Search for "Hoot" method and flush it that way. It is a flush where you use the transmission to pump out the old fluid as you fill it with new. It is very easy. Allison says not to use a machine as they might delaminate the clutch disks.
we have one here at the dealer. and use it every trans r&r, but never for a fluid change. i have used it to flush trans coolers though. how many miles on the truck. and if the fluid is not to burnt, which i bet it is not, just open the drain plug and let it rip. change the spin on and be done with it.
The truck has 23,000 miles on it. I was wanting to put the new Mobil DelVac Trans fluid in it, thus exchanging all the fluid would be nice. My thinking is if I have all the same fluid in the trans, the better off I will be and I can easily pull the plug and change the spin on filter myself as often as I like. Any thoughts?
As far as how to change, if you drain the stock pan, you'll drain about 7 qts out of a 12 qt system. You're not getting it all out, but this is the easy way. If you did this again, you'd be down to only a few qts of original ATF in the system.
If you want a complete flush, this isn't too hard to do and you flush out 99% of the old. Here's a link to the "Hoot Method".
The Hoot method starts at post #25: http://dieselplace.com/forum/showthread.php?t=14301&highlight=transmission+flus&page=3
Thanks, I appreciate the help. What about the internal filter? I know everyone says it only needs to be changed at overhaul. I spoke to a local transmission shop, claiming to be deisel experts, and they insist on changing the internal filter and wiping the inside of the pan out. Also, anyone using the new Mobil Delvac trans fluid?
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