Turbotoerentalsensor

Reports how fast the turbocharger shaft is actually turning, usually by sensing the compressor blades passing. It is a protective instrument before it is a control input, and that explains why most turbocharged engines do not have one: boost pressure is normally a good enough proxy, and the turbo is sized with margin to spare. A speed sensor appears where that margin has been engineered away — high-output diesels and downsized petrol engines running close to the limit — because the load trying to burst a compressor wheel rises with the square of its speed, so a modest overspeed is not a modest overload.

Not the same part as a transmission turbine speed sensor, which measures the torque converter's turbine inside the gearbox. The names are close enough that parts lookups and search results cross over constantly; this one lives on the turbocharger, in the exhaust stream.

Two diagnostic properties are worth knowing. Because it reads blades going past, a shaft moving on worn bearings produces a jumping signal — which makes an erratic reading an unusually direct early report of a turbocharger starting to fail. And because the module targets boost pressure rather than speed, a leak downstream makes it drive the turbo harder and harder chasing an unreachable target, so a genuine overspeed is often caused by a split hose rather than by anything wrong with the turbocharger.