If one of the lines to or from the transmission cooler were blocked, like in pinched or a secondary filter were plugged, where would the fluid go? I assume the hydraulic pressure would be high enough that it would escape or damage something rather than just stop, right?

 

Only a portion of the fluid flows through the cooler, the rest returns to tank. If the cooler circuit were to become blocked, the fluid would just follow the path to tank. This will create higher back pressure than normal, which may affect how it shifts, but nothing catastrophic will occur until it overheats due to lack of cooling.

 

The "overheat" is the main concern.

 

Certainly the 42RE has what’s called a ‘ drain back valve ‘ in the actual line to the radiator cooler. This little item is flagged by rebuilders as the most overlooked item that kills transmissions. It becomes totally clogged. I think the XJ ‘ might’ have this too. I just removed mine and bypassed it saw an improvement. I do need to first shift to N in the am for like 3 seconds to fill the converter.
that said, the transmission cooler in the radiator is also known to clog. I always thought the inside was just a 3/8” coil; it isn’t. It’s a complex field of very this slits that also plug up.
I bypassed the cooler and mounted a $20 eBay external cooler in front of the rad.

 

How is everything going now?