Diagnosis: Remove the Hall effect sender from the transmission/extension housing and inspect the drive key. Look for rounded corners on the square shaft, twisting, or metal shavings.

Verification: Reinsert the key fully into both the sender and the transmission output gear. Ensure there is adequate engagement length (typically at least 1/2 inch of key depth). If the key wobbles excessively inside the socket, replace the key or adapter driver.

Driven Gear / Drive Gear Wear in Transmission
Ford speedometer drive systems use a plastic driven gear driven by a worm gear inside the transmission tailhousing. High RPM can cause a worn or chipped gear to jump or bind under load.

Diagnosis: Inspect the plastic driven gear on the end of the speedo cable/sender assembly. Look for missing teeth, uneven wear, or a stripped center bore where the key inserts.

Verification: Shine a light into the transmission tailhousing hole and inspect the internal drive gear (driven gear clip/worm gear) while turning the driveshaft by hand. Ensure it is firmly secured to the output shaft and doesn't rotate freely on the shaft.

Mechanical Vibe or Shaft Wobble at High RPM
At higher speeds, driveshaft vibrations or internal transmission output shaft movement can cause the sender body or drive gear to lose consistent mesh.

Diagnosis: Check if the sender body is clamped securely to the transmission. If using an inline Hall effect sender with an adapter cable, check if the cable is binding or bent at a sharp angle.

Verification: Ensure any drive cables have a smooth radius (no bends tighter than a 6-inch radius). Ensure the clamp/fork securing the speedo assembly to the transmission case is tight.

Signal Noise or High-Frequency Dropout
Though less common given your symptom, electrical noise (RFI/EMI from ignition wires or alternator) can increase with engine RPM, or the Hall effect sensor internal pull-up resistor/ground may be fluctuating under load.

Diagnosis: Inspect the 3 wires coming from the sender (Power, Ground, Signal). Ensure the ground wire goes directly to engine block ground or sensor ground, not a daisy-chained chassis ground.

Verification: If the key and gears are mechanically sound, shield the signal wire or reroute it away from spark plug wires and the alternator.

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