Which of the following is an indication that an aneroid device has been shorted out on a hydromechanical highway truck diesel engine?

• A. Increased top end power
• B. Improved lug performance
• C. Smoking at rated speed and load
• D. Puff of exhaust smoke at each upshift

Answer: (D)

The key idea is how the aneroid device influences fuel metering in a hydromechanical diesel governor. The aneroid provides automatic compensation for changes in air density and engine load, adjusting the fuel delivery as conditions vary. If that device is shorted, the metering system can’t respond properly to transient changes—like those that happen when you upshift. Each upshift alters airflow and load suddenly, and without the aneroid’s adjustment, the engine may receive a brief overfueling, producing a visible puff of exhaust smoke with every shift. This transient smoking pattern is a telltale sign of the aneroid being effectively bypassed. The other options describe conditions that don’t align with the transient, shift-related nature of this fault, such as a general power increase or sustained lug improvements, which wouldn’t result in periodic smoke puffs tied to upshifts.