The 604 / 41TE has been around for some time, and we are all pretty familiar with it; however some things still pose a challenge on this transmission and one of them is leaks.
Over time we have seen numerous fixed in vehicle reports that have solenoid packs being changed for leaking. In response to that, three years ago we made a special test plate so our rebuilt solenoid blocks can be charged with compressed air, and then sprayed all over with a soapy water solution. Any air leakage would then show up as bubbles in the soap, and would expose the leak. Before that, we had an occasional one or two blocks that would leak on the dynos. Now that has gone away, and we don't see that problem anymore.
What hasn't gone away is solenoid blocks that are being changed and reported as fixed in vehicles for a leak. Of the blocks that we get back, we are usually unable to find any leaks, unless the pack has been externally damaged in some way.
It is my experience that the oil leakage that the block is being blamed for, usually comes from somewhere else and ran down onto or around the solenoid block. There are cooler lines (notorious leakers), a fill port, pressure taps, vent, and an input sensor, all just above and around the solenoid block. Plus, it could be just plain oil spillage from filling the transmission with fluid, easy to do. In any case all these places and possibilities should be investigated thoroughly before just assuming the block is leaking and changing it which is expensive if replaced, and it doesn't fix the problem.
Oil always runs from the highest point down, to find the source of the leak, follow the oil trail up as far as it goes, always look to the highest possible leak source first. This goes for just about any leak on anything, follow it up as far as the trail goes, and you'll usually find the culprit.
If you're not having any luck because the leak trail is hard to see, there are dies available to make it easier for you. Run the transmission for awhile to mix the die in with the transmission fluid, then with a black light the leak trail will show right up. If it happens to be a slow leak, you may need to ask the customer to drive it for a few days then bring it back for a black light inspection.
Illustrated are some of the areas to check for as possible leak sources. Don't forget to look at the back of the engine as well; motor oil leaks are often confused for transmission leaks.
Along with noises, and vibrations, leaks can be hard to find, but the one thing to remember when hunting them is to ?Always Look Up and follow the trail.