A method and a device for detecting the oil level in a hydraulic pulse unit at an impulse type power wrench, wherein the pulse unit comprises an oil filled pulse chamber, a motor driven inertia drive member, and a kinetic energy receiving member connected to an output shaft and intermittently coupled to the inertia drive member via an oil cushion to receive kinetic energy from the inertia drive member during repeated energy transfer phases. The rotation speed of the inertia drive member is measured both at the start of a pulse generating phase and at the end of that phase, wherein the two measured speeds are compared to each other, and when the speed at the end of the pulse generating phase exceeds the speed at the end of that phase by a certain percentage a faulty signal is emitted indicating a too low oil level in the pulse unit.