[stackj]

Ed,

This problem is perplexing.

I do not see any way a bad diode in that circuit could cause the master solenoid to engage. I can see a shorted diode causing the fuse to blow. Those diodes should be allowing current from the alternator field to flow back to the bus via either the alt field circuit breaker or back to the battery via the other diode, through the GEN fuse to the positive battery lead attached to the master solenoid.

The only way I can see the master relay engaging by inserting the GEN fuse is by having two faults.

1. the circuit between the two diodes is grounded, and

2. the wire from the fuse to the master solenoid is connected to the wrong terminal on the solenoid, i.e. the same terminal which the master switch grounds.

It might also be interesting to disconnect the master switch lead from the master solenoid to see if the symptom changes. This would be rare but could be caused by corrosion inside the master switch. Corrosion in electrical circuits does strange things when energized.

Try verifying the wiring connections and if that is OK, try disconnecting the master switch lead from the solenoid.

Also, be sure there is insulation between the diodes and the airframe. I think the metal case of the diodes is connected to their cathodes and if it is not insulated from the mount, it will be grounded. That coupled with the miswiring described above would cause the problem you have described.

I have not looked for the location of the other diode, but I expect it would be behind the panel somewhere between the master switch and the Run/Stby switch. There is also a resistor / diode circuit board behind the interior left paneling just forward of the circuit breaker panel. I don't think there is a third diode on this board though. This board disables the avionics bus when either starter is engaged.

Maybe I should ask an intelligent question. When you insert the GEN fuse, do you hear the master solenoid engage? If you don't, probably all I've said above is inapplicable for your problem.

Jim