I need to solve a long standing issue with trannies. Is it best to leave old burnt fluid in a trans, or flush it and change it out? I have been told by transmission guys that at some point, there is no turning back when the fluid is too bad: changing it would remove whatever is keeping plates and such together. Is this just b.s., or is there some truth to the matter?

 

Can't say for sure. But on my Ford F150 1986, it was like day and night, from very old fluid to new fluid. I did have to change it a few times, but after about 3 changes, the transmission felt like new. This is a transmission that have been along for 372.822 miles (600.000 km), and stil is going amazingly strong!

 

there is some truth to that. changing the can lead you to a rebuild quicker than you want. the new fluid acts like a detergent to the frictions cleaning the linings. either way if the fluid is that dark and smells burnt it is just a matter of time when the rebuild happens. i work in a tranny shop and we get customers asking that same question all the time. if it is working good you may want to keep using it, just be aware of what is around the corner!

 

Where much of the stigma comes from is transmission flushes. Flushing a high mileage transmission with an ultrasonic flush machine can definitely loosen up areas of sediment that has accumulated near seals, etc. This sediment can help those seals in their sealing duties, but it also can eat away at the seals in the process. Clean it out and the seal may now leak.

It is by far best to drain the pan and top off the fluid as part of routine maintenance.

 

Thank you, sir