<p>An algorithm is proposed for real-time detection of the natural frequencies of rotating shafts in the driveline using an onboard transmission control module (TCM). The algorithm, unlike previous methods, not only aims to detect the linear oscillating modes of the driveline but also incorporates the capability to identify both linear and nonlinear resonance modes. This algorithm is crucial for mitigating potential failures caused by cyclic loading, as it enables the detection of resonance modes beyond the linear range, which has been previously unexplored. By utilizing the fast Fourier transform (FFT) with respect to the revolutions of the transmission output shaft (order), the algorithm analyzes the vibration signals acquired from integrated transmission speed sensors. Experimental tests conducted on a medium-duty truck demonstrated the algorithm’s effectiveness in real-time execution on the transmission controller. Additionally, the algorithm’s capability to detect both linear and nonlinear resonance modes was validated through post-processing of additional data collected during these tests using signal processing tools. The results conclusively show that the proposed algorithm consistently and accurately detects the natural frequencies of rotating shafts in automatic transmissions across wide operating ranges.</p>