On changing automatic transmission fluid which of the following two would be preferred: remove the pan and changing the filter and fluid or removing the fluid with a machine and not changing the filter? I have heard talk of the latter way and then the fluid starts leaking at the seals.
If you have a machine force-feed the process without first changing the filter, what you will accomplish is to drive the sludge and particles out of the filter back into the extremely tiny passages of the transmission. The result can be disasterous.
There are only two good ways to do this if you want the transmission to continue to function properly:
Drop the pan, clean it out, change the filter, install a new pan gasket, and refill with the correct spec fluid.
Change the filter (which may involve dropping the pan anyway), and then have the machine process done.
Many of the better transmission shops do it as VDC described. They combine BOTH procedures. But if only one is on the plate, drop the pan and change the filter.
if you are going to do this yourself, the way to reduce the chances of leakage is to wash all the mating surfaces with slovent, alcohol, or keytone. This will cut to old oil and allow a clean dry seal of the gasket when you replace the pan.
ALWAYS drop the pan and change the filter with a service. RUN from the shop/s which dont do this.
While I prefer to do both, every time I did a second pan drop there was virtually no debris in the pan. The filter itself is a sieve. All suction side filters are sieves.
It’s not a bad idea to see if you’re shedding MORE stuff, but …