A DHC-8-100 inbound to a station with known icing conditions diverted back to their departure airport when the flight crew discovered one prop de-icing system was malfunctioning.

Narrative

I decided to perform a check of the ice protection systems in advance of reaching known snow and ice on the approach into our destination. We had not originated the airplane; so there was no reason to check the systems on the ground prior to takeoff. I am in the habit of using low workload periods in cruise to double check aircraft systems that I know will be safety critical during the high workload approach. For this reason; I 'double checked' all of the de-ice/anti-ice systems; including the #1 and #2 prop heats. During the course of the test; I noted that the #1 prop 'A' phase was not showing an indicator light and only displayed a .1 increase in load on the aircraft load meter. I attempted the check using both prop timers; even though I surmised (correctly; it turns out) that we probably had a broken lead to one of the blades. I was unsuccessful in getting the #1 A phase to work properly. After transferring the flight controls and the #1 COM to the First Officer; I contacted dispatch who told us to take the aircraft back to our departure station. I briefed the flight attendant and passengers; resumed flying pilot duties and we returned without incident.