Foretravel Owners' Forum

I am about to drain and change my transmission filters (myself) for the first time. I'm also converting over to Transynd with a 131,000 miles on the coach. Purchased the filters and fluid from Stewart Stevenson in Albuquerque. The guy at the counter recommended changing the filters, filling the transmission, and then disconnecting the input cooling line from transmission, start the engine and letting the old fluid still in the system drain into a bucket until the new fluid starts to emerge, then yelling to person at the front to "turn it off." I am much more inclined to drain, replace filters, and put in Transynd and then change again at the "non synthetic" interval. He said that would be okay, but he prefers the other method.

What say you of more experience than I? I've read several posts here on the forum and it seems most go with change and change again. I can see me getting into a serious mess with the change, purge, and refill method.

Your inclination is what Allison Corp recommends.  His method would be preferred if the fluid is really bad/contaminated.

And assume your new fluid is Transynd 668.

Seems like the line disconnect method has some undeniable positive aspects. 

One drain and fill rather than two.
Ultimately cleaner fluid and filters in the trans rather than semi contaminated with old fluid. 
Buy the fluid once, do the process once, dispose of old fluid once.
With a reliable helper, a careful systematic approach,  and remote start stop in engine bay, seems readily doable.

Just change the fluid and filters and drive it unless it's contaminated. It will mix perfectly fine with the old fluid. It might already have transynd. Probably has had the fluid and filters changed at least once.

We think all that new Transynd is enough to overcome some old fluid, so drain & fill and changed again in the future when 100% Transynd recommends which is a long time from now. Remove one of those old flare fittings just to drain, you may be finding a new problem. Change filters in the future again sometime. Be very careful with bolts, drain plug, etc as those steel threaded items are known to 'eat' the softer aluminum tranny bottom, similar to the M11 engine drain plug. Tranny and engine drain plugs have a reputation of taking pan threads with it. Allision drain plug hole problems are so common, Allison has published remedies, like a longer drain plug that grabs a second set of pan threads that are located up in the pan above the bottom threads. Also sell a ream & tap to use their oversize drain plug. Being very careful with torque measure when installing drain plug and bolts is no guarantee next time removal will not have problems.

 The gentleman that invented TranSynd will send a kit to send back to him a sample in Indiana at no charge if you are not sure of current fluid

My transmission has had three filter and fluid changes to my knowledge, all conventional. Barry, thanks for the cautionary note on the drain plugs. When I bought my coach eight years ago had to abort the first oil change because someone had strong armed the oil pan drain plug. That was not cheap. Now use a Fumoto drain and do it myself whenever possible. I'm probably going to do the drain and fill because I'm already dealing with a failed King Throttle Control. That's another post. Hoping to get things sorted in time to make the pilgrimage to Quartzsite.

[see King Control discussion here - KC throttle control (https://www.foreforums.com/index.php?topic=48749.msg492347#msg492347) - Michelle]