757 with an APU generator that would only engage for about two seconds before dropping offline
This problem is normally associated with the GCU (Generator Control Unit) sensing an out-of-tolerance condition. Over/Under Frequency, Over/Under Voltage, Missing Output Phase, or a Current Differential will cause the APU contactor to isolate the output from the aircraft distribution bus.
It's a guess...... but I believe the GCU also removes the field current from the generator which basically stops the generator from creating power.
I checked the output of the generator's PMG (Permanate Magnet Generator) with the APU running. This really shouldn't be the problem as the GCU can receive its power from a cockpit circuit breaker as an alternate source. The PMG should be primary power for the GCU. 62-64 VAC was observed on pins 2, 3, and 4 of the GCU's bottom plug. These measurements were taken to ground and not pin to pin.
Next up was to check the field coil in the generator. From the GCU middle rack plug, pins 1A and 1B a resistance of 12 ohms was noted. Each leg was also checked for a short to ground which was not present. (If the field coil was open, the generator should not produce power at all. Not even for 2-3 seconds.)
For the voltage sensing, pins 15A, 15B, and 15C of the middle plug were checked to ground. Pins A and B had readings of about 1-2 ohms to ground, pin C was open.
It was noted later that taking these reading from the GCU was not a true test of the voltage sensing wires. The readings really should be taken from the "L" (or input) side of contactor C905 with the wires isolated from each other for each separate phase. There are some other pick-off wires that might provide an alternate ground path that could possibly render the readings from the GCU rack as erroneous.
C905 is located in the P34 panel. It is partially obstructed by a circuit breaker box. This image is the top (or power input side) of the contactor. A Current Transformer assembly is located above it.
A reading from the contactor aft towards the generator verified the "C" phase power wire was open.
Upon removing the firewall plug, one broken wire and two fused pin/sockets were found.
Unfortunately, both the firewall receptacle and the APU side harness will require replacement. These are big pins on 1/0 guage wire. A pneumatic crimper is required for replacement. The receptacle is replaced from the Stabilizer Trim area and then mounted to the firewall.
Got lucky shooting from GCU. "C" phase is the only phase that doesn't have an extra pick-off at the contactor. If you look at the one picture, you can see two smaller wires on "A" and "B" phases and only one on "C". If the open was with one of those two other phases, I would of misread the meter results and the troubleshooting would of been much more drawn out.