# MATH Question For all you SUPER SMART people out there

#### MikeT

##### Subscriber
OK, I know there is some sort of formula for what I am going here but I don't know what it is. I am going this the hard way and I'm sure there is a formula for it. I would like for you to help me and see if you can tell me what the formula is. Here is the problem.

My van has 12 quarts of transmission fluid in it but when you drain it, only 3 quarts come out. Then I fill it with 3 new quarts of fluid, I now have 25% new fluid in my transmission.

2nd change If I circulate all the fluid around (mix it) and then do the same 3 quart change each quart I am pulling out has 1/4 new fluid and 3/4 old fluid. When I add the new 3 quarts in I now have 5-1/4 quarts of new fluid in the system and still have 6-3/4 quarts of old fluid in there. That means I have 43.75% new fluid in my system.

3rd change If I mix it again and do the same change, I have 57.75% new fluid in the system.

I know doing it this way I will never make 100% but the reason I do it this way is the dealer wants \$380 to change it and with 2 changes it only cost me \$48 and I have 43.75% new fluid.

So lets say I changed it 5 times. What is the formula that would tell me how much new fluid I have in it percentage wise? I know that 5 changes, I've purchased 15 quarts of fluid which is about \$120 and with each change I have a diminishing return on my "change investment".

Who can figure this out??

#### Okiewan

My van has 12 quarts of transmission fluid in it but when you drain it, only 3 quarts come out.
I'm already lost at this point. Why does only 3 of 12 quarts come out when you drain it?

#### Rich Rohrich

##### BioHazard
Okie - A bunch of the fluid is sitting in the converter.

Mike the idea that you're doing anything other than contaminating a bunch of new trans fluid with this approach made me smile. The idea of a math based solution to figure out how badly you are contaminating it made me spit up Pepsi on my computer I was laughing so hard.

Thanks for making my day, and BTW you owe me a keyboard.
Unless of course you want to send me one key cap a week and I'll change them out until the keyboard works again.

Let's see, 1keycap per week, 52 weeks a year divided by 12 ounces of Diet Pepsi, carry the space bar ... hmmmmm this is tougher than it looks ;-)

Mike - I'm sure you know this but for those who don't, the reason the dealer charges as much as they do is they use an expensive machine to actually force/flush all the of the old fluid out of the transmission and the fluid coupling (aka Converter) so the new fluid is just that NEW . I'll admit it sucks to have to pay that bill, but there isn't a good way around it if the plan is to accomplish the end goal.

Pete Payne

#### Okiewan

Uh... yeah. Reading comp is failing me... missed the "transmission fluid" tidbit.

Thanks for making my day, and BTW you owe me a keyboard.
Unless of course you want to send me one key cap a week and I'll change them out until the keyboard works again.
Expert puttage right there.

#### MikeT

##### Subscriber
Okie - A bunch of the fluid is sitting in the converter.

Mike the idea that you're doing anything other than contaminating a bunch of new trans fluid with this approach made me smile. The idea of a math based solution to figure out how badly you are contaminating it made me spit up Pepsi on my computer I was laughing so hard.
Oh I hear ya. I hate it too. On the Honda Odyssey that is the method that is RECOMMENDED by HONDA. They say to do a drain and fill every 30,000 miles which in the case of that vehicle is 2.5 quarts to 12 quarts. So the contaminating the new fluid with the old fluid is straight from the manufacturer.

Let's jump to TOYOTA now. They also have a drain and fill method that is outlined by a 12 page TSB that my friend who works at Toyota sent me. It requires a thermal temperature sensor and some other long drawn out procedure. Either way they also had the contaminated fluid procedure.

I wish that is was a clean drain it all and put all new fluid in there but they (as you said) want you to bring it in to them and I think it is absurd to have to spend \$380 to do a simple fluid change.

I say all this and when I bought the Toyota Sienna was told that the reason Toyota did not even have a dip stick to measure the fluid was that they placed LIFETIME transmission fluid in it that never needed to be changed. I said BOGUS on that. Lifetime until it blows up due to not changing the fluid is what I say.

So, I figure that the contaminated fluid change and doing it a couple of times is better than the "leave it in there until it blows" method.

Anyway, I found out wha the formula is. R = (9/12)n (that is to the nth power didn't know how to type it) 9 is what is left in the system (dirty fluid) and 12 is the total capacity of the system. So if I changed the fluid 3 times there would be 42.1% old fluid left in the system.

#### MikeT

##### Subscriber
I'm already lost at this point. Why does only 3 of 12 quarts come out when you drain it?
like you said, torque converter, trans cooler, bad design, etc.

#### Rich Rohrich

##### BioHazard
Something too consider for what it’s worth that I learned long ago working as a Ford line mechanic. ATF has really high detergent levels. As the fluid ages some of that detergent is broken down while at the same time band material (think clutch fiber plates) develop varnish and slightly different properties than they had when new. Reintroducing high levels of detergent into a transmission that has friction material on the bands that has aged can breakdown the varnish sand quickly destroy the bands making them slip. Toyota’s sealed “lifetime” system is a time honored approach going back to the 70s, ASSUMING you don’t seriously overheat the transmission and scorch the fluid.

When I worked for Ford our transmission expert confided in me that the only reason they sold ATF fluid replacement at a huge discount was because it sold lots of rebuilds down the road. For what it’s worth, if you have significant miles on the vehicle and you aren’t having trans issues, leave it alone.

#### truespode

##### Wheelie King
I buy Toyota's so I don't have to do this type of stuff :)

#### mx547

##### Ortho doc's wet dream
When I worked for Ford our transmission expert confided in me that the only reason they sold ATF fluid replacement at a huge discount was because it sold lots of rebuilds down the road. For what it’s worth, if you have significant miles on the vehicle and you aren’t having trans issues, leave it alone.
Interesting, I've never heard that. So my 2014 F-150 with 95k miles doesn't need a transmission flush?

#### Rich Rohrich

##### BioHazard
You can usually get a good indicator by pulling the trans dipstick and smelling the fluid. A burned or acidic smell usually means trouble. Here's where opinions from actual experts seem to differ. One camp says burned fluid should be replaced even if the trans isn't showing any signs of problems. The other camp says introducing that much detergent into a old system that isn't having problems will accelerate the time to a problem by breaking down the bands. I'll qualify this by saying I haven't worked as a auto mechanic for a very long time, but my own experience and the empirical data I've gotten along the way makes me tend to side with the don't fix what isn't broke crowd. This is coming from an OCD level oil & fluid changer. Take it for what it's worth.

Okiewan