Section III. UNIT TROUBLESHOOTING PROCEDURES
4.7 INTRODUCTION. The FDECU uses a printed circuit board (pc board) to automatically operate and control unit
components under various conditions. This pc board has a comprehensive monitoring, testing, and diagnostic capability
that can be used to perform much of the troubleshooting for the unit. The following procedures will provide information
and procedures to access the control components for diagnosing troubles, and a detailed explanation of the pc board
logic sequence with a layout of pertinent components on the board. Temperature potentiometers on the pc board are
used to establish temperature set points for various control functions and will affect troubleshooting if not adjusted
correctly. A procedure is provided to set these potentiometers if the settings are suspect. Thermistor temperature
sensors provide the pc board with temperature input signals. To properly troubleshoot the unit, a testing procedure is
provided to check these thermistors. General troubleshooting procedures are provided to address non-specific
troubleshooting items such as wire lead and harness testing, contactor and overload testing, motor winding testing, etc. A
table of branching logic tree symptoms and corrective actions is provided to troubleshoot specific fault symptoms.
Procedures are then given to secure the unit and return it to service. The following procedures cannot address all
possible faults that could occur with the unit but they do cover the most common symptoms and should give enough
general information to help logically diagnose other symptoms effectively. When performing any troubleshooting functions
or checks refer to the necessary equipment setup procedures found in Section I of this Chapter then operate per
Lethal voltage levels are used in operating the FDECU. Disconnecting power will
reset the fault indicators. Therefore it may be necessary to access the pc board
and power box assembly components when power is connected to effectively
diagnose the trouble while the unit is still in the fault mode. Be sure to observe
extreme caution when accessing electrical assembly if power is connected.
The pc board contains high voltage on control circuits as do the contactor coils.
Injury or death can occur if personnel contact any electrical terminal while the unit
is connected to power source.
b. Loosen eight captive screws (1) and carefully pull the electrical assembly (2) out.
c. Turn to release three latches (3) and open power box assembly cover (4).
d. Loosen eight captive screws (5) and carefully remove panel (6).