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I just purchased a trans cooler for my 96 f150 and would like to know which factory line do I hook it up to,the top one or the lower one?In other words depending on the direction of flow, does the fluid pass through the radiator or the aux. cooler first?Or does it really matter?BTW the cooler is rated to 18000 GVW is this big enough for towing a 4000lb boat& trailer on 95% level ground 300 miles round trip 2x a month here in louisiana( 80- 95 degree temps).
charlez is correct. The preferred installation is 'in series' with the OEM cooler. The inlet to the OEM cooler is usually the upper connection, the outlet usually comes from the lower one.
One good way to confirm this is to trace out the supply and return lines from the transmission. The high pressure supply will come off near the torque converter. The low pressure return will go back in near the tailshaft.
You want to connect your auxilury cooler between the outlet of the OEM cooler and the transmission return line.
Here's a simplified flow diagram that should explain it: