TM 9-2320-363-20-1 Table    3-1. Troubleshooting (Cont) Malfunction Test   or   Inspection Corrective Action ELECTRICAL SYSTEM   (CONT) Panel    Light    Circuits    (Cont) Step    3. Disconnect lead 29A from heater control light. Check for   +12    VDC at lead    29A    from    panel fuse. . If   +12 VDC is present, repair lead 29A to heater control light. If no voltage is present, repair lead 29A from    fuse (page 3-2). 4. ONE OR    MORE GAGE LIGHTS NOT OPERATING. Check panel    gage lamp(s). . If   defective, replace lamp(s) (page    4-176). 5. FIBER OPTICS NOT OPERATING. Step    1. Check    fiber    optic    light   source lamp. . If   damaged, replace lamp    (page    4-193). Step    2. Check    for   continuity between lamp    socket    and   ground. . If   continuity is   indicated, go   to   step   3.    if   no   continuity    is indicated, repair    ground    lead    (page    3-2). Step    3. Disconnect power lead    to   fiber    optic   light   source. Check    for   +12 VDC at power    lead. . If   +12 VDC is   present, replace fiber   optic   light   source light (page 4-1 93). If   no   voltage is   present, repair    power    lead (page 3-2). 6. TACHOGRAPH LIGHT NOT OPERATING. Step 1. Check    for   continuity between ground    lead    and   ground. . If   continuity is   indicated, go   to   step   2.    If   continuity    is   not indicated, repair    ground    lead    (page    3-2). Step    2. Disconnect lead    29A from   tachograph. Check for   +12 VDC at   lead    29A. . If   +12 VDC is   present, replace tachograph (pages    4-178, 4-1 82). If no voltage is present, repair lead 29A (page 3-2). 7. PANEL LIGHTS DO    NOT DIM. Disconnect lead    420B from    electronic dimmer switch.    Check    for   +12    VDC at   lead 420B with    main    light   switch    in   dim   mode. . If   +12 VDC is   present, replace main    light   switch   (page 4-170). If no voltage is present, replace electronic dimmer switch (page 4-168). 3-60