Final answer:
The secondary fuel filter is usually located on the pressure side of the transfer pump to protect the engine by removing any remaining contaminants in the fuel.
Step-by-step explanation:
The secondary fuel filter in a diesel engine system is typically located on the pressure side of the transfer pump. This placement ensures that the fuel is filtered right before it enters the engine's high-pressure components, thereby providing a final cleaning stage to remove any small contaminants that could damage sensitive engine parts. Because its location is crucial for protecting the engine, the secondary fuel filter helps maintain a clean fuel system and prolongs the life of the engine.