of use
2.25 Qts w/ 15,000 miles of use
3 Qts w/ 0 miles of use
This trend will continue and you will always have 3.8 Qts of 12 Qts with 60,000 miles of use or more
After performing your eighth Drain & Fill @ 120,000 miles your fluid consist of:
1.2 Qts w/ 120,000 miles of use
0.41 Qts w/ 105,000 miles of use
0.53 Qts w/ 90,000 miles of use
0.71 Qts w/ 75,000 miles of use
0.95 Qts w/ 60,000 miles of use
1.26 Qts w/ 45,000 miles of use
1.69 Qts w/ 30,000 miles of use
2.25 Qts w/ 15,000 miles of use
3 Qts w/ 0 miles of use
3.8 Qts w/ 60,000 miles of use or more
So, i've decided i will be performing option #2 when i reach 60,000 miles.
2.) Drain, Fill, Pump + Repeat - Enables essentially a full flush:
ATF Exchange (using the cooling line)
Thoughts?