The magnetic exploder was to be used from the beginning of the war. July 4th 1943 Lookwood ordered the deactivation of the magnetic detonators. Still, the impact detonators had issues. The rod that pressed the detonators upon impact were found to be shorter than required. However, some skippers deactivated the magnetic detonators before the order to do so because they knew these did not work. This was done at sea where BuOrd could not know about it.
Nice link to quickly see the dates and time for torpedo modifcations and introduction of new electric torps.
http://74.125.47.132/search?q=cache:...&ct=clnk&gl=us