How is trans fluid sucked in to your motor?

Transmission Fluid can be sucked into your oil if the modulator valve fails on the transmission. A modulator valve is mounted on the side of a transmission and is hooked to a vacuum line that is connected to the engine. If the diaphragm in the valve ruptures then you end up sucking transmission fluid via the vacuum line into the engine. If not repaired promptly you can very easily double the volume of "oil" in your engine as the transmission fluid mixes with the oil. A simple test is to disconnect the vacuum line that runs down to your transmission and if it is filled with trans fluid the diaphragm in the valve has ruptured. A modulator runs less than $20 and takes about ten minutes to replace (if you can get at it). On some vehicles there is an access plate cut into the transmission hump (under the carpeting) that allows you to access it from within the interior of the vehicle. 1994 Mazda B3000 for example.