Proper hose routing is from the transmission - past the temperature gauge sender - into the radiator wet cooler - then onto the supplemental dry cooler and back to the transmission. The gauge sender should be immersed in the fluid flow between the transmission and wet cooler inlet rather than in the transmission case or pan. Your primary interest is in the working temperature of the fluid exiting the transmission, which represents actual transmission heat production or waste. When the temperature sender is inserted into the fluid flow, the driver receives immediate feedback of realtime changes in trans temp, which can forewarn of developing transmission trouble. However, care must be taken so as not to install the sender such that it impedes fluid flow.
According to Hayden Automotive (http://www.haydenauto.com/Featured%20Products-Transmission%20and%20Engine%20Oil%20Coolers/Content.aspx), automatic transmission life doubles for every 20° F drop in transmission fluid temperature. Automatics enjoy their greatest service life when routinely run below ~180° F.
Some installers go so far as to place a redundant sender in the outlet (return) side of the coolers to monitor cooler effectiveness. The gauge can then be flipped back-and-forth between the two senders using a toggle. Redundant senders aren't really necessary, but satisfy curiosity.
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As previously stated, there are many approaches to line routing. Here is just one option: Layout and install two 1/4" NPTF x 3/4" bulkhead fittings (pictured) through the radiator support. Then install two 1/4" NPTM x 3/8" barbed 90° elbows into the front of the bulkhead fittings for the dry cooler hoses. When installing the new dry cooler, do not install the hose barbs pointing down - this prevents air from being trapped inside the cooler and maximizes heat transfer. Route and secure the hoses through Adele clamps to keep them from chafing and away from the hood safety latch.
On the backside of the bulkhead fittings install two 1/4" NPTM x 1/2-20 inverted flare fittings. Connect the dry cooler line that returns to the transmission to one of the bulkhead fittings through a 1/2-20 inverted flare union and a double flared 5/16" steel brake line of the appropriate length.
Connecting the "outlet" of the radiator wet cooler to the bulkhead fitting requires a little more ingenuity. Procure a 5/16" double flared steel brake line that is sufficiently long to connect between the inverted flare bulkhead fitting and the radiator fitting.
- Notice that the radiator is mounted in rubber cushions. The 5/16" steel line MUST BE LONG ENOUGH that you can bend a double pigtail into the new 5/16" line between the radiator and the bulkhead fitting to allow for radiator movement (look at the brake lines that attach to the master cylinder for an example). If you don't incorporate the pigtail into the 5/16" line, you run the risk of breaking the line at the fitting and possibly damaging the radiator tank!
Alternatively, you can use rubber hose or AN hose with the appropriate fittings instead of steel line.