What is the maximum allowable deviation of speeds during an ASM test?

What is the maximum allowable deviation of speeds during an ASM test?

• A. Plus or minus five mph
• B. Plus or minus ten mph
• C. Plus or minus two miles per hour
• D. Plus or minus one mph

Answer: (C)

During an ASM test, keeping the engine speed tightly controlled is essential because small changes in speed alter engine load, air-fuel balance, and exhaust behavior, which directly affect emissions readings. The target speed can drift only within a narrow range—plus or minus two miles per hour. This tight tolerance helps ensure the results are consistent and comparable, reflecting the test cycle rather than unintended speed variations. If the speed moves beyond two miles per hour, readings can shift, making the test data unreliable and often requiring a redo.

During an ASM test, keeping the engine speed tightly controlled is essential because small changes in speed alter engine load, air-fuel balance, and exhaust behavior, which directly affect emissions readings. The target speed can drift only within a narrow range—plus or minus two miles per hour. This tight tolerance helps ensure the results are consistent and comparable, reflecting the test cycle rather than unintended speed variations. If the speed moves beyond two miles per hour, readings can shift, making the test data unreliable and often requiring a redo.