Tightening the speedometer cable nut has no effect on the speedometer adapter seal effectiveness, so I would not crank it down.
Also, at the risk of sounding like Captain Obvious, I would double check the fluid level to ensure the trans is not overfilled.
Its been like 40+ years since I touched a THM350, but I seem to recall two seals on the speedometer gear adapter. An O-ring that seals the adapter to the extension housing, and another one that seals the speedometer driven gear to the adapter -which helps keep ATF out of the speedometer cable.
Call me a skeptic, but it just seems suspicious to me that the speedo adapter did not leak until the seal was replaced by the trans shop. Is it possible that during installation, the shop either nicked the O ring, or a mating surface ?
Question to all the trans gurus out there: I have heard that converter drainback is a fairly common issue on older THMs. However, it is unclear to me if this is mainly just a problem that shows itself when the seals are not 100%? Some reports claim that when various seals were replaced, the leak was corrected. Maybe it just depends what degree of drainback occurs? (i.e. perfect seals can tolerate a lot of drainback, marginal seals some, and worn seals none ? )
Seems like by design, the seals would have to be able to hold back a fair amount of fluid. What if the vehicle were parked on an incline ?
With regards to the selector shaft, could there be a burr, nick, or pitting on the shaft that could be tearing up the new seal? Or could it too have been nicked during installation?
Best Regards...
