"When using the impact pistol and then the CTD, were there any enemy surface or air assets around, shooting at the Asashio?"
No,it was night, rough seas. I continued on without shooting the Asashio at bearing 335, later attacked and sank a Kongo 6 hits with magnetic, sank an Akagi with stern tubes, again magnetic, then dove and avoided four Asashio and later came up to periscope and decided to shoot a trailing Asahio, 2700 yds astern, hit it, magnetic, explosion, and sank it. Go figure, now there was no CTD
