vince
New Member
Posts: 3 
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Post by vince on Aug 30, 2020 11:59:54 GMT -9
Partner forgot to raise flaps after flight. Went back to hangar put on master switch and no response (no gear lights, turn&bank gyros, door open light, nada. Tried overhead light and came on bright so battery and hot bus OK, Checked all CBs, and all were in. Hooked up external power unit and still no indication of power to acft bus. After reviewing wiring diagram in service manual I see 3 possibilities: both master and external power contactors failed (not likely), 1N1183 diode across master contactor failed, or open on cable to main bus. What say you please. By the way I believe that diode is there to provide current path from external contactor to main bus and to prevent reverse current. Is this correct? This airplane is early bird SN PA31-198. Later models did away with this diode.
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Post by klausnw on Aug 30, 2020 14:12:52 GMT -9
The Master Solenoid (another term is 'Contactor') is most likely faulty. Listen for the solid 'Click' when you switch the Master Switch - ON. Another test, run a jumper from the battery side of the Solenoid to the Aircraft system side. If you do the jumper test be aware that the flap motor and other electrical items will all come on at the same time and make a big spark. Be sure the Avionics are ALL turned - OFF.
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Post by Vinny on Aug 30, 2020 18:40:51 GMT -9
I'll give that a try but I don't see why that would keep the GPU from powering the main bus.
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Post by vincec on Sept 2, 2020 11:50:32 GMT -9
Found the problem. The conductor from the Master contactor to the shunt failed at the shunt. Apparently the bolt/nut attaching it to the shunt had come partly loose allowing sparking to occur. Shunt was partially burned and melted with most of the phenolic insulator turned to dust. One of the alternator shunts had the same problem to a lesser degree, but there was evidence of overheating. Attachments:
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Post by klausnw on Sept 2, 2020 16:06:13 GMT -9
It's getting tougher and tougher to troubleshoot these aircraft as they get older. Shunts usually are a simple enough item they don't fail but, as the connectors and wires age they create higher resistance. It might be a good time to disconnect the battery and check the tightness of all the other electrical connections throughout. I like to use a small paintbrush and paint all the terminal ends, connections and such with CorrosionX or ACF-50.
Especially grounds, disassemble where they connect to the aluminum then clean, lube and reassemble.
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