The transmission yoke that the mechanic speaks of is the part that attaches the universal joint (at the front of the driveshaft) to the transmission output shaft. On the outside diameter of the yoke is a "tone wheel" which looks like evenly spaced teeth on the yoke. The VSS (Vehicle Speed Sensor) is mounted with it's probe in very close proximity to the tone wheel. As the trans output shaft (with yoke & tone wheel attached) rotates, the VSS creates a pulse each time one of those teeth on the tone wheel passes by it. These pulses are "read" by the ECM which sends a signal to the speedometer based on how many pulses are occurring in a given amount of time, causing the speedometer to display your MPH (or KPH).
Since the VSS is in such close proximity to the tone wheel on the yoke, a loose yoke can cause the tone wheel to actually strike the VSS probe, destroying the VSS. When that happens, the ECM puts your engine into "limp mode" since it is no longer receiving a signal from the VSS.
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