Why would there be transmission oil in the radiator?

Transmission oil in the radiator indicates a catastrophic failure of the automatic transmission cooler. Automatic transmissions use a cooler, often integrated into the radiator, to dissipate heat. This cooler is usually a separate circuit within the radiator, but a leak in the transmission cooler lines or the cooler itself will allow transmission fluid to mix with the coolant.

The most likely causes are:

* Transmission cooler failure: The cooler itself can develop a leak, either a crack or a seal failure. This is the most common cause.

* Transmission cooler line failure: The lines connecting the transmission to the cooler can rupture or develop leaks due to corrosion, age, or damage.

* Radiator failure: Less common, but a leak in the radiator itself, near the transmission cooler section, can allow the fluids to mix.

This is a serious problem requiring immediate attention. Driving with mixed transmission fluid and coolant will damage both the transmission and the engine, likely leading to costly repairs.