Wheel slip was found to be a problem with the more powerful 500-series motors, so in the late 1960s through early 1970s the "weak field" power generation, which used the back-EMF from the motors, was disconnected. 547M was fitted with an experimental wheel-slip solution, but the final implementation as applied across the fleet would close off the power supply, then re-open it bypassing the acceleration control when wheel-slip was detected, with limited success. The weak-field system was reconnected in the late 1970s to early 1980s in order to keep up with amended timetables, but this brought back the wheel slip issues.