Both answers in here are half right, so to split the difference: on the 100 the radiator section is a pre cooler and the stack in front of the condenser is the main cooler. Running only the radiator section around town in mild weather will not immediately hurt anything — the A343F will live at reasonable temps at light load — but the margin is gone the second you're in stop and go on a hot day, on a grade, or with any load, and heat is what kills these transmissions. There's no A/T temp warning you can rely on as an early alert either; by the time you'd notice, the fluid has already been cooked. Before you decide, confirm you actually have a leak. Discoloration on the fins after a wash is very often just the residue that has been sitting on there for years plus the oily film that everything up front picks up. Wipe it clean, run the truck to temp, then look at the cooler and the two line fittings with a light — real cooler leaks show at the tube-to-tank ends or as a wet fin bloom that comes right back. You can also pressure test it off the truck with shop air and a bucket of water if you want to be sure. If it is leaking, the OEM cooler is still available and a used one from a parted truck is cheap. If you go aftermarket, get a plate and fin unit rather than a tube and fin, mount it in the same airflow path, and keep the flow direction the same. Bypassing entirely is fine as a get-home measure, not as the plan.
