AIR TRACTOR AT-502A, Maintenance Section

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Maintenance Page 56 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
Troubleshooting Procedures
Static Tests -
Before going into specific Symptoms which will result in operational “squawks” it is
productive to point out a static test routine that is “quick and easy” and will result in localizing some problems
without starting the engine. This procedure entails removing the connector from the GCU and checking with
the ohmmeter from designated sockets in the aircraft side of the GCU connector to various other designated
points in the system. Bear in mind that the ohmmeter test leads will usually have a small resistance, say .1 to
.4 ohms, so before starting, short the probes together and get this reading. Subtract this reading from all
point to point readings.
Initial Setup- Master Switch OFF, Generator Switch ON.
From To Normal Notes
Value
GCU J S/G B 0 If high, check Start Switch, Generator Switch, 15A C/B, and
all associated wires and connections.
GCU L S/G E 0 If high, check wire and connections from L to ground.
GCU G S/G E 0 If high, check wire and connections.
GCU M S/G A 0 If high, check wire and connections.
S/G A S/G E 2 If high, suspect S/G field is open.
If low, suspect that S/G field is shorted or GCU M to S/G A
wire shorted to ground.
GCU D S/G D 0 If high, check wire and connections.
GCU H LCR X2 0 If high, check wire and connections.
LCR X2 S/G E 50 If high, suspect LCR Coil is open.
If low, suspect LCR Coil is shorted or GCU H to LCR X2
wire is shorted to ground.
GCU A LCR A2 0 If high, check wire and connections.
GCU B LCR A1 0 If high, check wire and connections.
In the following, the expression “GEN will not come on line” is evidenced by a low Lower Instrument
Panel Voltmeter reading (25V or lower) and the LV Light staying on if so equipped.
Symptom - After engine start, GEN will not come on line unless the Start Switch is engaged momentarily
after the Generator Switch is turned on.
Probable Cause - Field Flash function of GCU inoperative.
Fix - Repair or replace GCU.
Notes - The system can be operated as described without creating further detrimental effects until a
convenient time to have the GCU replaced or repaired.
Symptom -
During engine shutdown the starter “motors on” if the Generator Switch is left on.
Probable Cause - Reverse Current Protection function of GCU is defective.
Fix - Repair or replace GCU.
Notes - Turning the Generator Switch off prior to engine shutdown will correct this condition as a stopgap
measure but the operator should be aware that if this precaution is overlooked damage to the
starter generator can result.