Originally Posted by EMAW

Here is an excerpt.

Approval for single pilot operations with use of an operative
approved autopilot system under FAR 135.105 gives an operator an
additional option in the conduct of operations. It does not
mandate that all future flights be conducted in that manner. The
operator can elect to fly trips with two pilots, as is otherwise
required for flight in IFR conditions under FAR 135.101, using
the second in command instead of the autopilot.

Your second question asks if, under the circumstances given
above, the SIC can log time as SIC when the designated pilot in
command is flying the aircraft. The answer is yes, as long as
the certificate holder is using the SIC as a crewmember instead
of exercising the autopilot authorization. In other words, the
certificate holder elects not to conduct an IFR flight using the
single pilot with a functioning autopilot option, but rather
conducts an IFR flight using two qualified pilots. The two
pilots are then "required by the regulations under which the
flight is conducted", FAR 61.51(c)(3), and the assumption is that
the second pilot (SIC) will function as a required crewmember,
and SIC time may validly be logged. However, if for some reason
another qualified pilot "rides along" and does not function as a
crewmember, then second-in-command time may not be validly
logged.

This doesn’t say that the autopilot has to be inop. If you are trained and functioning as a crew member, SIC time is loggable under part 135 passenger carrying ops. The way I read it, 135.105 and A015 gives operators an “option” if the don’t have a second pilot on staff or available.