What would likely cause the pointer of a magnetic drag tachometer to oscillate?

What would likely cause the pointer of a magnetic drag tachometer to oscillate?

• A. Excessive engine RPM
• B. Battery failure
• C. A dry cable connecting the indicator to the engine, or a kink in the cable housing
• D. Loose ground connection

Answer: (C)

Magnetic drag tachometers get their motion from a drive cable that spins a magnet inside the instrument, with magnetic drag providing the damping. If that drive cable is dry or kinked, it won’t transmit torque smoothly. The cable binding or snagging changes the input to the tachometer unpredictably, so the needle can flutter or oscillate around the actual RPM instead of moving smoothly. Replacing or repairing the drive cable (and ensuring proper lubrication and routing) typically fixes the issue. In contrast, excessive RPM would push the needle to a higher reading in a steady way, and battery or ground issues wouldn’t cause this specific flutter in a magnetic drag system.

Magnetic drag tachometers get their motion from a drive cable that spins a magnet inside the instrument, with magnetic drag providing the damping. If that drive cable is dry or kinked, it won’t transmit torque smoothly. The cable binding or snagging changes the input to the tachometer unpredictably, so the needle can flutter or oscillate around the actual RPM instead of moving smoothly. Replacing or repairing the drive cable (and ensuring proper lubrication and routing) typically fixes the issue. In contrast, excessive RPM would push the needle to a higher reading in a steady way, and battery or ground issues wouldn’t cause this specific flutter in a magnetic drag system.