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Don't you mean Allison transmission/Duramax engine?
What is a major transmission service on an Allison transmission? I have a 2012 Silverado 3500HD dually. I talked to my dealer a while back about getting my ally transmission serviced and what it entailed. He said all they do is replace the spin on filter and top off the fluid. I said that's it. That's all you do? He said that's all that is required by the manufacturer. I didn't believe him so I called an Allison transmission dealer in my area and asked them about getting it serviced and what they do. He said the same thing. I said you're kidding, you don't drop the pan or flush it out. He said no, you will cause problems and could damage it if you flush it. He said unless you are having problems with it there is no reason to mess with it. Just change the filter and top it off.
A lot of dealer service advisors will try to " up sell" you things you don't need because they get commission on it. A friend's son was a service writer for a dealership and quit after a few weeks because they kept pushing him to " up sell " especially to woman and elderly customers. He quit because he didn't want to do it..