03 Trailblazer 4L60E; closing on 250K miles. Original trans, no service other than a previous fluid/filter change.
Trans was slow to engage reverse, only when cold (below freezing). Worked great 3-seasons, slow to hit reverse first thing in the morning in winter.
Took it to a local, trusted trans shop. They had no idea; thought maybe the valve body warped and didn't seal when the cold contracted the metal.
Ignored the problem for about ten years. So long that I've kinda lost track of when this issue started, but "years and years" Every winter, slow to engage reverse first-thing in the morning, every spring/summer/fall the trans worked beautifully.
A year and a half ago, I thought I was getting some shift-flare under heavy throttle. Not every time. As soon as I felt the flare the third time, I remembered that I was way overdue for a fluid flush and filter change. Fluid was getting maroon rather than "red" like it should be, but not "burnt" or "brown".
Dropped and cleaned the pan, (it had very little debris) replaced filter, reinstalled pan. Dropped a cooler tube, flushed all the old fluid until the fluid coming out of the cooler tube was virgin-new looking. Reconnected cooler tube, topped-off fluid level. 15 quarts of Dex/Merc (Dexron III equivalent) altogether.
This is the second winter that I've had no problems engaging reverse. Practically a miracle-cure.
