It seems like it could be such a far stretch but your engine coolant, or also known as anti-freeze can get into the automatic transmission fluid. This coolant regulates the temperature of transmission fluid is regulated inside the engine cooling system. When the fluid passes through a small tank inside the engine’s radiator it cools the engine. If there was a rupture of the internal radiator tank can cause the coolant to mix with and contaminate transmission fluid. The engine coolant can also become polluted by the transmission fluid, as the pressures of either liquid vacillate. The extent of damage to either circuit depends on the severity and longevity of the internal leak.
All transmissions have a cooling line that runs to the radiator. A separate fluid passage circulates the transmission fluid through a portion of the radiator to cool the fluid. Sometimes, a leak develops in the radiator between the coolant passage and the transmission fluid passage, and the coolant gets into the radiator. It is odd if you are to find transmission fluid in your coolant. They are separate lines and there is no way for the lines to have been crossed if you recently had maintenance done to your engine. The best explanation is user and human error. The next option is that there is a leak in one of your systems and it is leaking over into the opposite system.
The best thing would be to bring your vehicle to our trained mechanics at Newman’s Auto, we will be able to determine what is going on with your system by tracing the problem back to the source. Our technicians have extensive training to ensure that every step of the system is tested correctly, and we can identify the problem before more problems arise from having transmission fluid in your coolant.