In most aspects of mechanical design related to a motor vehicle there are two ways to treat dynamic fatigue problems, that is: time domain vs. frequency domain approach. Time domain approaches are the most common and most widely used especially in the automotive industries and accordingly it is the method of choice for the fatigue calculation of welded structures. In previous paper frequency approach has been successful applied showing a good correlation with the life and damage estimated by the time one; in this paper the same comparative process has been applied but now extended specifically to welded structures. Both frequency domain approach and time domain approach are used for numerically predicting the fatigue life of the seam welds of a thin sheet powertrain installation bracketry of a commercial truck submitted to variable amplitude loading. Predictions results are then compared with bench tests results, and their accuracy are rated. Ultimately the key question addressed within this study is whether the frequency domain approach can be a better alternative when compared to the conventional time domain approaches, when used to determine weld fatigue durability. It is demonstrated yet that when the fatigue damage is efficiently evaluated using the frequency (PSDs), it permits to better understand the importance of the dynamic characteristic of the actual system, isolating the mode contributions to the total damage, guiding how much a test can be accelerated or even how this damage can be minimized by changing the normal modes of this system.